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Best Marine Boat Batteries – A Complete Guide

Best Marine Boat Batteries

Below is our selection for the best marine batteries. While these are our top picks, be sure to read the remainder of the article to ensure you are purchasing the right battery for your needs. Links will open in a new tab.

Best Cranking / Starting Marine Battery

Best Marine Boat Battery Optima Starting Cranking Deep CycleOptima BlueTop Starting Battery

For a starting battery, you need one that’s going to be reliable, durable, and able to perform when you need it to. That’s why the Optima BlueTop Starting battery is our top pick for a cranking/starting battery. Optima is a well-known brand and uses spiralcell technology to build their batteries which offers a more energy-dense cell.

>> Check the Price on Amazon


Best Deep-Cycle Marine Battery

Best Boat Battery Renogy Deep Cycle Gel Renogy 100Ah Deep Cycle Pure Gel Battery

Renogy is better known for their solar power charging kits, which you should check out if you’re in the market for one. However, they make a pretty damn good battery too. With or without a solar kit, this battery simply performs. Utilizing high purity lead and a patented gel electrolyte, this battery is the best deep-cycle option available. A perfect option for building a battery bank.

>> Check the Price on Amazon


Best Trolling Motor Battery

Best Marine Boat Battery GreenLife Lithium Ion Deep CycleGreenLife GL Lithium-Ion Series or Universal Deep Cycle AGM

For small watercraft weight tends to play a factor in the battery you purchase. If you have the funds, purchasing a lithium-ion battery is ideal. Lithium ion batteries are incredibly lightweight, and are very compact compared to other types. Depending on your power needs, a 50Ah or 100Ah is the way to go.

For 50Ah (pictured), try the GreenLife GL50 50Ah Lithium-Ion Battery. For 100Ah, try the GreenLife GL100 100Ah Lithium-Ion Battery.

>> Check the Price on Amazon


Best Marine Boat Battery Universal Deep CycleIf the price for a lithium-ion battery is a bit out of your budget, try Universal’s 100Ah deep cycle AGM battery. It’s a great battery at a reasonable price.

>> Check the Price on Amazon





Best Lithium-Ion Battery

Best Marine Boat Battery Renogy Lithium ion Deep CycleRenogy Lithium-Iron Phosphate 100Ah Battery

Renogy takes another spot! Offered both in 100Ah, and 170Ah options, their lithium-iron (yup, you read that right, lithium-iron) phosphate batteries are a fantastic option for storing energy for multi-day use. If you plan on building a battery bank and have a larger budget, this is the way to go. The 100Ah model weighs under 30lbs, and the 170Ah model weighs less than 50lbs.

>> Check the Price on Amazon


Best Boat Battery Value

Best Marine Boat Battery Universal Deep CycleUniversal 100Ah Deep Cycle AGM Battery

Nothing fancy about these batteries, but they’re tough and they’ll do their job. This is the economic option for a deep cycle AGM style battery. Each puts out 100Ah and if you’re working on a budget battery bank or need to replace another deep cycle battery and keep it cheap, here’s your best option.

>> Check the Price on Amazon



Best Battery for Boat Lift Motor

Best Marine Boat Battery ExpertPower Boat Lift Deep CycleExpertPower 33Ah Deep Cycle Battery

You’ll want to consult the owner’s manual for your boat lift, but you generally won’t need a battery with a lot of power. ExpertPower’s battery is an AGM 33Ah deep-cycle battery. Hook this up to a drip feed solar charger and you’ll have all the power you need to operate your lift whenever you need it.

>> Check the Price on Amazon



Best Dual-Purpose Marine / RV Battery

Best Marine Boat Battery Odyssey Thunder Dual PurposeOdyssey Thunder Marine Dual-Purpose Battery

Don’t have the space for a cranking battery and deep-cycle? Here’s your next best option. Odyssey batteries are built to last. They’re strong, reliable, and have a strong warranty behind them. This model in particular provides 880 CCA and a RC of 135 minutes. Perfect for a boat or RV.

>> Check the Price on Amazon



Types of Marine Batteries

Buying a battery for a marine vessel is a little bit different than buying one for your car or truck. There’s different types of batteries and different needs that need to be taken into account throughout the purchase process. By the end of this guide, you’ll have answers some of the most common questions like:

Cranking (aka starting) vs. Deep-Cycle Boat Batteries

Cranking Starting Batteries Vs Deep Cycle Batteries

What is a Cranking  or Starting Boat Battery?

