Choosing an Inverter or Inverter Charger for a Boat

By Tom Burden, Last updated 1/12/2024

What Inverters Do

Inverters change the DC current supplied by a boat's batteries into AC current to power electronic devices and "household" appliances on a boat. Whether you want to blend up a margarita, charge your laptop or power up a drill, installing an inverter can make all this possible when unplugged from shorepower away from the dock.

While this article focuses on how to select an inverter, if you prefer the space-saving convenience of inverter/chargers that combine the functions of a battery charger and a power inverter in one device, our West Advisor Selecting a Battery Charger for Your Boat can help you choose an inverter/charger that includes a battery charger to meet your needs.

Inverters versus Gensets
How big should your inverter be?
What to Look for When Choosing an Inverter
Modified versus Pure Sine Wave Efficiency
How much battery capacity will you need?
Installation
Conclusion

Inverters versus Gensets

As stated, inverters are used to change direct current (DC) into alternating current (AC) used to power common household tools, electronic items and appliances. Generators also produce AC power, so why not use a generator to do this instead?

While both inverters and gensets (generators) create AC electricity, inverters are better for variable demand and loads from 2,000 to 3,000 watts. Unlike noisy, fossil fuel-driven gensets, inverters are silent, and require virtually no maintenance, On the other hand, their effectiveness is limited by your battery capacity.

Gensets are better for large, continuous loads like air conditioning, heating, and refrigeration. While many of our customers would love to run air conditioning off of an inverter, it is pretty impractical unless you are cooling a small area for a relatively short period of time. If your 120V energy requirements exceed 2,000 watt-hours per day, a genset is probably needed.

Another option is to use an inverter/charger with a genset. The genset is operated during meal times, and other high-load times, during which it recharges the house batteries using the inverter’s battery charger. During off-peak times, the inverter takes over, silently supplying power for smaller loads.

How big should your inverter be?

Inverters range from tiny 100W models, good for laptops, to 4,000W models that can run electric galleys and refrigeration systems. The size you select is governed by the maximum power needed at one time. A computer (200W), TV (200W), and microwave (800W) all running at the same time will need an inverter that delivers more than 1200W of continuous power. Inverters typically fall into three categories:

Portable inverters generally connect to your DC electrical system using the common cigarette lighter socket. These DC sockets are often problematic due to poor contact or an insufficient gauge wire constricting the flow of current and contributing to DC voltage drop at the socket, but we digress. Portable inverters are instant gratification products: just plug them in and start using their modest AC outputs (typically up to 300 watts). Ideal for laptops, small lights and tools. AC appliances are plugged into the front of the inverter. Portable inverters are also excellent for family trips in the minivan or SUV.

Permanent-mount inverters (400 watts or more) are hardwired directly to the battery bank. Inverters in this range usually have receptacles for appliances to plug into, but do not integrate with your boat’s existing AC wiring. Some allow you to easily wire more receptacles to place in remote locations. Top-of-the-line models include built-in transfer switches enabling full integration with existing AC wiring.

Inverter/chargers have outputs that rival small generators (2,000 to 4,000 watts) and include a powerful battery charger and associated battery monitors and remote controls. Inverter/chargers almost always include an automatic transfer switch, so that when your inverter/charger detects another source of AC power (because you plugged into shore power or started your genset) the inverter/charger shifts gears from inverting to charging batteries. If the outside source of AC goes away, it automatically switches back from battery charger to inverter.

Inverter/chargers should be professionally installed because they draw a lot of amps, with the potential for dangerous shorts and electrical fires if they are wired inadequately. They become an integral part of your boat’s electrical system and can rapidly replenish battery banks in the 400 to 800Ah range. We find that many trawler owners are drifting away from a large diesel generator, and are instead installing a large inverter/charger for their AC loads complemented by propane ranges and DC refrigeration systems.

What to Look for When Choosing
an Inverter

Wave Form: Inverters today generally produce one of two types of AC power: Modified Sine Wave (MSW) or Pure Sine Wave (PSW). Affordable MSW inverters are the most popular type, and work well with common appliances you are likely to use. PSW inverters are more expensive, but will run all AC loads, and are best for stereos, computers and other sensitive electronics. Portable tool chargers (especially Makita, according to Magnum Energy), light dimmers, variable speed tools, plasma screens, home theater sound systems and certain other items may not perform as well with MSW. MSW inverters are prone to electronic noise known as harmonic distortion, which can cause disturbances in some electronic circuits. Modified sine wave has only one step per cycle.

Pure sine wave inverters, on the other hand, are extremely sophisticated and can produce AC power at or above the quality levels of shoreline power grids. According to Magnum Energy, a pure sine wave inverter can eliminate static lines in televisions and improve the quality of a sound system. And a good pure sine wave inverter can have as many as 1,333 steps per cycle and have a total harmonic distortion (THD) of less than five percent, typically around two percent.

