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Inverters can enhance the comfort and convenience of your boat when it is away from the dock. They convert 12- or 24-volt DC electrical energy from on-board batteries to household-type 110-volt AC electrical power. This permits you to use appliances, under way or at anchor, that you could otherwise use only when shore power is connected at the dock, or if you operate an onboard generator.

One of our customers wanted to use his microwave oven, toaster and television while cruising Chesapeake Bay aboard his 35-foot Beneteau sailboat. We installed a Heart Freedom Marine 30 3,000-watt inverter and two 8D batteries for energy storage.

The inverter supplies enough power to run all three appliances at once, in a package that measures 12 inches long by 11.5 inches wide and about 9 inches high. It operates silently and is fully automatic. When he disconnects from shore power and departs, it begins providing power to his AC convenience outlets. When he returns to the dock at the end of the cruise and connects shore power, the inverter automatically changes to "charger" mode and recharges the battery bank. Moreover, it is a "smart" charger, with controlled charge rates specially designed to enhance battery life. Our customer was so pleased with his setup that he had us install a five-speaker home theater system with a woofer and a second-zone control for on-deck speakers. Now he can watch movies at anchor with state-of-the-art surround sound.

Not all inverters are designed for large loads, nor do they all provide the automatic features of the aforementioned large inverter. Smaller and less expensive inverters can be used to power a single AC outlet that you intend to use for small tasks, such as recharging your PC or cell phone batteries, or running the stereo. These small inverters do not include a battery charging feature. Xantrex's xPower 300 is a portable power source that includes a built-in AGM battery and a 300-wat inverter that will run a television or small stereo. It sells for around $170.

(Wattage is a measure of power - the rate at which electrical energy is being provided to the user, having been converted from some source of stored energy. Inverters provide wattage from energy stored in batteries. By comparison, gensets provide wattage from chemical energy stored in fuel.

Nothing is perfect, and the inverter concept is no exception. The first limitation is that batteries are heavy and bulky, which limits the energy that can be stored for use by the inverter. Our customer needed to plan his energy use with this in mind. For example, he can't use his electric hot water heater while cruising because it would rapidly consume all of his stored energy. Running his air conditioner likewise is out of the question. Compare that to a generator, which typically derives its energy from the same fuel as the main engines and in such small amounts as to be generally negligible. We can assume that total stored energy for the generator is unlimited for practical purposes.

Another limitation is maximum rated continuous power output. The largest inverters compare only with the smallest gensets in output wattage. The conclusion is that if large, continuous loads are to be supported, an appropriate sized genset is needed.

Yet for boats that have generators, inverters can still serve a purpose: They provide 110-volt AC power during periods of low demand when the genset can be turned off. When the generator is cranked up again, the inverter recharges the battery bank for the next quiet period. Trace's SW Series inverters will automatically start and stop the generator as needed to recharge the battery bank.

Gensets operate best with continuous loads and at a significant fraction of their rated capacity, say at least 40 percent. Operating the generator during periods of heavy demand, and switching to an inverter for low- and variable demand periods makes best use of the strengths of both units.

The inverter-generator partnership can lead to lower genset maintenance costs and longer genset life. And if your genset can be heard throughout the yacht, then the quiet periods of inverter use might provide a welcome respite.

Inverters come in a variety of sizes and features. If you decide an inverter might be right for your boat, take a careful inventory of your expectations and compare them with the capabilities of the inverter installation you envision.

You need to consider your installation from two perspectives: whether the inverter has sufficient output wattage to run the appliances you expect to use, and whether your battery bank will store enough energy to carry you through from recharge to recharge.

Make a list of the appliances you expect to use and the total time you expect to use each one in the interval between full recharges of the battery bank. Write beside each one the manufacturer's estimated power requirement. Now add up the watt requirements of the appliances you plan to use simultaneously.

Use the table as an example. If you plan to use a 1,200-watt microwave oven, a 1,300-watt toaster, and a 150-watt television simultaneously, you need an inverter with a capacity that exceeds 2,650 watts. A 3,000-watt inverter from a reputable, established company should meet your needs. (Some inverters fall short of their rated output as battery output voltage drops even slightly. Keep in mind that bargain-priced inverters might not be a bargain.)

