Calculating Cost and KwH

If you’ve ever paid an energy bill before, you probably noticed the word ‘kilowatt hour’ somewhere on your monthly statement (sometimes abbreviated as kW·h, kW h or kWh). If you’ve ever been confused by what this means, you’re in good company—it’s not as straightforward as a gallon or a degree or a day. And that’s because a kilowatt hour is actually measuring two different things at once: energy (in watts) and time (in hours).

So if you have a 1000 watt microwave, and you decided to pop a bag of popcorn for an hour, it would use one kilowatt hour, or 1 kWh. It would also smell really bad.

Many appliances and gadgets are labeled with the maximum wattage they draw to operate, but the best way to know exactly how much energy your stuff uses is to measure it with a watt-hour meter, like the wonderfully named Kill-a-Watt, which you can buy here.

Once you know the watt usage, it’s time to closely estimate two additional figures: the number of hours per day and the number of days per month the device or appliance is used. The final equation will look like this:

Watt Usage X Hours/Day X Days/Month, divided by 1000 = Kilowatt Hours used that month

For my hair dryer, this would look like:

2000 watts X .5 hours X 15 days = 15,000 watts, divided by 1000 = 15 kWh per month.

At a rate of 9.6 cents per kWh, it costs about $1.44 a month to have dry hair.

Now, consider larger appliances like air conditioners given the above information. Hopefully it’s a little easier to understand just how much your personal preferences can impact your bill—and how much control you truly have over your energy use.