Water Heater Element: An attractive solution is to use a low voltage water heater element in your onboard hot water heater. Now if you are making excess power you can use it for something more useful than drying socks. Without going deeply into the math, 300 watts will produce slightly over 1000 BTU/hr. One BTU will raise one pound of water 1 degree Fahrenheit. A 10 gallon water tank contains about 80 pounds of water. If we wish to raise 80 pounds of water from 70 to 140 degrees, a 70 degree temperature rise,  we will need 5600 BTUs 5600 divided by our available 1000 tells us that we would need 5.6 hours to attain our desired temperature. Unfortunately nothing is free or easy in the world of thermodynamics. The 1 BTU/pound/degree relationship is only accurate at the maximum density of water which occurs at 39.1 Fahrenheit and so the time will be slightly different. More important is tank loss. The average 10 gallon marine water tank will lose about 15 BTU/hour so in 6 hours we have lost about another 100 BTU. But assuming we have 300 watts available which with most small wind generators would require 30 knots or more of wind, it will still have to blow for 6 or more hours before the shower is hot if there are no other loads on the system. With the average wind generator of the size commonly found on boats,  even in the  Trade Winds or the Caribbean with 15 knot winds we would only have about 100 watts (7.5 amps at 13.5 volts), with no other loads and the batteries fully charged. Now we could stand around for 18-20 hours waiting for that 5 minute shower. This is not to say that a water heater element is necessarily a bad idea but we can’t change the laws of physics.

A different  consideration is that if the boat were left unattended for an extended period of time in high winds the water in the tank could boil. creating steam and potential explosion and damage. Even if the tank has a pressure relief valve, if we lose the water the element will go dry and burn out and the generator will now be looking at no load at all with potential damage to the generator itself. The long and short of it is that the boat should not be left unattended for an extended period with out disabling the generator.  If you do decide to use a water heater element you should choose one with a wattage capacity 20% greater than highest  potential generator output as a safety margin. For example, a 400 watt maximum generator should have a 500 watt element.