Whenever starting any sort of large engine, whether a boat or a car, there’s a large amount of energy needed in a short amount of time. In order to provide this burst of energy, cranking, or starting, batteries are designed with this intent in mind. No need to get into what these batteries look like on the inside just yet, we’ll cover that shortly.

What to look for

Most likely you’re boat came with a battery, which should be a good indicator for the kind of Marine Cranking Amps, or MCA, you’ll need for you new battery. Regardless, it’s recommended to check the owner’s manual to be sure the battery in previously was providing enough power. The MCA rating on the new battery needs to meet or exceed the recommended rating.

If your boat is recommended to have a 700 MCA battery, you’ll need a 700 MCA or 700+ MCA rated battery. A 800 or even 900 MCA battery will work just fine. Just be sure to take measurements of the old battery, or better yet, take it with you. The new battery will have to fit in the limited space the old battery was living in.

CCA vs. MCA vs. HCA Cranking Amps

What is a Deep-Cycle Boat Battery?

If you have a trolling motor on your boat or plan on constructing a battery bank, a deep-cycle battery is the way to go. A deep-cycle battery is different from a starting battery in it’s meant for a longer, slower pull of power. Instead of a quick burst in a second or two, these batteries are built for hours of gradual pull.

What to look for

If all you’re powering is a trolling motor and a few small accessories, you’ll be able to add up the power usage from those, the estimated time you’ll be using them and make some calculations. There are other factors you will need to account for such as depth of discharge. DoD is basically how much of your battery use before recharge. Since it’s not good to run batteries all the way to 0%, most manufacturers recommend not going anywhere below 20%-25% of power. Doing so significantly reduces the battery’s lifespan.

The exception to this rule is lithium-ion batteries. A lithium-ion battery can run 100% DoD, without harm to the battery. However, with all batteries, the shallower the discharge, the more cycles you’re able to get out of the battery. For example, if you get get 1000 cycles out of a battery at a 50% DoD, then you may be able to get 1200 cycles at a 70% DoD, ignoring other factors.

For even smaller batteries, some manufacturers do not recommend going below the 50% threshold. Your functional power is essentially cut in half.

If you have a lot of gadgets to power, has a simple calculator. (Opens in a new tab)

A Realistic Example

Let’s say you have a single trolling motor that provides around 40lbs of thrust. This is roughly 2/3 of a HP, 500 watts, or 40 amps at 12.6 VDC. Here, were rare assuming 746 watts equals a single horsepower and 58lbs of thrust equals a single horsepower.

You decide to purchase a deep cycle wet cell battery which has a 100Ah (Amp hour) rating at 20 hour rate, meaning it functions at a draw of 5 amps per hour. Since this is a smaller battery, you won’t want to discharge this past the 50% point, leaving you with about 50Ah of functional power.

If you use the battery for 30 minutes every time you’re out fishing, the result is 20Ah of use (.5 x 40 amps). With 50Ah of functional power, you could take 2, 30-minute trips before needing to recharge the battery. However, these calculations don’t take into account other factors, like battery age, temperature, etc.

In a perfect world, you could get 2, 30-minute trips. However, in reality you’re safe getting in that single trip with some extra time to spare if you want to take the long way home. Rate of discharge and other accessories will draw power from the battery and simply said, you’ll want to charge the battery after every trip.

A Word of Caution

Do not switch out one battery type for the other. These batteries will be clearly labeled and if for some reason the label has become unreadable, don’t risk it. Using a cranking battery in a deep-cycle application will cause it to overheat, fail, and / or possible start a fire. A deep cycle battery in a cranking application typically won’t work. In the event it does, it’s likely to not start up the second time since deep cycle batteries take a significantly longer amount of time to charge. Better to be stranded ashore, then on the water.


Other Factors


Batteries don’t like the cold! In colder climates, and generally any temperature below 70 to 80 degrees Fahrenheit, you’ll likely see a drop in performance. In sub-freezing temperatures, wet-cell batteries can even freeze and become useless. There isn’t a hard-and-fast rule for battery usage in colder climates, but just know you’ll likely need a stronger battery if the weather around you isn’t 70+.

Additionally, wet-cell batteries self-discharge during warmer temperatures at around 1% every 24 hours. In a few weeks’ time, it’s not unlikely the battery could have lost 20Ah of power. It’s recommended to purchase a solar panel charger in order to prevent this. More on solar chargers here.

