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If you use a diesel generator as the power source for your boat's electrical systems, you will need to know how many kilowatt-hours of electricity your generator produces. No matter where you are in the heavens or on Earth, a kilowatt hour equals 0.746 horsepower. Knowing this, all you need to convert your diesel generator's output to kilowatt-hours is the generator's horsepower, and possibly a calculator.
The Generator Engine's Horsepower
Locate your generator engine's horsepower information, usually found on the manufacturer's identification plate on the generator's diesel engine. While this location may vary from engine to engine, it's most often found on the end of the engine's heads or on a special plate that may be bolted upright, like a flag on a flag pole, to the front edge of the intake manifold. You can also find the horsepower information for the generator engine in the operator's guide for the generator set or in the engine operator's guide, if you purchased the diesel engine separately from the generator set.
Beginning the Calculation
Write the horsepower down or enter it into a calculator. While it may seem unnecessary, recording the information obviates the need for a later search and saves time. Even if you record the information -- and plan to do the math -- on a piece of paper, working with a calculator has the advantage of speed. As with any other kind of computer, the information you enter must be correct to avoid an error in the final answer.
Horsepower-Hours and KiloWatt Hours
If you plan to convert a diesel engine's horsepower to kilowattt-hours, you can't compare apples to oranges. If a 25-horsepower diesel engine drives a generator for 1 hour, the engine has produced 25 horsepower-hours. If a generator produces 1 kilowattt of electricity for 1 hour, that's one kilowattt-hour. Whether you are discussing mechanical power or electrical power, you are talking about effort for a specified amount of time. The horsepower rating -- the effort -- is a variable, but the time for both the horsepower used and the kilowattts calculated is a constant one hour. This allows you to convert the diesel engine's rated horsepower directly to kilowattt-hours.
Makiing the Calculation
When used to run a generator, one horsepower generates 0.746 kW. This means that, if you multiply the horsepower by 0.746, you immediately arrive at the number of kW represented by the horsepower. If the diesel engine drives the generator set -- or "genset" -- for one hour, 0.746 kWh are produced for each horsepower-hour, since the time factor of the work is one hour in both variables. For example, a 400-horsepower drive engine can, with the right genset, produce 298.4 kWh of electricity -- 400 x 0.746 = 298.4 -- enough to power a neighborhood ashore, or to run every piece of equipment you might wish to install on your boat, assuming your boat is large enough to accommodate a 400-horsepower diesel engine just to run the generator.
Will Charpentier is a writer who specializes in boating and maritime subjects. A retired ship captain, Charpentier holds a doctorate in applied ocean science and engineering. He is also a certified marine technician and the author of a popular text on writing local history.