Electricity cost calculator

Watts, watt-hours and the kWh on your bill

Two units cause most of the confusion around electricity costs. A watt (W) measures power: how fast a device uses energy at a given moment. A watt-hour (Wh) measures energy: the amount used when that power keeps flowing for an hour. A 100 W lamp left on for 10 hours uses 1,000 Wh, which is one kilowatt-hour (kWh). In SI terms one kWh is exactly 3.6 megajoules, but electricity bills normally count kWh, sometimes printed as “units”.

Your bill charges for energy, not power. So the running cost of a device depends on both its wattage and how long it runs, which is why the calculator asks for:

• Power (watts), from the rating plate, the manual or a plug-in meter. A rating in kilowatts is multiplied by 1,000 (2 kW is 2,000 W).
• Hours per day of actual use, from 0 to 24.
• Days, the period you want to cost: 30 for roughly a month, 7 for a week.
• Price per kWh from your bill or tariff, in your currency.
• Number of devices, for several identical items such as light bulbs.

It returns the cost for the period, the total kWh, the kWh and cost per day, the figures for a full 365-day year, and the working line with every number used.

Formula

Worked examples

Rated wattage versus actual draw

The wattage on a rating plate or charger is a rating, not a measurement of use, and the Virginia Cooperative Extension guide points out that the power actually consumed depends on the setting in use. A laptop charger marked 65 W may draw far less once the battery is full; a heater on its low setting uses part of its rated power; a refrigerator or freezer switches its compressor on and off to hold its temperature. For a refrigerator, the same guide suggests counting about a third of the time it is plugged in as running time.

For a real figure, use a plug-in energy meter, which sits between the socket and the appliance and shows the power drawn and the kWh used. Run it for a full day or a full cycle, then enter the measured average in watts with 24 hours a day, or read the kWh directly. Plug-in meters suit appliances that use an ordinary socket; hard-wired equipment such as an electric water heater cannot be measured this way.

Standby power

Many devices draw a small amount of power whenever they are plugged in, even when they appear to be off: chargers, televisions, set-top boxes, game consoles and anything with a clock or remote control. These loads are small for one device but continuous, so they are worth checking with a meter. Switching the device off at the wall or with a power strip removes them.

Costing mistakes to avoid

• Confusing W with kWh. A 2,000 W kettle does not use 2,000 kWh. Used for 3 minutes, it uses 2,000 × 0.05 ÷ 1,000 = 0.1 kWh.
• Entering 24 hours for something that cycles. A 150 W refrigerator counted at 24 hours a day comes to 3.6 kWh a day; counted at about 8 hours of compressor time it is 1.2 kWh, a third as much.
• Using a price that leaves out part of the bill. Many tariffs add a fixed daily charge or several rates by time of day. Use the energy rate that applies when the device runs; fixed charges do not change with use.
• Typing minutes as hours. Twenty minutes is 0.333 hours, not 0.2. The time duration calculator converts minutes to decimal hours.
• Applying one month’s figure to the whole year. Heating, cooling and lighting change with the seasons, so a winter month is a poor guide to annual cost.

Sources

1. Energy Series: Estimating Appliance and Home Electronic Energy Use (BSE-301NP), Virginia Cooperative Extension, Virginia Tech.
2. Guide for the Use of the International System of Units (SI), NIST SP 811, Appendix B, National Institute of Standards and Technology.