This chart shows that as the temperature drops, so does a battery’s ability to hold a full charge. On the other hand, the battery’s ability to hold that charge increases. Remember Grandpa storing his batteries in the freezer? | Source: Donovan D. Devore –

Power Draw

The rate at which power is drawn from the battery also has an effect on the battery’s lifespan and use. This pertains more so to deep-cycle batteries. When a lot of power is drawn in a short amount of time, this not only shortens the lifespan, but also lessens the total functional power you can utilize. A significant portion of that energy is lost as heat.

Dual-Purpose Batteries

If you’re town between a cranking battery and a deep-cycle battery, there is a middle ground! Dual-purpose batteries give the best of both worlds without having to worry about frying a battery in the process. If possible, it’s best to have a separate starting battery and a separate deep-cycle if you have the need for both types. However, there scenarios where that isn’t possible, like if you’re limited on space.

What to look for

For a dual-purpose battery, you’re looking for the same factors and specs you would on a cranking or deep cycle battery. You need to make sure your CCAs meet or exceed the requirements of your motor, and make sure the reserve capacity will suffice for your intended usage.


Four Types of Batteries: Wet-Cell (aka Flooded, Flooded Cell, Lead Acid), Gel, AGM (Absorbed Glass Mat), and Lithium-Ion

Wet-Cell Batteries

Wet-cell, or flooded-cell batteries are your most basic batteries in boats, cars, SUVs, trucks and pretty much anywhere you need to draw a large amount of power in a short amount of time. They’re the most affordable option available, but also tend to require the most maintenance.

These batteries contain many lead plates that live in a mixture of distilled water and sulfuric acid. While we won’t get into how they work (that was for high school physics), the important thing you need to know is they’re capable of up to 1000 discharge/recharge cycles. This means they can last for years if well taken care of, at a low cost.

These batteries are less likely to be damaged by overcharging and for weight to energy output, they tend to weight the least. The problem with these batteries is you need to be checking the water levels to ensure they’re full. The additional problem with having to check these levels is insides of the battery are accessible which means they could potentially spill or leak battery acid in applications where there’s lots of movement – like a boat. When opened these batteries release hydrogen gas, which is flammable, so maintenance needs to be done in a well-ventilated area.

Lastly, these batteries do self-discharge. So when left off of a charger and not being used they typically discharge somewhere between 1% a day to 7%-8% per month, depending on factors like age and temperature.

Gel Batteries

Gel batteries solve many of the problems wet-cell batteries have, but at a cost. These batteries are sealed at the time of production, so these batteries are maintenance free. Instead of lead plates living in an acidic solution, these batteries are filled with a liquid electrolyte that is gelled with silicates.

As mentioned earlier, these batteries are meant to be used (or discharged) at lower rates over longer periods of time. Because of these, they can be stored for extended periods of time without worrying about losing power. Compared to a wet-cell’s of up to 1% per day, these batteries lose less than 1% of power per month.

Downside to these batteries is they’re typically more expensive and a special charger, or at least one with a gel-setting needs to be used. These are recommended to be charged to 13-14 volts.

AGM Batteries

AGM stands for Absorbent Glass Matting, meaning these batteries are composed of a dense filling of glass threads that are woven together to form a literal mat. These designed to wick the battery electrolytes between the lead plates.

Similar to gel batteries, these are sealed at production, meaning essentially no maintenance is required but some external cleaning. The downside to these batteries is they are sensitive to overcharging (rendering them virtually dead if overcharged), and are more expensive than their wet-cell cousins.

Lithium-Ion Batteries

Whether or not you know it, you use lithium-ion batteries on a daily basis. They’re in our cars, phones, power tools, and other electronics. The largest benefit with lithium-ion batteries is their lifespan and the energy density. Compared to other battery types they can hold more energy in a smaller amount of space and for a longer period time. When it comes to weight, these batteries win again. A 100Ah battery will usually weigh around 30lbs or so, compared to a AGM that will weigh 70lbs+. Lastly, there isn’t any reason to worry about depth of discharge. Lithium-ion batteries can be fully discharged without damage – this makes them great for building a battery bank.

The downside is, these batteries are far from cheap, however, they will typically last up to 7-10 times longer than some of the batteries already mentioned.

Other Specifications

CCA vs. MCA vs. HCA

Marine Cranking Amps and Cold Cranking Amps are essentially the same thing, just with a 32 degree Fahrenheit difference. You may see that your car battery has a CCA rating. What the CCA rating means is it tells how many amps the battery can send for 30 seconds at a 0 degrees Fahrenheit before the voltage drops to 1.2V per cell or 7.2V for a 12V battery.

Thus a 12V battery rated at 800 CCA, will provide 800 amps for 30 seconds at 0 degrees Fahrenheit before the voltage drops to 7.2V.

