Power  Inverting

Installing an inverter is easy.

It’s knowing which one to use that takes work.

While small boat owners may be accustomed to “roughing it” a bit more than those on larger craft, we’re always on the look-out for ways to make boating more like cruising and less like camping.  One example of this is installation of an AC power inverter, which allows you to bring some of the comforts of home onboard, such as a coffeemaker, blender or even a small microwave.  Let’s take a look at some of the do’s, don’ts, and other considerations of choosing and using an AC inverter.

Inverter101

In basic terms, an inverter converts your boat’s 12 or 24 volt DC battery power to 115V AC electricity. Unlike a generator (which actually generates AC), power output of an inverter depends on the output rating of the inverter and available battery capacity. If you’re looking for lots of continuous AC power a generator is your best bet, but for intermittent or small long-term AC power needs (brewing that pot of coffee or running a portable sander for example) an inverter is an attractive option.
The key to successful inverter nirvana is a reasonable expectation of just what your inverter can (and can’t) do. Being able to use a blender is probably a reasonable expectation, but don’t expect to power up your air conditioning unit with a 50 watt inverter and a group 24 starting battery.
Although limited by available battery capacity, inverters have advantages that even boats 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 (or waking up the neighbors).

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, to permanently mounted units wired directly to your vessel’s batteries for 2,000 or 3,000 watts.
Portable units are easy to use and work fine for powering smaller items (cell phones, laptops, tools, etc) 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. Many of these also have built in battery chargers, allowing you to keep your batteries topped off when shore power is available.
Most inverters simply have an outlet in the case to plug in your appliance, however some of the more high-end units have built-in transfer switches, providing you the option to power remote AC outlets. Inverters designed to integrate 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 want to always disconnect it from the AC system prior to doing any work or maintenance.

Choosing an inverter

Inverters are rated in watts and selection is based on the maximum amount of AC power you’ll need at any given time. Most AC powered appliances and electronics list wattage requirements on their data tags – simply add up the watts for each appliance you need to operate at once.
For example, if you’re anchored out and plan on running a TV/DVD unit (200 watts) for movie night and a small microwave (800 watts) at the same time to pop the popcorn, you’ll need an inverter that delivers more than 1000 watts of continuous power.
Some appliances may also have a power surge requirement that has to be taken into consideration.  That TV/DVD may have a power requirement of 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.
Finally, you’ll also need sufficient battery capacity to power the inverter. A good rule of thumb is that battery capacity (amp-hours) should be equal to at least 20% of your inverter’s rating.  Building on our movie night example, you would need at least 200 amp-hours to power a 1000 watt inverter at full power for one hour (two hours at half power).

Inverter location

You’ll want to follow all instructions provided by the manufacturer, but in general permanently installed inverters should be located within 10 feet of your batteries (to avoid voltage drop or the need for oversized wires) and in an area that is cool, dry, well ventilated and away from the corrosive gasses that can be generated by batteries while charging. Unless labeled as being ignition protected, inverters must not be installed in a gasoline powered vessel’s engine compartment due to the risk of explosion. Even if ignition protected, many inverter manufacturer’s recommend against installation within engine compartments or spaces containing fuel, so read your instruction manual carefully before choosing a location.