Electricity Cost Calculator

About the Electricity Cost Calculator

Electricity bills arrive as a single total, which makes it hard to know where the money goes. Is the space heater in the spare room costing a fortune? Is it worth switching off the games console at the wall? How much does it cost to run a dehumidifier all winter? The answer for any single device comes from just three numbers: how much power it draws, how long it runs, and what you pay for each unit of electricity.

This electricity cost calculator works out the running cost of any electrical device. Enter its power in watts, the number of devices, the hours of use a day and your electricity price per kWh, and it gives the cost per hour, per day, per month and per year, along with the energy used in kilowatt-hours.

How to Use the Electricity Cost Calculator

Enter the device's power in watts. It is on the rating plate or label, or in the manual.

Enter the number of devices — useful for light bulbs or several heaters.

Enter the hours of use a day. For devices used for minutes, enter a decimal: 15 minutes is 0.25 hours.

Enter your electricity price per kWh, from your bill or tariff.

How the Cost Is Worked Out

  power (kW)     =  watts × number of devices ÷ 1,000
  energy a day   =  kW × hours a day               (kWh)
  cost a day     =  kWh × price per kWh
  cost a month   =  cost a day × 30
  cost a year    =  cost a day × 365

Step-by-Step Example

A 1,500 watt electric heater used 4 hours a day, with electricity at 0.16 per kWh.

  Power:        1,500 ÷ 1,000           =  1.5 kW
  Energy:       1.5 × 4                 =  6 kWh a day
  A day:        6 × 0.16                =  0.96
  An hour:      1.5 × 0.16              =  0.24
  A month:      0.96 × 30               =  28.80
  A year:       0.96 × 365              =  350.40  (2,190 kWh)

Watts, Kilowatts and Kilowatt-Hours

Watts measure power — how fast a device uses energy at any moment. A kettle might draw 2,000 to 3,000 watts, a laptop about 50, an LED bulb under 10.

A kilowatt is 1,000 watts.

A kilowatt-hour (kWh) measures energy — power multiplied by time. A 1,000 watt device running for an hour uses one kWh; so does a 100 watt device running for ten hours. Electricity is sold by the kWh, which some bills call a "unit".

Finding the Wattage

The rating plate is usually on the back, bottom or inside the door of an appliance, or on the power adapter. It may show watts directly, or volts and amps: multiply them to get watts, so a 230 volt device drawing 2 amps uses about 460 watts. For devices with a range, such as "1,800–2,200 W", use the higher figure for a worst-case estimate. Chargers and adapters often show their maximum output, which the device only draws while charging hard.

Why Real Use Is Often Lower

The wattage on the label is the most a device can draw. Many appliances cycle on and off. A heater with a thermostat switches off once the room is warm; a fridge's compressor runs only part of the time; an oven heats up at full power, then maintains its temperature with short bursts. For these devices, the true cost can be half the calculated figure or less. A plug-in energy meter, which sits between the socket and the plug, measures the real energy used over a day or a week.

Typical Power Ratings

Cutting Running Costs

Use the most efficient device for the job: heating one room with a small heater for a few hours can cost less than heating the whole house, but a heat pump is far cheaper than a plain resistance heater for heating a whole home. Switch off at the wall devices that sit on standby for long periods. Replace old bulbs with LEDs. And check your tariff — a cheaper rate per kWh, or a time-of-use tariff that rewards running devices at night, lowers the cost of everything at once.

Peak and Off-Peak Tariffs

Some tariffs charge different prices at different times of day, with cheaper electricity overnight and more expensive electricity in the early evening peak. On such a tariff, the same device can cost half as much or twice as much to run depending on when it is used. Enter the price for the time you run the device, or work out a weighted average: a dishwasher run entirely overnight on a 0.08 rate costs half as much as at a 0.16 daytime rate. Washing machines, dishwashers, tumble dryers and electric car chargers are the easiest to shift, because timers let them run while you sleep.

Shared Houses and Offices

In a shared house, the calculator can settle arguments about who uses what: a housemate's electric heater running every evening can cost far more than everyone else's laptops and lamps combined.

Understanding Your Result

Monthly cost is the cost over a 30 day month.

Per hour gives the cost and energy for each hour of use.

Per day gives the daily cost and energy.

Per year gives the yearly cost and energy over 365 days.

Worth knowing explains why thermostat-controlled devices often cost less.

When Should You Use This Calculator?

Checking a new appliance's running cost before buying.

Comparing two devices, such as heaters or lamps.

Finding what drives a high electricity bill.

Working out the cost of running equipment for a business.

Sharing electricity costs fairly in a shared house.

Common Mistakes

Confusing watts with kilowatt-hours.

Entering minutes as hours.

Forgetting to divide by 1,000.

Assuming thermostat devices run at full power all day.

Using an old electricity price instead of your current tariff.