What's a Watt?

Energy and Power Conversion Calculator

We are confronted every day by the notions of Energy and Power: Cars and motors are sold by Horsepower, lightbulbs by Watts, natural gas by Therms, electricity by Kilowatt-Hours, and air conditioners by Tons or BTUs per hour. Is Energy the same as Power? If not, what's the difference, and how do all of these units relate to each other?

It's really simpler than it looks. You can think of Energy as a "stuff" that can't be created or destroyed, but can be converted in form. The form might be a moving mass, light, heat, an elevated mass, particular chemical combinations, or electricity in a wire. Power is simply the rate at which energy is converted (for example, a Watt is one Joule per Second).

Here are some of the ways that you will see Energy and Power represented: Modify the value in any box and see what happens...

Energy Time Power 

This is the average power in full sunlight falling on a square foot surface, directed toward the sun, and collecting hours each day.

Watt Hours Kilowatts
Kilowatt Hours Horsepower
Therms tons

The following table illustrates how various energy storage systems would store this amount of energy:

Chemical gallons of gasoline ( pounds)
pounds of natural gas ( cubic feet at psi)
pounds of Hydrogen Gas ( cubic feet at psi)
A Lead-Acid battery weighing pounds
A high-tech flywheel weighing about pounds
A mass weighing pounds, raised to a height of feet.
A pound car traveling at miles per hour
Thermal Melting a block of ice weighing pounds
Heating a gallons of water by degrees F.
Evaporating pounds (about gallons) of water
Heating a pound slab of concrete by degrees.


Always remember that, in every energy conversion, some of it goes where you want it, and some goes elsewhere (usually heat). The ratio of what you want to the amount that you start with is called efficiency. Here are some typical efficiencies encountered in everyday processes:

  • burning fossil fuel to get useable heat - about 85% (running a gas-fired water heater, or making steam to power a turbine...some heat goes up the smokestack).
  • burning fossil fuel to get electricity - about 33%
  • sunlight to electricity in a PV cell - about 10%
  • putting electrical energy into a battery (charging it) and pulling it back out: about 90%
  • converting electrical energy into mechanical energy with an electric motor about 85%

I have made considerable effort to present correct information.

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