How To Select & Install A DC-To-AC Power Inverter

Want to enjoy the plug-in comforts of home aboard your boat? Here's what you need to know.

When it comes to spending time on the water, nothing elevates your boating experience from “camping on the water” to “gloating” (the nautical version of “glamping”) like the on-demand access to AC power an inverter provides. Even if your boat has a generator, there are numerous benefits to having a DC-to-AC inverter as well. Here are some tips on choosing and installing an inverter that’s right for you.

What They Do

Inverters convert the direct current (DC) power stored in your batteries (typically 12 volts) to 120-volt alternating current (AC). This allows you to use common household devices such as coffee makers, power tools, TVs, and other appliances without the need to plug into shore power or crank up that noisy generator.

Unlike a generator (which actually generates AC power), output for an inverter depends on the output rating of the inverter and available battery capacity. If you’re looking for lots of continuous AC power, then a generator is your best bet. For small, intermittent, or long-term AC power needs (such as brewing a pot of coffee or running a portable sander), an inverter is an attractive option.

Although limited by available battery capacity, inverters have advantages that even boaters with generators can appreciate. They’re silent, virtually maintenance-free, and don’t produce exhaust fumes. Having an inverter installed in addition to a generator allows you to operate smaller, AC-powered items without the hassle of firing up the generator (and waking up the neighbors in that quiet anchorage).

The key to successful inverter nirvana is a reasonable expectation of just what your inverter can and can’t do. Being able to power a blender or microwave is a reasonable expectation for many boaters, but don’t expect the use of your AC-powered air conditioning unit without checking for (and possibly adding) sufficient battery capacity and that the inverter can handle the load.

Portable Or Fixed Mount?

Inverters are available in a wide range of sizes from portable units (typically 300 watts or less) that plug into a cigarette lighter or similar DC outlet, to permanently mounted units wired directly to your vessel’s batteries.

Portable units are easy to use and work fine for powering smaller items (e.g., cellphone chargers, laptops, small tools) but there can be issues with poor socket contact and smaller wiring, which limits their efficiency.

Inverters greater than 400 watts are typically designed to be permanently mounted and are the focus of this article.

Choosing An Inverter

There are two primary factors when selecting an inverter – output wattage and the type of AC power produced.

Let’s talk watts first. Inverters are rated in watts, and selection is based on the maximum amount of AC power you’ll need at any given time. The more watts an inverter produces, the larger the AC-powered device it can operate, or the number of smaller ones it can operate simultaneously.

Smaller, portable inverters of around 300 watts are a popular choice to power smaller electronics, such as laptops or phone chargers. For permanently installed units, 2,000-watt inverters are popular and a good all-around power choice.

But how do you determine just how much power you need? Most AC-powered appliances and electronics list wattage requirements on their data tags. To calculate your needs, simply add up the watts for each appliance you plan to operate simultaneously.

For example, let’s say you’re anchored out and plan on running a TV (200 watts) for movie night and a small microwave (800 watts) at the same time to pop the popcorn. Simply adding the two will show that you’ll need an inverter that delivers at least 1,000 watts of continuous power.

Some appliances may also have a power-surge requirement that has to be taken into consideration. That 200-watt TV may need 500 watts for a few seconds when turned on from a cold start. If so, you’ll have to use this in your calculation, rather than the 200 watts needed during normal use if running both appliances at the same time.

Frugal load management can go a long way toward maximizing inverter use. Using the microwave to pop popcorn, then using the inverter to power the TV afterward (rather than trying to run both at the same time) allows you to achieve the same result using a smaller inverter.

Size Matters

The flip side to selecting the size inverter you want (or need) is ensuring you have sufficient battery capacity to support its operation. Your inverter should have access to a battery bank that’s at least 20% as large in amp-hours as the inverter output watts.

This means that a 2,000-watt inverter would require at least 400Ah of battery capacity. The logic here is that the inverter uses roughly 200Ah of electricity for every 2,000-watt-hours of use. Operating the inverter at maximum capacity for an hour would consume 200Ah and discharge the 400Ah battery bank by 50% (assuming it was fully charged to begin with). Again, this is the absolute minimum, with more battery capacity being better.