MCA on the other hand, is same measure, but at 32 degrees Fahrenheit. Sometimes ‘MCA’ is simply referred to as ‘CA’, or ‘Cranking Amps’. So, a battery rated at 1000 MCA (or CA), will provide 1000 amps for 30 seconds at 32 degrees Fahrenheit before the voltage drops to 7.2V.

HCA is less common, but sometimes you’ll see this rating too. HCA stands for Hot Cranking Amps and refers to the same measurement, but at 80 degrees Fahrenheit.

Reserve Minutes

Earlier we went through the calculations for a battery powering a trolling motor. There is another measure you can use for either building a battery bank or powering a trolling motor and that’s the reserve minutes, or also called the ‘reserve capacity’. This measure is expressed in minutes and tells how long a fully charged battery can provide a constant load (typically 25 amps), before it’s fully discharged.

For a 12V battery, it is fully discharged, once it has fallen to 10.5 volts.

Group Size

The group size is a categorization given by Battery Council International, or simply BCI. These sizes are apply to all battery types, including automotive, utility, and commercial applications. There’s a lot of different sizes and if you really want to dive through all of them, a chart is on this site here.  Group size isn’t super important, just make sure you take your old battery with your to compare sizes and purchase a battery with the same or higher CCAs.


Tips for Optimal Battery Performance

General Maintenance

This is specific for wet-cell batteries but cleaning any sort of corrosion that builds up around the terminals is a must. Too much corrosion can lead to a bad connection. Use a disposable shop towel to wipe away corrosion and other debris into a trashcan – don’t sweep this into the water.

If the corrosion is really bad, you’ll want to remove the connections and take a wire brush to terminals and wires. Once the metal has been cleaned, using a battery corrosion preventative such as NCP2, is highly recommended.

Lastly, doing some basic visual checks can go a long way. Make sure the case isn’t bulging or cracked. This could indicate overcharging and you will want to check your charger and alternator as well.

Winter Storage

If you’re winterizing your boat, you will want to full charge your battery and then disconnect them from the terminals. If you have a flooded cell battery, be sure it has enough water before charging. Depending on how long the battery is in storage, you’ll want to check it periodically and ensure it’s fully charged. If you don’t have the ability to check/charge the battery on a least a monthly basis, it’s highly recommended to purchase a trickle charger or a solar trickle charger if left out in the sun.

Consider a Battery Box

A battery box is exactly what it sounds like – it’s a protective housing for your battery. A battery box is excellent protection against the elements while still allowing the cables to be ran inside of the box. Boxes are typically vented to allow any gasses that might need to ventilate. These boxes will collect any battery acid that may leak and they make it incredibly easy to tie down and secure the battery.

Charging and Discharging

As was mentioned earlier, batteries should not be pushed past their depth of discharge as specified by the manufacturer. For deep cycle batteries, they should not go below a 20% capacity, with starter batteries, they generally shouldn’t go below 70% of their charge.

Always charge your batteries after use and do not mix different types of batteries together. Whenever replacing a battery, be sure all the batteries in your system are replaced at the same time. Older batteries tend to decrease the lifespan of new batteries. Never leave batteries deeply discharged for any significant period of time.

What to do with old batteries?

Whenever you purchase a new battery, automotive, marine, or whatever, there will be a core charge added to the cost. This will depend on the size of the battery, but it’s typically around $10-$20 in the US. This core charge is refunded when an old battery is returned to be recycled.

Where To Recycle Old Batteries Graphic

Most of the time, if you have multiple batteries, retailers will be happy to take those batteries off your hands for recycling. Home Depot, Lowe’s, AutoZone, and Rural King are a couple just to name a few. Be sure to call ahead to check. Otherwise, check with your city’s recycling program. and  Call2Recycle.Org are also 2 additional resources that can point you in the right direction.


Boat Battery FAQ

Can a boat battery get wet?

A marine grade battery can get wet, but only to an extent. Don’t forget, we’re dealing with electricity here. Electricity and water don’t mix, so avoid contact as much as possible. No battery should ever be sitting in any pool or puddle of water. However, most batteries will be fine if they get a little bit of rain on them. It’s highly recommended to store your battery in a battery box to protect it from the elements.

Can a boat battery be used in a car?

It depends on the application. If you plan on using it to start the car, then you need to use a cranking battery that meets the CCA required by your vehicle’s 12V electrical system. In general, if you’re using a cranking battery and the boat battery meets or exceeds the vehicle’s requirement for CCA, then yes you can.

Can you overcharge a boat battery?

