water heaters

tauras · #1 (**permalink**) 12-16-2003

Need help. Just purchased a 65'' schooner which has never had a water heater on board.
We have four 8d agm batteries with a 3500 watt inverter plus a new Yanmar 10kw gen set.
Does it make sense to install an on demand
water heater? Will the inverter handle it? Or what type of water heater makes the most sense. Can a 10 to 15 gallon electric water heater run off the inverter? opinions please.
Thanks
Ray

Sailmc · #2 (**permalink**) 12-17-2003

Multiply the amps of the water heater you are interested in by 120 volts. This will give you the wattage the water heater requires. If this number is less than 3500 watts your inverter will work. You then need to multiply the water heater amperage by a factor of ten. This will be the amperage draw on your batteries for however long it take to satisfy the thermostadt on your water heater. This will be a sustantial portion of your available amp hours necesitating engine or generator run time. If you know how long the water heater needs to run and the output of your alternators you can determine how much run time will be needed to replace the power used by the water heater.

mcain · #3 (**permalink**) 12-17-2003

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.

svGandalf · #4 (**permalink**) 12-31-2003

Why bother with hooking it into your electrical system at all?

Have you considered the propane "instant on" water heaters such those made by Bosche?

This would be by far my choice.
Electricity can be as precious as fresh water.