American Mini Splits

What the heck is a kilowatt hour?

Before we see how much electricity costs, we have to understand how it's measured.

Watts

The rate of electrical use at any moment is measured in watts. For example:

• A 100-watt light bulb uses 100 watts.
• A typical desktop computer uses 65 watts.
• A central air conditioner uses about 3500 watts.

If your device lists amps instead of watts, then just multiply the amps times the voltage to get the watts. For example:

2.5 amps   x   120 volts   =   300 watts

Watt-hours

To know how much energy you're using you have to consider how long you run your appliances. When you run a 1-watt appliance for an hour, that's a watt-hour. It's abbreviated Wh. For example:

• One 100-watt light bulb on for an hour is 100 watt-hours (100 Wh)
• One 100-watt light bulb on for five hours is 500 Wh
• Five 100-watt light bulbs on for an hour is 500 Wh

Kilowatt-hours

1,000 watt-hours is a kilowatt-hour (kWh). For example.

• One 100-watt light bulb on for an hour, is 0.1 kWh (100/1000)
• One 100-watt light bulb on for ten hours is 1 kWh (1 bulbs x 100W x 10h= 1000Wh = 1 kWh)
• Ten 100-watt light bulbs on for an hour, is 1 kWh (10 bulbs x 100W x 1h= 1000Wh = 1 kWh)
• Ten 50-watt light bulbs on for an hour, is 0.5 kWh
• Ten 100-watt light bulbs on for 1/2 an hour, is 0.5 kWh
• Running a 3500-watt air conditioner for an hour is 3.5 kWh.

 

Take a moment to understand the difference between kilowatts and kilowatt-hours. The former is the rate of power at any instant. The latter is the amount of energy used A light bulb doesn't use 60 watts in an hour, it uses 60 watt-hours in an hour.

Trivia The average U.S. household used 10,654 kWh a year in 2001, or 888 kWh/mo. (Dept. of Energy) The U.S. as a whole used 3,883 billion kWh in 2003, or 13,868 kwH per person based on a population of 280 million. (Dept. of Energy) On a peak day in 1999, California used 50,743 megawatt-hours of electricity, or 50,743,000,000 watt-hours.

The "-hours" part is important. Without it we'd have no idea what period of time we were talking about. If you ever see a reference without the amount of time specified, it's almost certainly per hour.

 

How much does electricity cost?

The cost of electricity depends on where you live, how much you use, and possibly when you use it. There are also fixed charges that you pay every month no matter how much electricity you use. For example, I pay $6/mo. for the privilege of being a customer of the electric company, no matter how much energy I use.

Check your utility bill for the rates in your area. If it's not on your bill then look it up on the utility's website.

The electric company measures how much electricity you use in kilowatt-hours, abbreviated kWh. Your bill might have multiple charges per kWh (e.g., one for the "base rate", another for "fuel") and you have to add them all up to get the total cost per kWh.

Most utility companies charge a higher rate when you use more than a certain amount of energy, and they also charge more during summer months when electric use is higher. As an example, here are the residential electric rates for Austin, Texas (as of 11-03):

These figures include a fuel charge of 2.265¢ per kWh.

The average cost of residential electricity was 9.86¢/kWh in the U.S. in March 2006. The average household used 888 kWh/mo. in 2001 and would pay $87.56 for it based on the March 2006 average rate. (Dept. of Energy)

The cost of electricity varies by region. In 2003 the price ranged from 5.81¢ in Tennessee to 12¢ in California, 14.314¢ in New York, and 16.734¢ in Hawaii. In Summer 2001, electricity was a whopping 20¢/kWh in parts of California.

Exercise #1. Go get your electricity bill and see how many kilowatt hours you used last month. Also see if it lists how much you're paying per kilowatt hour.

Exercise #2. Assume that the lights in your kitchen and living room together use 400 watts. How much does it cost if the lights are on 24 hours a day, for a whole month? How much per year? Assume 10¢/kWh. (see answer)

Exercise #3. Assume your window AC uses 1440 watts. How much does it cost to run it continuously for a month? How much per year? Assume 10¢/kWh. (see answer)