Yes you can. While certain batteries are more sensitive to overcharging, like the AGMs mentioned in this article, all batteries can be overcharged. Once overcharged, they can be dangerous to handle and rendered almost useless in certain situations.

What boat battery do I need?

The type of battery you will need depends heavily on the use and requirements of your boat. Generally speaking, if this battery is only used to start up the motor, then a cranking battery is needed. If you are using a trolling motor or looking to power electronics on board, you’ll need a deep-cycle battery.

How long will a boat battery last?

The lifespan of a boat battery will vary greatly based on age, climate, use, and other factors. As each year passes, the total lifespan of the battery loses its ability to hold a charge. For example, a battery that could provide 100% of power at the start of its life, will likely only be able to provide 75% of its total power in 3 years. This number begins to increase significantly has time goes on.

How do I test a boat battery?

To check the voltage of any battery, you’ll need a multimeter. These can purchased fairly inexpensively from Amazon or your local hardware store. To test the battery you’ll want to disconnect the battery from the boat, set the multimeter to DC (or direct current), and connect the alligator clips to the correct terminals.

A fully charged battery will be around 12V or higher. If the reading comes less than 10.7V to 11V, it may be time to replace the battery. You may opt to purchase a multimeter than can perform a load test. A load test is simply a measure of how the battery performs under simulated use. While under a load test, the reading should be somewhere around 9.5V to 10.5V for 30 seconds total.

How do I charge a boat battery while out on the water?

To charge a boat battery while on the water you must have a charger or alternator built into your engine or some other means of replenishing power such as through a solar charger.

Do outboard motors recharge the battery?

Yes, most modern outboard motors have an alternator built-in to recharge the battery.


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Best Power Inverters – The Complete Guide

Best Power Inverters

Best Power Inverter Brand

Best Power Inverter Brand For Boat RV ProMarinerProMariner

Marine or land vehicle, ProMariner is one of the leading brands out there. They’ve been in the battery and power supply industry for over 35 years and own almost a dozen brands that cover a variety of battery and electrical applications. Their brands include Ancor, BEP, Blue Sea Systems, CZone, Lenco Marine, Marinco, Marinco Power Products, Mastervolt, ParkerPower, Progressive Industries, and ProMariner. They specialize in both marine and RV electrical systems.

>>Browse ProMariner Inverters on Amazon



Best Power Inverter for a Boat

Best Power Inverter For Boat RV ProMarinerProMariner True Power Plus Inverter

For a marine application you need an inverter that is not only shock resistant but can also stand up to the moisture around it. ProMariner’s TruePower Plus Power Inverters are the ideal choice. This model comes in 1000W Pure Sine, 1200W Modified Sine, 1500W Modified Sine, and a 2000W Pure Sine options.

Each model has all the safety mechanisms built right in including protection from circuit overload, overheating, short circuit, and reverse polarity. There is an audible alarm and automatic shut down for too low or too high of DC voltage. An on/off remote is included with the 9 foot cable and DC covers are included to protect the connections from the elements.

Additionally a on-unit dual color display provides immediate feedback on the DC to AC output, DC input and system status message area. This model is the perfect setup for a boat or even RV.

>>Check ProMariner True Power Plus Inverter Prices on Amazon


Best Power Inverter for a Car or Truck

Best Power Inverter For Car Truck EnergizerEnergizer EN (pictured) and ENK Series Power Inverters

It’s tough to go wrong with a trusted brand like Energizer. With all the cheap Chinese models available online, going with a name you can trust is a must. Energizer offers a 100W, 500W, 1100W, 1500W, 2000W, 3000W and all the way up to a 4000W for your car, truck or SUV.

These energizer power inverters are modified sine wave inverters, but as mentioned earlier, should be fine for most applications. The 1100W model should suffice for most car and truck drivers, although the higher rated models are also available. Just be sure to check your car or truck’s electrical system beforehand to see if it can handle the power that the inverter will draw.

>>Check Energizer Power Inverter Prices on Amazon


Best Power Inverter for a RV

Best Power Inverter Brand For Boat RV MagnaSine Magnum EnergyMagnum Energy MagnaSine 3000W Power Inverter

ProMariner power inverters are listed as the best brand, but Magnum Energy comes in close second. Their power inverters are incredibly reliable, offer versatile mounting positions, and are overall just well made products. For a RV, you probably won’t need a marine grade inverter, however you’re more likely to have a greater power need.