If you expect to use appliances with electric motors or other components that have high start-up surge demand, such as large refrigerators, freezers and pumps, you'll also need to take that into consideration. Most inverters will handle a surge demand significantly greater than their nominal rating, but you should obtain the information from the manufacturer so you won't be disappointed.

Next, determine the energy storage requirement by multiplying the wattage for each appliance by the time in hours, or fraction of an hour, that you expect to use it in the interval between full recharges of your battery bank. Add these up for all your appliances to get the number of watt-hours you expect to use. The table shows this calculation. The expected usage shown in the table is 1,370 watt-hours. To determine the needed capacity of the battery bank, we need to allow for efficiency losses in the inverter. We add 20 percent as a conservative estimate, bringing us to 1,644 watt-hours.

Battery storage capacity is measured in ampere-hours, which is current multiplied by time. To obtain the watt-hours stored, we need to multiply the battery ampere-hours by the battery voltage. A good deep-cycle 8D battery (12-volt) will have a capacity of more than 200 ampere-hours, or 2,400 watt-hours.

Since we’ve been conservative at each step, we would consider a single 8D battery sufficient for the needs listed in the table. Keep in mind that battery life is shortened each time the battery is fully discharged. Battery life can be prolonged by sizing the battery bank to avoid full discharge. By dividing the watt-hours needed (1,644) by the battery’s watt hours (2,400) our estimated draw-down is 68.5 percent, which leaves the battery about 30-percent charged in a worst-case situation.

One last consideration in the choice of your inverter is the waveform. Most inverters produce a modified sinusoidal waveform slightly different from that produced by a generator or provided by your utility company. It can be thought of as containing higher frequency components than the expected pure 60 cycles per second. Nearly all appliances will be unaffected by the presence of these impurities. At greater expense, you can purchase a pure-sinewave inverter, such as Statpower's PROsine series. If you have any doubt about a particular appliance, such as a sensitive sound system, then you should check with the manufacturer or compare notes with someone using one with an inverter.

Inverter prices vary depending on features. For example, the Freedom Marine 30, part of Heart Interface's affordable Freedom line, sells for around $1,500 in marine discount catalogs. The higher-end PROsine 3.0 sells for around $2,800.

In selecting the location of your inverter installation, some common sense rules apply. For example, the inverter should be fastened securely to the boat in a dry, ventilated space. If your needs are modest and require only a small inverter, with peak DC input of 30 amps or less, install it close to the load and connect it to the main DC panel with appropriate marine electrical cable and circuit protection.

One good approach is to connect your small inverter to one or two convenience outlets dedicated strictly to the inverter. Then you can hook up your chosen appliances at convenient locations on the boat. These outlets should be marked "powered by an inverter. " This safety placard is a warning to someone who might otherwise assume the outlet is harmless when shore power is disconnected.

Keep the power to your small inverter switched off when it isn't needed because there will be some energy loss even when there is no load on the inverter.

For large systems with automatic features, the inverter needs to be connected directly to the battery bank. The inverter and battery bank need to be installed in a ventilated, dry and accessible location. The inverter should be installed as close as possible to the battery bank, but not above it. Do not store anything on top of your batteries.

These big systems require large electrical cabling and ideally should be installed by an ABYC-certified (American Boat and Yacht Council) technician. The positive conductor must be protected by a high-amp class T fuse. The chassis ground wire should be no smaller than 1 gauge under that of the positive battery cable.

If you plan to install the inverter yourself and don't have the tools to make up cables of the required sizes, then have a commercial shop make them up for you. If you do, be sure the cable sizes are specified using AWG (American Wire Gauge) and not SAE (Society of Automotive Engineer) standards. The cables should be marine quality. Cabling is a bad place to cut corners.

ABYC's "Standards and Recommended Practices for Small Craft" devotes a chapter to inverters and another to battery chargers. (The latter will apply if your chosen inverter includes a charging feature.) While these standards are voluntary, they contain requirements that will make your installation safer and more reliable. Whoever installs your inverter should be familiar with ABYC standards and follow them. The ABYC standards also include sample wiring diagrams.

Large inverters with automatic functions are installed between the AC shore power connection and the main AC distribution panel. In this location, the inverter senses the presence of shore power and automatically changes from inverter mode to battery-charger mode, and vice versa when shore power is connected or disconnected. In addition to recharging its own battery bank, there is usually a provision for the inverter to charge one or more of the boat’s other battery banks, such as those that start the engines or generator.

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Last modified: March 01, 2002