Basic Power Conversion

When we’re talking about electricity, we talk about power in terms of Watts (W) or kilowatts (kW). The Kilowatt Hour (the power of a kW used for one hour) is a measure of how much energy is used, and also how your home electric bill is computed. Now when’s the last time you heard somebody say about their car, "this baby has 200 kW under the hood!" Probably never.
Horsepower (hp) is also a unit of power, so it can be directly converted into kW, and vise versa. The conversion factor is:

1 hp = 745.7 W, or 0.746 kW

Since car buyers and manufacturers who pay attention to horsepower tend not to care about fuel mileage, it’s not a standard thing to think in terms of how much energy is used, or horsepower hours, because after all, you can just refill the tank when you run out. When people do think about fuels in terms of available energy, the traditional unit is the British Thermal Unit (Btu), and it’s a little smaller than the Watt:

1 Btu = 0.293 Wh or 1 kWh = 3412 Btu

Just for reference, a ton of coal contains about 25 million Btu or 8076 kWh. A barrel of crude oil contains more than twice as much energy, about 58 million Btu, while a cubic meter of gas contains just over a thousand Btu.

When we’re talking about energy available to do work (to produce a force through a distance), it’s critical to know not just how much energy is available from a generation or storage system, but how quickly it can be drawn. One reason gas guzzlers are so popular is because they can produce their peak horsepower just by revving them up to the right RPM range and dumping the clutch. Batteries, on the other hand, will overload and fail if you try to suck out all of their available energy at once through a short circuit, and will produce less energy when drawn on heavily rather than moderately.