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 Interactive Energy Calculators

Our calculators help you understand energy production and consumption in a whole new way. Use them to develop a personal profile of your own energy use.

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What's a Watt?

What's a Watt?

We are confronted every day by the notions of energy and power: cars and motors are sold by horsepower, light bulbs 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 amount of energy that is converted in a unit of time.

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

EnergyTime Power
Joules
Watts

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
BTU BTU/hr
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 pound car traveling at miles per hour melting a block of ice weighing pounds heating a gallon tank of water by degrees F. evaporating pounds (about gallons) of water heating a pound hunk of concrete by degrees.

Efficiency

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%

The What's a Watt energy conversion calculator is sponsored by the Texas State Energy Conservation Office (SECO). The calculator can also be accessed at Martindale's Calculators On-Line Center, along with other energy calculators.