 # Eco Brief: The Kilowatt Hour

January 13, 2016

Today’s Eco Brief explains the term kilowatt hour, abbreviated as kWh. Residential solar panels and other alternative energy sources are becoming more and more common throughout the US. Here are the basics on an essential term to help you determine which renewable energy solutions are best for your home.

Kilowatt Hours Let’s start with the kilowatt hour (kWh). First think about it as two separate terms: kilowatt and hour. One kilowatt is one thousand watts, and watts are units of power, the same watts as lightbulb watts. A unit of power is a unit of energy divided by a unit of time. When a car travels 50 miles per hour it is traveling a unit of distance, 50 miles, divided by unit of time, one hour. With a watt we are measuring joules, a unit of energy, per second, a unit of time.

Different phrasing: One watt is one joule per second. It takes 60 joules to illuminate a 60 watt lightbulb for one second. So 60 joules (the energy) are flowing out of a light socket every second, and the bulb is converting those joules into light.

If you leave that 60 watt light on for a second, you have used 60 watt seconds of energy. If you leave it on for an hour, you have used 60 watt hours of energy. One kilowatt is equal to 1000 watts. So, the 60 watt hours of energy used by a 60 watt lightbulb in an hour, is equivalent to 60÷1000, or .06, kWh.

Another way to visualize the difference: the kilowatt is how fast the water is coming out of the faucet, kilowatt hour is how much water is in the tub. The flaw in this analogy is that there is no tub full of energy at the end of the month because the energy comes out in the form of light, heat and mechanical movement like the twirling of blender blades. Other ways to think about it:
-One kilowatt hour would power a 100 watt bulb for ten hours.
-Energy Star certified dishwashers use approximately of 200-300 kWh in a year (run at normal settings, four times a week).
This chart gives a sense of around how much energy your appliances use.

The most important things to remember are that the kilowatt is the rate of energy use (1000 watts per hour) and the kilowatt hour is the amount of energy consumed in a period of time.

When it comes to solar panels, the panels will generate a certain amount of electricty at a given moment. This will be measured in
kilowatts. They will also generate a certain amount of energy over
a period of time, which will be measured in kilowatt hours. Now that
you understand the difference, it will be much easier to pick a solar
array that is best suited for your energy needs.

-Aidan Kelly

Photo Credit: Flickr/saragoldsmith, Flickr/Tom Taker, Flickr/liz west