How do I find out how much electricity something uses?

For most devices you can just look at the label! Nearly everything you can plug into the wall has a label that says how much electricity it uses. (It may be printed directly into the plastic or metal.) You may have to hunt for the label. It’s often located on the bottom or side of the device, or possibly where the power cord enters the unit. If the device is powered with an AC/DC adapter, the electrical rating is usually listed on the adapter itself.

If the label only gives the number of amps and not the number of watts, then just multiply the amps by 120 to get the number of watts. (Amps x Volts = Watts, and most U.S. electricity is 120 volts. So a hot plate that uses 6 amps uses 6 x 120 = 720 watts. Most other countries use 240 volts instead of 120, so outside of North America and Japan use 240 instead of 120 in your calculations.) Note that if a device is powered by a transformer (one of those great big plugs), then the transformer has converted the electricity from AC to DC, so you need to multiply by the DC voltage, not the AC voltage of 120. For example, if the device says “INPUT 9V, 0.5A”, then that’s 9 volts x 0.5 amps = 4.5 watts.

You may have noticed that appliances may be labeled 110, 115, or 120 volts. Appliances are actually designed to accept a range of voltages, between 110-120 volts, and the exact voltage coming out of your electrical socket can vary depending on conditions at the power plant and in your own home. Let’s just agree that when we say 120 volts, we understand that it’s actually a range from 110-120. And just use 120 for your calculations (unless you’re outside of North America or Japan, in which case you probably have 240 volts).

Your device might actually list a huge voltage range, like 100-240V. That just means that it will work with any country’s voltage. For your calculations, use the voltage for the country where you’re plugging the device in.

Some important caveats:

1. The amount of electricity listed on the label is the maximum amount that the appliance will ever use. For example, a 300-watt refrigerator will only run at 300 watts when the compressor’s running (which is when it makes that humming sound, indicating that it’s actually chilling the air inside). Most of the time the fridge just sits there, using only 5 watts or so for its electronics. If the amount of work done by a device varies up and down, then so does its energy use. (e.g., a stereo that can be turned up or down, an oven that can be set at various temperatures, a fridge that sometimes runs and sometimes doesn’t, a computer that sometimes spins its various drives and sometimes has to use more of its brainpower, etc.) The label on computers is particularly useless; a computer labeled at 300 watts probably uses only about 100. (More on computers’ electrical use.) In just a bit we’ll cover how to measure the actual amount of electricity being used by a device.

2.

Heating – 26,500 watts

Elec. furnace, 2000sf, cold climate – 7941 watts

Elec. furnace, 1000sf, warm climate – 1440 watts

Electric space heater (high) – 900 watts

Electric space heater (medium) – 600 watts

Electric space heater (low) – 750 watts

Gas furnace (for the blower) – 1100 watts

Waterbed heater – 450 watts

Waterbed heater (avg. 10 hrs./day)

Cooling – 3500 watts

Central Air Conditioner (2.5 tons) – 1440 watts

Window unit AC, huge – 900 watts

Window unit AC, medium – 500 watts

Tiny-ass window unit AC – 750 watts

Central AC fan (no cooling)

More efficient cooling – 400 watts

Evaporative cooler – 350 watts

Whole-house fan – 100 watts

Floor or box fan (high speed) – 90 watts

52″ ceiling fan (high speed) – 75 watts

48″ ceiling fan (high speed) – 55 watts

36″ ceiling fan (high speed) – 24 watts

42″ ceiling fan (low speed)

Major appliances – 4400 watts

Washing machine – 3800 watts

Water heater (electric) – 200-700 watts

Refrigerator (compressor) – 57-160 watts

Refrigerator (average) – 3600 watts

Dishwasher (washer heats water) – 2000 watts

Electric oven, 350°F – 1178 watts

Electric oven, self-cleaning mode(takes 4.5 hrs, 5.3 kWh total) – 1200 watts

Lighting – 60 watts

60-watt light bulb (incandescent) – 18 watts

CFL light bulb (60-watt equivalent) – 5

Night light – 0.5

Computers (see more about electrical use of computers) – 150-340 watts

Desktop Computer & 17″ CRT monitor – 1-20 watts

Desktop Computer & Monitor (in sleep mode) – 90 watts

17″ CRT monitor – 40 watts

17″ LCD monitor – 45 watts

Televisions & Videogames – 340 watts

50-56″ Plasma television – 260 watts

50-56″ LCD television – 170 watts

50-56″ DLP television – 270 watts

42″ Plasma television – 210 watts

42″ LCD television – 125 watts

32″ LCD television – 55-90 watts

19″ CRT television- 45 watts

HD cable box – 194 watts

PS3 – 185 watts

Xbox 360 – 70 watts

Xbox – 30 watts

PS2 – 18 watts

Microwave oven or 4-slot Toaster – 900 watts

Coffee maker – 800 watts

Range burner – 4 watts

Clock radio – 3 watt-hours

Total power stored by an alkaline AA battery. This is to put batteries into perspective. If you could power your clock radio with a AA battery, it wouldn’t even last an hour. We have more on batteries on our Guide to Household Batteries.

See data for specific models of appliances at the Power Consumption Database.

Many consumer items are advertised according to their power output, not input. That means the stereo that says 30 watts on the box might actually require 50 watts to make 30 watts of sound (assuming the volume was cranked), and your 900-watt microwave oven might actually use 1400 watts (on its highest setting). That’s because all electrical devices are inefficient — they have to use some extra energy to do what they do.

3. Knowing how much electricity a device uses at a given moment doesn’t tell you how much it’s using in a month, because it’s probably not running 24/7 (and if it is running 24/7 like a fridge, it’s probably not using the maximum amount of electricity, as we discussed earlier). To measure how much electricity something uses for a certain period of time (like a week or a month), you can use a watt-meter.

4. Some devices use a small amount of electricity even when they’re not on. For example, VCR’s and microwaves draw a small amount to power the time display. This amount is often 5 watts or less. Devices which run off transformers also draw a small amount of power.

And of course, electricity consumption of a device varies from brand to brand, and condition to condition.

Exercise: The power adapter on your laptop computer says its output is 24V and 1.875 amps. The input specs aren’t listed for some reason. What’s the maximum number of watts your computer could ever use? (see answer)

24V x 1.875a = 45 watts

Your adapter could draw a little more than this, because it’s a little inefficient at converting the 120V to 24V, but that doesn’t really matter because your computer will rarely need the full 45 watts anyway.

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