Join Date: Jun 2003
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Sailmc has the arithmetic right on the water heater. For example: We just installed an 11 Gallon electric water heater tank. Has a 1500 watt element. At 120V this is 13Amps. (All arithmetic here is approximate and ignors losses due to inefficiency in voltage conversions). It takes a half hour for the water to get warm and an hour for it to get hot. So, at 120V, 13 amps, one hour gets the water hot. At 12V (using the inverter), it takes 130 amps. For one hour is 130 AH. 8D batteries are probably 200 AH capacity each, or so, and using a conservative 35% utilization factor, you can then use 280 amp hours without significant impact to the batteries--total capacity of the bank. So, to heat that 11 gallons of water takes half your available battery capacity.
So, while it can be done, in our case (having similar capacity), we don''t like to do that. Instead, we make sure the engine is running, we are at the dock plugged in, or the gen set is running while heating up water or using a lot of it--like for showers. Just keeping it hot will use much less.