Marine Inverter Basics

Magnum Energy MS Series Inverter ChargerWhat is an inverter?

At its most basic, a marine inverter takes DC power from your boat’s battery bank and converts it into AC power, so that you can use “household” items on your boat without being hooked up to shore power or facing the great expense of installing a generator. Inverters come in a variety of sizes with a multitude of features, which we will cover more in depth below.

Why does my boat need an inverter?

If you plan to watch TV, make some popcorn in the microwave, brew some Starbucks in the coffee maker or maybe tackle that project that needs the power drill – you will need an inverter to run these items if you are out at anchor without a generator.

While there may be some die hard “old salts” out there that think all inverters should be thrown overboard, the vast majority of modern day boaters have become pretty attached to the AC gadgets we use every day. So if you desire the conveniences of home while you’re out on the water – an inverter will make that possible… long as your battery bank is sufficient to handle the load.

Let’s get a little more technical…

Mass Combi Inverter/Charger from MastervoltAs stated before, the inverter will take the 12 (or 24) volt DC current and increase the voltage by about 10 times to create 120/240 volt AC current, as well as changing the current from “direct” (DC) to “alternating” (AC). As part of that process, the inverter will either produce a Modified Sine Wave (MSW) or a True Sine Wave (TSW – also known as Pure Sine Wave/PSW).

  • Modified Sine Wave – This type of power basically steps the wave – similar to a staircase going up and down – in an effort to emulate the pure wave of AC power. Inverters which use MSW will normally run the majority of your household gadgets without issue. That said, they will likely have issues with “electronic noise” and/or some items may not run at their full power. Even more important, sensitive electronics need a pure sine wave to run properly and you may cause permanent damage if you try and run them with MSW – so think hard about what you’ll be using your inverter for before you make your decision.
  • True Sine Wave – This type of inverter produces a pure wave – similar to a rolling wave on the ocean – going up and down in a smooth line like AC power. While inverters using this type of sine wave are usually more expensive, they are significantly better at dealing with sensitive electronics and/or making sure your items operate at their top performance level. If you plan to use your inverter for charging a computer, using a printer, dimming your lights or watching that new flat screen TV onboard – you should select a TSW inverter.

In addition to the wave type, inverters are also available in different levels of output power – which will often dictate whether they are portable or fixed mount. Most of the smaller, portable units are made to plug into a 12 volt socket style outlet – like the cigarette lighter in your car. These inverters will usually provide up to 300-400 watts of power and are suitable for powering laptops, cell phones or some small appliances.  

Larger fixed mount inverters can provide up to 4000 watts of output power, depending on the model and brand you select. Fixed mount units will normally be hardwired to your battery bank and may even integrate directly with your AC wiring – which is why it’s best to leave their installation to the experts! At higher outputs they may come with a remote panel and often will be combined with the ability to perform as a charger when shore power or a generator is being used – highly convenient!

Additional Features to Consider

Outback FX Series Inverter/ChargerHere are some further things to consider when deciding which inverter to purchase:

  • Surge Capability – while most appliances that we run have an estimated wattage, many of them will exceed that estimate (sometimes by 2-3 times) when they start up. It’s very important to make sure you have an inverter large enough to cover these surges for the items you intend to run. Most manufacturers will refer to this in their specs as the “Surge” or “Peak” output to indicate how much load you can exceed over the stated wattage.
  • Transfer Switches – a transfer switch is useful if you have purchased a combined battery charger/inverter. If it’s a built in switch, it will automatically sense when there is AC power available (such as when you are using shore power or a generator) and will move from inverting to charge mode. If it’s not built in, the unit will usually have a switch that you flip to change from one mode to the other.
  • Efficiency Ratings – all inverters will lose some amount of power during the conversion process – some more than others. Efficiency ratings were developed to give a better apples to apples comparison to see just how much power you might be losing (or not) between models. Efficiency ratings are usually expressed as a percentage of the output – such as ? 90% – which would mean it turns greater or equal to 90% of the DC power used into AC power. The normal range is 85 to 95% efficiency, with most inverters coming in at around 90%. If you plan to use your inverter to power something that needs to run all the time (such as refrigeration), you want as high a rating as possible.
  • Multistep Battery Charging – again, this is only for inverter/charger combination units, but basically this means the charger portion is utilizing the 3 step process of Absorption/Bulk/Float to properly charge your batteries. This will ensure your batteries have a much longer life and are charged at the right amounts during the entire charging process.

How do I choose an inverter?

TruPower Plus Inverter from Pro MarinerOkay – now that you know what they are, how they work and what features to choose from, your last step is to figure out how big an inverter (output wattage) you need.

To do this, you’ll need to add up the total wattage of all of the items you might be running at the same time. Most items will list their wattage somewhere on the bottom or near the cord. If they only show the amps, you can figure out the watts by using this conversion: Volts x Amps = Watts (with Volts being 120 for AC). So if you have something that runs at 10 amps, you will need a 1200 watt inverter to run it (120 x 10 = 1200).  

After adding up your needs, you should choose the next highest unit available. For instance – if you need a total of 1400 watts, you could use a 1500 watt inverter – but you would be much better off with a 2000 watt inverter to give your system a little breathing room.

Final words of wisdom…

Xantrex PROwatt InverterLast – but most definitely not least – it’s extremely important to make sure your battery bank capacity is large enough to support these needs and that you can maintain your battery voltage while using your inverter. The higher the wattage and the longer you need to use the item may overrun your battery bank’s capacity to cover those needs – so you must take your battery bank’s amp hours into consideration when deciding what you can (and can’t!) run on your boat with an inverter.  

Fisheries Supply carries a wide variety of inverters in all shapes and sizes from the best manufacturers – including Magnum EnergyMasterVolt, Newmar, Outback, Pro Mariner and Xantrex. At Fisheries, we have an inverter to meet the needs of every boater no matter how big or small!

We hope you’ve found this Navigator useful – but if you have additional questions, feel free to contact our product experts at (800) 426-6930.


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