For a RV our pick is the Magnum Energy MSH3012M 3000W Pure Sine Inverter Charger Hybrid. This inverter is everything you’ll need and then some. It has all the typical protection points you’ll need and has a 5-stage charging capability. If you plan on being on the road a lot, this is the best option. Even if you don’t have a use for 2000W-3000W you can rest easy in the future knowing your system can handle the extra TV, the kids’ gaming system, or whatever unplanned item finds its way onto your RV.

>>Check Magnum Energy MagnaSine Prices on Amazon


What is a Power Inverter and What Does It Do?

Simply said, a power inverter allows you to use nearly any device, that requires your standard home wall outlet, in your boat, car, or RV. The current that runs through your typical wall outlet runs is called an Alternating Current or AC. Cars, Boats, RVs, Trucks and other vehicles use what’s called Direct Current, or DC power.

In a metaphorical sense, AC power speaks one language, say Spanish, and DC power speakers another, say English. An inverter is the translator between Spanish and English and allows one current to be converted to the other. For most applications, DC power is being converted into AC so people can watch TV, charge their phones, run refrigerators, and use other items that require an alternating current.


Types of Inverters

Standalone Inverter

Your standard inverter is called a ‘standalone inverter’, or simply a ‘power inverter.’ This takes the direct current and converts it into an alternating current.

Inverter-Charger Combo

An inverter-charger combo is different from a regular power inverter in that it has 2 modes. The first being the regular inverter mode in which it converts DC power to AC power. The second, differing mode is standby or charging mode. In this mode, the inverter converts incoming AC power to charge the DC source, or use the incoming AC power to power the AC devices.

Think of it as a two-way street vs. a one-way street. A standalone inverter can only move electricity one way, from DC to AC. While an inverter-charger can convert DC to AC, and AC to DC.

Modified Sine vs. True / Pure Sine Inverters

You’ll notice in the title or description inverters are labeled as ‘true / pure sine’ or ‘modified sine’ inverters. Without getting too technical, pure sine is essentially a more accurate AC current. It’s truer than a modified one and many electronics require a true sine inverter to function.

The main difference between the two is price and performance. Modified sine inverters are significantly cheaper than pure sine inverters, however their performance suffers in certain electronics. Radios or speakers may sound worse due to interference and damage may be caused by the additional heat caused by slight inefficiencies in a modified sine inverter.

So, which should you buy?

If you ask me, it all comes down to use-case. A lot of electronics will work just fine with a modified sine inverter. However, if you plan on powering sensitive medical equipment, like a CPAP machine (especially one with a humidifier), a radio or other item where sound quality matters, or a device or appliance that uses a brushless motor or rectifier, buy a pure sine inverter. Otherwise, a modified sine inverter will usually work just fine.

Continuous vs. Peak Power Ratings

A word of caution – be sure to pay attention to the wording used in listings online or on a package. “1000 Watts Peak Power!” is much different from “1000 Watts Continuous Power!” Peak power is what the inverter is able to handle in short bursts for a very short period of time. Continuous power is what the inverter can handle for a longer, sustained period of time.

The only time peak power will be relevant is on start up of some devices where there is a larger wattage or amp pull. Although, there would have to be a significant gap in the peak handling power vs. the continuous


Watts, Amps, and Volts – What do they all mean!?

Watts, Amps, and Volts are all essentially ways of expressing energy in electrical form. This post isn’t meant to be a science lesson, so here’s the practical information you’ll need to understand how they interact with each other.


Yup, that’s it. Watts is equal to amps times volts. Most boats, cars, trucks and RVs run on a 12 volt system, so we’ll use that to calculate the your power needs down below.

Sizes of Inverters

Inverters are sold based on their wattage size. They typically range from a few hundred watts, all the way up to several thousand watts. If you only need to power a few small electronics like a phone, toaster, or charge a laptop, a 700 or so watt inverter will be more than enough, and your current battery will support it most likely. However, for larger applications, you will want to make specific calculations, which you can find help with below.

You need to make sure your current system can support the size of inverter you purchase. Other factors besides the posted rating will affect your current system’s output like age of battery, temperature, condition, etc. A general rule of thumb for 12V systems is you need a 1:10 ratio of DC input to AC output. So, for a 1000 watt inverter, you’ll need at least 100amps from the battery source.


What Size Inverter Do I Need?

Devices have different have different initial pulls and continuous use. What you’ll want to do is create a list of all the electronics you plan on running off the inverter. Add up the wattage and amperage displayed on those devices.

For example, let’s assume you plan on hooking up a microwave, 32” television, portable refrigerator, and your laptop charger. Open up Excel and create something like the setup below.

