Adding an Inverter

By Don Casey

An inverter can be a marvelous convenience, but take care not to overlook the load it places on your boat's batteries. AC appliances draw 10 times their rated amps when running on 12 volts. For example, a 1,000-watt coffeemaker draws 8.3 amps at 120 volts (1,000 / 120), but 83 amps at 12 volts (1,000/12). Add about 15% for inverter inefficiency and the actual battery draw for that pot o' Joe is close to 100 amps.

This likely exceeds your starter-motor draw, so the effect on your batteries of a 10-minute brew cycle is roughly the same as cranking the starter for that long. Unless you are willing to run the engine while the inverter is in use, limit inverter wattage to five times your battery's rated amp-hour capacity. In other words, don't expect a 100 Ah battery to power an inverter larger than 500 watts.

Don Casey has been one of the most consulted experts on boat care and upgrades for 30 years, and is one of the BoatUS Magazine's panel of experts. He and his wife cruise aboard their 30-footer part of the year in the eastern Caribbean. His books include Don Casey's Complete Illustrated Sailboat Maintenance Manual, and the recently updated This Old Boat, the bible for do-it-yourself boaters.


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