Electricity Cost Calculator

E = P × h × d / 1000

P

Power rating of the appliance (W)

h

Hours it runs per day

d

Days in the period you are costing

E

Energy used over that period (kWh)

Dividing by 1000 converts watts to kilowatts, because the unit you are billed in is a kilowatt-hour. Multiply the result by your price per kWh to get the money. The same expression with d = 1 gives the daily figure and with d = 365 the yearly one, which is why the page shows all three at once.

1. A 1000 W appliance for 4 hours a day

   1. Power: 1000 W. Hours per day: 4. Period: 30 days.
   2. Energy per day: 1000 × 4 / 1000 = 4 kWh.
   3. Energy for the period: 4 × 30 = 120 kWh.
   4. Cost per day: 4 × 0.15 = 0.60.
   5. Cost for the period: 120 × 0.15 = 18.00.
   6. Cost per year: 4 × 365 × 0.15 = 219.00.

   Enter the price in whatever money your bill is in — the calculator never attaches a currency, so 0.15 here means 0.15 of your unit per kWh.

2. A 60 W device left on for 8 hours a day

   1. Power: 60 W. Hours per day: 8.
   2. Energy per day: 60 × 8 / 1000 = 0.48 kWh.
   3. Energy for 30 days: 0.48 × 30 = 14.4 kWh.
   4. Cost for the period: 14.4 × 0.25 = 3.60.
   5. That is 0.12 a day, or 43.80 over a full year.

   A small load running for a long time adds up: 14.4 kWh a month is more than a 2000 W kettle used daily for 15 minutes.

3. A 2000 W kettle for 15 minutes a day

   1. 15 minutes is 0.25 hours — switch the time unit to minutes and enter 15 instead of converting by hand.
   2. Energy per day: 2000 × 0.25 / 1000 = 0.5 kWh.
   3. Energy for 30 days: 0.5 × 30 = 15 kWh.
   4. Cost for the period: 15 × 0.2 = 3.00.

   High power for a short time is usually cheap. The kettle draws 2000 W, more than thirty times the 60 W device above, and still costs less over a month — but not because it uses less energy. At 15 kWh against 14.4 kWh it uses slightly more; the saving comes from the unit price, 0.20 against 0.25.

This is an estimate from the rating plate, not a prediction of your bill. Appliances rarely draw their rated power for the whole time they are switched on: a fridge's compressor cycles on and off, a washing machine heats only during part of its cycle, and a laptop's charger draws far less once the battery is full. Standby loads are not counted at all unless you enter them. The price box takes a single figure, so a tariff with peak and off-peak rates, or one with a standing charge, will not be reproduced exactly — enter an average and read the answer as an estimate.

How is the running cost worked out?

Energy in kilowatt-hours is the power in watts, multiplied by the hours it runs, divided by 1000. A 1000 W appliance running 4 hours a day uses 1000 × 4 / 1000 = 4 kWh a day. Multiply that by your price per kWh — at 0.15, that is 0.60 a day, 18.00 for a 30-day period and 219.00 for a year.

What price per kWh should I enter?

Divide a recent bill by the kWh on the same bill. That gives the average price you actually paid, which is the right figure for this calculation. Avoid the headline rate from the tariff leaflet: it usually excludes taxes, network charges and the standing charge, so it understates what a marginal extra hour of use costs. If your tariff has different rates at different times of day, enter the rate that applies when the appliance runs.

What exactly is a kilowatt-hour?

It is one kilowatt of power sustained for one hour. 1000 W for 1 hour is 1 kWh; so is 100 W for 10 hours, and 10 W for 100 hours. Electricity meters count kilowatt-hours, and they are what appears on the bill, which is why this page multiplies watts by hours and then divides by 1000.

Why is my real consumption higher than this?

Because the rating plate gives the maximum, not the average. A fridge's compressor runs perhaps a third of the time, a washing machine only heats during part of the cycle, and anything with a remote control keeps drawing a watt or two in standby. If you have a plug-in energy monitor, use the measured figure in the power box instead of the label — the rest of the calculation is unchanged.

Does a device left on all day really cost more than a powerful one used briefly?

Often, yes. 10 W running 24 hours a day is 7.2 kWh a month, while a 2000 W kettle used 15 minutes a day is 15 kWh — but a 100 W device used for one hour a day is only 3 kWh. What matters is the product of power and hours, which is exactly what the reference table above sets out.

Is this how my supplier calculates the bill?

The arithmetic is the same — they multiply kWh by a price. The difference is the price: real tariffs add a standing charge, sometimes tier the price by how much you use, and often charge less overnight. The figure here is what the appliance costs at the price you entered, which is the useful number for deciding whether to run it.

• Measuring electricity — U.S. Energy Information Administration
• Prices and factors affecting prices — U.S. Energy Information Administration