Note: When looking at any electronic that has some sort of heating element, look at the input power. The input power is what the device actually draws from the power source. A microwave labeled as 900W is likely pulling 1300W or more. You can find this in the item’s manual.

Note 2: Some electronics won’t provide the ampere pull, so you’ll have to do a little math to figure that out with the Watts=Amp(Volts) formula.

Note 3: Items that have motors or compressors will usually have higher ratings than what is printed on them. For example, in the case of a refrigerator with a compressor, it will have a higher pull (sometimes called a ‘surge’) rating each time it has to kick on vs. the average or continuous rating it may indicate. You will want to contact the manufacturer to obtain the startup wattage and amp pull. This is sometimes also referred to as “surge.” It’s common in vacuums, power tools, and other items with a brushless motor.

Microwave (FARBERWARE FMO07ABTBKQ): 1050W / 8.75A / 120V

32” TV (TCL 32D100): 45W / 2.67A / 120V

Portable Refrigerator (Whynter FM-45G): 65W / .75A / 115V

Laptop Charger (Dell Replacement HK65NM130): 65W / 3.33A / 19.5V

Total: 1225W / 15.5A

Now that we have our total, 1225W and 15.5A. This is assuming all electronics are running at exactly the same time. The microwave is heating something, the TV is on, the fridge’s compressor is running, and a laptop is charging – not recommended to run all at once unless your system can handle it, but not unlikely. The big thing to pay attention to here, is what is drawing the most and how much.

Looking at the numbers, the microwave is drawing the most amount of electricity and this item will not be running the entire time. Likely, only a few minutes at a time, if that. Furthermore, you probably won’t want to run the microwave until the generator/alternator is running or you’re connected to shore power.

We don’t notice it on land or at home, but microwaves draw a ton of power relative to other appliances.

For practical purposes, let’s just assume you’ll only be using the microwave while the engine is running. All other items are running from the power inverter.

Removing the microwave, that puts the total at 175W and 6.75A. A very reasonable number and one that is easy to work with.

So you only need a 200W inverter right?


Here’s why.

Inverters need power to operate themselves, and 100% efficiency isn’t available just yet. You’ll need to account for these factors along with electronics you may want to plug in and use apart from those just mentioned. A good rule of thumb is to purchase an inverter that is at least double what your planned usage is.

A 400W inverter would be a good choice here, although 500W would probably be better.

What about the microwave?

Microwaves pull a lot of power and if you’re not connected to shore power or running a generator, then that bag of popcorn can drain your batteries very quickly. At minimum, a 3000W inverter would be ideal for the setup mentioned here. It’ll be ideal to have a small battery bank of sorts, which I’ve covered more in depth here.


Why do Amps matter?

But wait, why do the amps matter?

Amps matter as they will provide info to how long your battery (or battery bank) will last without it being charged, or while being used by the inverter. There has to be an external source recharging the power source, like an alternator for power to be harvested and that has to be equal to or above what is being drawn from the battery if using without a generator or recharging on shore power when low.

To put it in simpler terms, if you have $10 in the bank and work deposits $10 in your checking account every week and you pull $11 out every week, you’re pulling money at a -$1 deficit and will eventually run out of money. The same goes for the battery, it has to be continually replenished or very full, in order for power to be drawn from it.

In order to continually run your electronics, you will need to have a generator replenishing that electricity, such as a high output alternator, recharging at shore power when low, solar panels, wind generators, etc.

Calculating Amp Usage

Now you need to estimate how much time each of your electronics will last before the next charge. Insert your estimated time usage in column H in the excel file. Now use the following formula to calculate the amp hours you will be pulling from the battery supply.

(Total Watts/12)(1.1)(Hours) = Amp Hours

Let’s not forget – there are other factors that will also play into this calculation. For one, it’s not good for the overall life of the battery to run it completely down to zero. Instead, only 80% or less of the battery’s total life should be used. Furthermore, different types of batteries (lead acid, zinc-air, carbon zinc, etc.) have different rates of discharge. I will be covering those in this article.

In our example, without the microwave, we’d use 288.75 amp hours if using each device for 6 hours total before another charge. As already mentioned, it’s not good for a battery to be completely drained of its reserve, then quickly recharged. Nor is it good for a battery to be drained below roughly 20% of its total power.  There are batteries specifically built for these purposes called, ‘deep-cycle batteries,’ but those are covered in another article.

Instead, you should double the total amp hours you have calculated, (288.75)(2) = 577.5, and then add another 20%, (577.5)(1.20) = 693. This will help to ensure your batteries last long and do not fall short of their intended lifespan.