Remote controls: Being able to control, monitor at a glance and program your inverter from inside the cabin is an important consideration, especially when the inverter is in a hard-to-reach place. Remote panels are a must-have for inverter chargers.

Powerful surge capability: An inverter must be powerful enough to handle the “surge” or starting requirements of appliances. A 125 watt TV might need 625 watts for a few seconds when starting cold. If you are using an inverter without the adequate surge capability, it won’t start the appliance.

To quote Don Wilson, the Xantrex Tech Doctor, “My advice? Use common sense here, but always err on the side of the inverter. After all, nobody wants to have to remember to turn off the entertainment center when using the microwave to pop the popcorn! If there is any possibility of the loads running concurrently, include both in your calculations.”

Low electrical interference: High-frequency inverters frequently produce some EMI, or electromagnetic interference. This can cause problems with SSB and Ham reception and weather fax reception. However, many of the newer designs, especially true sine wave inverters, are meeting FCC Class A requirements so there is great improvement in this area. Of course, a simple solution to the problem of radio interference is to turn off your inverter.

Modified versus Pure Sine Wave Efficiency

MASTERVOLT's CombiMaster 12/3000-160 (120 V) Inverter/Charger

Inverter efficiency is a rating of how much of the DC energy gets converted into usable AC power. Inverter efficiency ranges from about 85 to 95 percent, with 90 percent being about average.

Efficiency ratings often are created using a resistive load (basically something like a light bulb or electric heater). When running loads such as motors, we split efficiency into two parts: the efficiency of the inverter we mentioned above, and the efficiency of the waveform. Waveform efficiency means that most motors and many electronic appliances run better and use less power with a pure sine wave. Typically, an electric motor (such as a pump or refrigerator) will consume from 15 to 20 percent more power with a modified sine wave than with a true sine wave. When choosing an inverter based on efficiency, you should also consider the type of appliances you’ll most often be running.

Power Factors and Generators: The charger in an inverter/charger presents a large, inductive load for a small generator (under 8kW). Chargers with a high “power factor” are desirable because they are less sensitive to peak voltage and need less AC power. We recommend that a generator be sized at 2.5 times the power draw of the charger.

How much battery capacity will
you need?

Design Rule: To be effective, we recommend inverters have access to a battery bank that is 20 percent as large in amp-hours as the inverter size in watts. This means a 1000-watt inverter should be supported by at least 200Ah of battery capacity.

The rationale behind this rule is that inverters use about 100Ah of electricity for every 1,000-watt-hours of use. So if a 1000-watt inverter were used at its maximum capacity for an hour, it would consume 100Ah. This would discharge a 200Ah battery by 50% if it started out fully charged. This is a recommended minimum: as usual, more battery capacity is better.

Calculating electrical loads: Many appliances and electronics have their wattage on a back panel. If only the amp rating is provided, use the following conversion formula: Volts x Amps = Watts. A 7-amp microwave at the standard North American voltage of 115V will need 805 watts of power to run. Multiply the wattage of each appliance by the hours the appliance will run per day then divide the resulting number by 10 to find the approximate amp-hours consumed from your batteries.

If an inverter is going to run a 500-watt load, it will draw around 50A DC. The math behind this rule is as follows: 500-watts AC ÷ 85% efficiency ÷ 12.5V = 47A DC. Keep two things in mind: 500W is not very much power (think about a small cabin heater that draws 1500W or a large hair dryer that draws 1000W); and 50A is a heck of a lot of DC current use. Not many DC loads on boats 40' and under would draw 50A or more for more than a few minutes. Therefore, an inverter frequently becomes the largest DC load on most boats, and may require substantial changes in the battery capacity and wiring of the boat in which it is installed.

Installation

We recommend installing a fixed-mount inverter no more than 10' from the battery bank, to avoid a voltage drop or oversized wires. Inverters are not ignition protected, so are not safe for installation in gasoline engine compartments, but can be installed in a diesel engine space. In general, you should pick a location that is cool, dry, well ventilated and safe from explosive gasses produced by batteries, engines and fuels.

When installing an inverter or an inverter/charger, for your own safety and that of others, it is imperative that you carefully follow the manufacturer's instructions and that your installation comply with applicable American Boat and Yacht Council (ABYC) standards.

Conclusion

Depending on your expectations, when considering an inverter, you might have to make a tradeoff. Compared to a genset, an inverter can be a better choice for powering loads up to around 3,000 watts. Of course, in line with the inverter you select, your boat's batteries must have enough capacity to back your inverter up. For some boaters, this means that they will either have to increase their battery capacity, supplement their needs with a genset, or choose a less powerful inverter.

Choosing an Inverter or Inverter/Charger