How to measure real appliance electricity use

Does your electricity bill show what each appliance uses?

Usually not. A bill is normally based on your utility meter, which records the electricity used by the whole home, so it normally gives one total for the billing period rather than a figure for each appliance. Some advanced smart meters can track individual devices, so check what your supplier offers. If your bill or app does show a breakdown, find out whether each figure is measured or estimated.

If your price changes by time of day and your bill lists the kWh used in each price period, for example day and night, those figures are also for the whole home. To price one appliance on such a tariff, measure the kWh it uses in each period, then enter its energy and the share used at the second price in the two-price option of the kWh calculator.

To find out what one appliance uses, you can:

• measure it with a plug-in electricity monitor;
• compare meter or smart-meter readings with the appliance off and on;
• estimate it from its wattage and hours of use with the appliance cost calculator.

Watts, watt-hours and kilowatt-hours

• Watt (W): power, the rate at which electricity is being used at a given moment.
• Watt-hour (Wh): energy, one watt used for one hour.
• Kilowatt-hour (kWh): 1,000 watts used for one hour. Utilities generally measure electricity in kWh.

So energy (kWh) = power (W) × hours ÷ 1,000. A 40 W bulb left on for 5 hours uses 40 × 5 ÷ 1,000 = 0.2 kWh. A measurement that gives you energy (kWh) over a known time is more useful than a single power reading, because most appliances change their power as they run.

Choose a method

• Plug-in electricity monitor: for appliances with an ordinary plug, such as refrigerators, freezers, washing machines, dishwashers, televisions, computers and portable heaters. This is the most direct method for a single appliance.
• Your electricity meter or smart-meter data: for hard-wired appliances, or when you have no monitor. It measures the whole home, so the appliance's share has to be worked out.
• Rating plate or manual: not a measurement. Rated input power is a value the manufacturer specifies for set operating conditions, often the maximum draw, not an average. If the plate shows only volts and amps, multiplying them gives volt-amperes, which can be higher than the real power, so treat the result as an upper estimate.

Using a plug-in monitor

1. Check that the monitor is rated for the appliance. Its manual gives the maximum current or power it can handle; high-power appliances such as heaters and kettles can be close to a socket's limit.
2. Plug the monitor into the wall socket and the appliance into the monitor. Reset its energy total.
3. Use the appliance normally for a representative period:
   • a complete program for a washing machine, dishwasher or dryer, run the way you usually run it;
   • several full days for a refrigerator or freezer, including normal door opening;
   • a typical day or evening for a heater, air conditioner, television or computer.
4. Note the energy (kWh) and the time it covers. Ignore any cost the monitor shows: it uses its own price setting.
5. Repeat on another day if use varies, and use the average.

Very small loads. Standby power can be very small. Check the accuracy stated in the monitor's manual: at low power a reading can be rounded or unreliable. Measuring energy over a long period, such as several days, gives a more stable figure than reading the watts at one moment.

Turning a measurement into calculator inputs

Illustrative example: refrigerator (energy over several days)

A monitor records an illustrative 2.1 kWh over 3 full days. The yearly figure is 2.1 ÷ 3 × 365 = 255.5 kWh. Enter 255.5 in the refrigerator calculator, which uses yearly kWh.

Illustrative example: washing machine (one program)

One run of your usual program reads an illustrative 0.65 kWh. Enter 0.65 kWh per cycle and the cycles you run each week in the washing machine calculator. Measure each program you use often, because hotter or longer programs can use more.

Illustrative example: heater on a thermostat (effective power)

A heater rated at 1,500 W runs on its thermostat for 6 hours and the monitor records an illustrative 4.2 kWh. Its average power was 4.2 × 1,000 ÷ 6 = 700 W, which is 700 ÷ 1,500 = 46.67% of its rating. In the space heater calculator, enter 1,500 W, 6 hours and an effective power of 46.67%, or simply enter 700 W at 100%. The same approach works for the air conditioner calculator. One day's measurement reflects that day's weather and settings only.

Illustrative example: standby (a long, low reading)

A television left in standby records an illustrative 0.1 kWh over 125 hours. Its average standby power was 0.1 × 1,000 ÷ 125 = 0.8 W. Enter 0.8 W as the standby power in the standby power calculator.

Using your electricity meter or smart-meter data

Your utility meter records all the electricity your home uses, and smart meters can report that use in real time. To estimate one appliance:

1. Keep other use as steady as you can, and avoid starting large appliances during the test.
2. Read the meter, or note your usage data, for an hour with the appliance off. This is your baseline.
3. Read it again for an hour with the appliance running.
4. Subtract the baseline from the second reading.

Illustrative example: the home uses 0.4 kWh in the baseline hour and 1.9 kWh in the hour with a heater running, so the heater used about 1.9 − 0.4 = 1.5 kWh, an average of 1,500 W. Refrigerators and other appliances switch on and off by themselves, so repeat the test and treat the result as approximate.

Hard-wired appliances and safety

Water heaters, built-in ovens and hobs, central air conditioning and some heaters are wired directly into the home's electrical system. Do not open wiring, switchboards or appliance panels to measure them. Use the meter method above, the label or manual, or ask a qualified electrician. Never use a plug-in monitor with a damaged plug or socket, or beyond its rating.

Common input errors

• Entering a reading in watts where the calculator asks for kWh, or the other way round.
• Entering watt-hours (Wh) as kilowatt-hours: divide Wh by 1,000 first.
• Measuring for too short a time, such as a few minutes of a fridge or one part of a wash program.
• Entering minutes as hours: 30 minutes is 0.5 hours.
• Counting standby twice: in the standby calculator, plugged-in hours already include the hours in use.

Limitations

A measurement describes the period you measured. Seasons, settings, the load and the age of the appliance change use, and plug-in monitors have their own accuracy limits. A measurement still gives a better basis for your home than an assumed percentage. To convert between watts, hours and kWh, use the kWh calculator. For label figures, see how to read appliance energy labels, and for how the calculators work, the methodology.

Sources

These sources support specific statements on this page. They do not review or endorse this calculator or its results. Last reviewed 5 November 2026.

• U.S. Energy Information Administration (EIA): Measuring electricity. Defines the watt, watthour and kilowatthour, gives the 40 W bulb example, and explains that meters record use, smart meters report it, and some advanced smart meters can track individual devices.