In an absolutely optimal scenario, you will need roughly 700Ah of power to run those 3 items at those specs for 6 hours before needing to turn on a generator 0or the motor. You can have a smaller bank of power, but keep in mind they will need to be charged more frequently in order to run all of the electronics accounted for at the 6 hour time frame indicated.

Learn more about types of batteries click here.

Features to Look For

You’ve done the calculations and now know how much power you’re going to need. What additional features should you be looking for on top of just having an inverter?

Pure Sine vs. Modified Sine

As mentioned earlier, pure sine is preferred, but if you’re working on a budget a modified sine inverter will work fine for most applications.

Remote Control / Display

Being able to know current usage is incredibly important with any sort of electrical system. Some inverters will have a built-in display while others, like Magnum’s Magna Sine inverter, have a remote display that can be attached just about anywhere on the boat. This is a wired remote, not a wireless one.

Low Electrical Interference

EMI stands for electromagnetic interference which can cause problems with all sorts of important equipment on your vessel. While I won’t get into the specifics of what an inverter can cause problems with, most new high-end inverters have little to no interference. Some cheaper, Chinese made inverters can cause electrical interference problems.

Voltage Alarms

Having too high of voltage or too low of voltage can cause damage to your system. An inverter with a built-in voltage detector can shut off the inverter before it causes damage to the entire unit.

Overload warnings / protection

During regular usage some may forget how many items are actually running off of the inverter. All it takes is one electronic that has a startup spike or surge, to cause a fuse to blow in the inverter, rendering it useless until replaced. An overload warning helps to notify you there is a spike and you need to lessen the load, or it will shut down automatically to protect itself from damage.

120V/240V capabilities

Depending on what kind of system you have, having 120V or 240V capabilities offers you some flexibility.

Reverse Polarity Protection

Not as common on mid to lower end models, but just as important. While the name sounds confusing, it really just means protection against accidentally misconnecting wires. The positive and negative ends of a battery indicate the polarity, and if accidentally mixed up, can essentially fry the power inverter, rendering it useless.


Difference Between AC and DC Power

Going back to high school trigonometry, AC and DC currents can be expressed as waveforms. If you recall, which I didn’t until entering it into my old but trusty graphing calculator, sin(x) is a perfectly sloping wave going up and down at a regular interval.

Notice how, in the image, the current alternates up and down (or in a diagram, forward and backward) . Your electronics are fed a constant supply of electricity that alternates back and forth. This probably doesn’t make a whole of sense by that sentence, so I highly recommend checking out water analogy, shown by Spark Fun Electronics (opens in a new tab).

In that example it becomes much clearer how electricity flows. You can see that it moves forward and backwards relative to the power supply.

On the other hand, car battery systems use a current system that supplies a direct and consistent level of voltage. Again using a water analogy, imagine a bucket of water with a hole on the bottom connected to a hose. If you fill this with water, the water will flow out of the bottom of the hose until it’s empty. This is essentially how DC power works. There’s a power supply that will feed whatever device it’s connected to until it’s empty.

This is why cars, RVs, trucks, and boats need to have some sort of charger in their system. In most cases, this is what is called the alternator. The alternator runs off energy produced by the gas and engine to charge the battery while the system is running, since your car’s electrical system is continually pulling power from the battery. Think of the alternator as a person that helps to refill that bucket and keep it from going empty.

For anyone that has ever worked on cars, finding the alternator is an easy job, but what about a boat? Most boats nowadays have an alternator or some other sort of charger built into the motor, especially for outboard ones.



A few final recommendations. As with everything, safety first. If you are unsure of what you are doing, heck even if you are sure, have a professional nearby for instruction and supervision. Electricity isn’t anything to mess around with.

Secondly, always buy at least a couple hundred volts up from what’s planned. You never know when you’ll have a guest, or extra electronic that makes its way onto your boat or RV that could end up overloading your system. Additionally the calculations made in this article do not take into account other factors that will affect your system such as wire length, current battery health, etc.

Third, document everything! Be sure to draw diagrams, keep manuals, and have all this information close by in the case of a breakdown or emergency. Proper documentation now, will save you frustration and time in the future.

Lastly, while there may be nothing wrong with the no-name brands on Amazon, Ebay, or elsewhere online, keep in mind you will likely not have a warranty with any of them. In the event the no-name inverter fails, fries your current system, or damages your electronics, you probably won’t be able to receive any customer service let alone compensation for damaged goods. Buy from a trusted brand in the first place so you don’t run into bigger issues down the line by trying to save a few hundred dollars.