## What is the difference between Watts and Watt-hours?

Before we see how much electricity costs, we have to understand how it’s measured. When you buy electricity they charge you by the kilowatt-hour (kWh). When you use 1000 watts for 1 hour, that’s a kilowatt-hour. For example:

medium window-unit AC | |||

large window-unit AC | |||

small window-unit AC | |||

42″ ceiling fan on low speed | |||

light bulb | (i.e., all month) |
||

CFL light bulb |

To get kilowatt-hours, take the wattage of the device, multiply by the number of hours you use it, and divide by 1000. (Dividing by 1000 changes it from watt-hours to kilowatt-hours.) That’s exactly what I did in the table above.

Here’s the formula to figure the cost of running a device:

**wattage x hours used ÷ 1000 x price per kWh = cost of electricity**

For example, let’s say you leave a 100-watt bulb running continuously (730 hours a month), and you’re paying 15¢/kWh. Your cost to run the bulb all month is 100 x 730 ÷ 1000 x 15¢ = $10.95.

If your device doesn’t list wattage, but it does list amps, then just multiply the amps times the voltage to get the watts. For example:

**2.5 amps x 120 volts = 300 watts**

Exercise #1. Go get your electricity bill and see how many kilowatt hours you used last month.

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 15¢/kWh.

**Answer**

• 400 watts x 24 hours/day x 30.5 days/month = 292,800 Total Watt-hours

• 292,800 Wh / 1000 Wh = 293 kwh

• 293 kWh x 15¢/kWh = $44/mo.; $528/yr.

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 15¢/kWh.

**Answer**

• 1440 watts x 24 hours/day x 30.5 days/monh = 1,054,080 Total Watt-hours

• 1,054,080 Wh / 1000 Wh = 1,054 kWh

• 1,054 kWh x 15¢/kWh = $158/mo.; $1897/yr.

**Watts vs. watt-hours**

What is the difference between watts and watt-hours. Here’s the difference:

- Watts is the rate of use at this instant.
- Watt-hours is the total energy used over time.
Here’s a question I frequently get, which makes no sense:

“You say that some device uses 100 watts. What period of time is that for?”

It’s not for any period of time, because watts is a rate at that instant. One might as well ask:

The difference is:

- We use watts to see how hungry a device is for power. (e.g., 100-watt bulb is twice as hungry as a 50-watt bulb.)
- We use watt-hours to see how much electricity we used over a period of time. That’s what we’re paying for.
So, just multiply the watts times the hours used to get the watt-hours. (Then divide by 1000 to get the kilowatt-hours, which is how your utility charges you.) Example: 100-watt bulb x 2 hours ÷ 1000 = 0.2 kWh.

Here is a guide to energy used by appliance

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