Not All AC Power Is Equal

The second part of the inverter selection process is the type of AC power produced. There are two output choices in this regard: modified sine wave (MSW) and pure sine wave (PSW).

The primary advantage of an MSW inverter is cheaper cost. Everything is a trade-off, however, and the cheaper cost of an MSW inverter is no exception. An MSW is not a true sine wave, but rather a stepped wave that’s less smooth than a pure sine wave. Disadvantages of this stepped wave output include buzzing when powering radios, lighting, or audio equipment, and even possibly visual distortion in some televisions and computer issues.

A PSW inverter provides improved performance and is better for powering motors, pumps, and sensitive equipment or electronics. While they cost more, PSW inverters provide better service and are the best choice overall.

Installation Considerations

Some permanently mounted inverters simply have an integral outlet built into the case, allowing you to plug your appliance directly into the unit. High-end inverters will typically have a built-in transfer switch, allowing them to be wired into a vessel’s existing AC system to power remote AC outlets. Inverters designed to be integrated into your existing AC system come with additional installation requirements, such as a visible means of determining that the inverter is online and a warning at the main AC panel stating that the system includes an inverter.

AC from an inverter is just as dangerous as your household AC, so you’ll always want to disconnect DC power to the inverter and disconnect the inverter from the vessel’s AC power system (if so connected) prior to working on the AC power system.

Along those lines, unless you’re proficient with electrical systems, consult with or hire a qualified marine electrician to assist with the installation. While you’ll always want to follow the specific instructions provided by the manufacturer of your particular inverter, here are a few basic considerations that can be applied to most any inverter installation:

1 . Choose a suitable location for the inverter. Manufacturers are very specific regarding where and how to install their inverters. According to American Boat & Yacht Council (ABYC) standards and recommendations, the location should be well-ventilated, easily accessible, and meet the manufacturer’s environmental requirements for humidity, water intrusion protection, and ambient temperature. They must also be located well away from heat sources (such as dry engine exhaust manifolds).

Inverters should be installed as near as practical (but not directly above) the battery bank that serves them. This minimizes the length of the wire run and reduces the diameter of the conductors needed, both of which result in increased efficiency and reduced installation cost.

Lead-acid batteries generate corrosive gasses when charging, which is why the inverter should never be mounted directly above a battery or bank. Inverters are also not ignition-protected, meaning they can’t be installed in a potentially explosive environment (such as the engine compartment of a gasoline-powered vessel).

2. Install proper fuses. The positive DC supply cable for the inverter requires installation of a fuse between the inverter and battery bank. Fuse type and placement are provided by the manufacturer’s instructions and should also follow ABYC standards and recommendations. For a typical installation, these call for the fuse to be installed no more than 7 inches from the battery. They must also be a “class T” fuse, rather than the cheaper, more common ANL-style fuse.

3. Power cutoff switch. In addition to fuses, the installation must also include a battery cutoff/disconnect switch installed in the DC positive cable. This allows you to cut off power to the inverter for servicing or in the event of an ­emergency.

4. Install a suitable sized chassis ground wire. As a marine surveyor, one of the most common problems I find with inverter installations is lack of a chassis ground wire or one that is too small. The chassis ground must be equal in size to the DC-positive conductor. This allows the full amount of DC power to be shunted to ground in the event of an inverter failure.

5. Provide wire run chafe protection and support. To prevent wire chafe and the potential fire hazards associated with it, install adequate chafe protection and ensure that all wires are properly supported (every 18 inches) along their entire run.

6. Install a remote control and monitoring panel. A remote control panel allows you to easily monitor inverter status, output, faults, and so on, as well as battery charge levels. Many also allow you to adjust the performance of your inverter and change some settings. This is particularly handy if the inverter is installed in some out-of-the-way location.

7. Install a warning sign or placard at the main AC breaker panel. This lets technicians know that the system includes an inverter, which must be turned off (along with any breakers) and disconnected prior to working on or near the AC-power system