Electricity Cost Calculator

What this tool is used for

• Working out what leaving an appliance on actually costs per month.
• Comparing two appliances on running cost rather than purchase price.
• Checking whether replacing something old would pay for itself.
• Estimating the cost of a always-on device such as a router or a server.
• Working out a bill component from a meter reading.

Frequently Asked Questions

How is the cost calculated?

Power in kilowatts × hours × the tariff rate. A 2 kW heater for three hours is 6 kWh, which at 30p is £1.80. The only conversion needed is watts to kilowatts, and forgetting it is a factor-of-a-thousand error.

What is standby power actually costing?

Less than it used to and more than nothing. Regulations pushed most devices below 0.5 W, but a household with fifty always-on devices still burns 20–50 W continuously — around 300 kWh a year, which is a real line on the bill for no benefit.

Does a device draw its rated wattage constantly?

Usually not. A rating is the maximum, and thermostatically controlled devices cycle — a fridge rated 150 W runs perhaps a third of the time. Using the nameplate figure as a continuous draw overstates the cost by a factor of two or three.

What are time-of-use tariffs?

Rates that vary by hour, sometimes dramatically. Where they apply, shifting a dishwasher or a car charge to an off-peak window changes the cost of the same energy substantially — which is why the tariff structure matters as much as the consumption.

How do I find a device's real consumption?

A plug-in energy monitor, which measures rather than estimates. It is the only way to capture duty cycle and standby together, and the results routinely contradict the nameplate — usually downward, occasionally alarmingly upward for older appliances.

Common errors and gotchas

• Using the unit rate without the standing charge, which on low usage dominates the bill.
• Confusing watts with kilowatt-hours, where one is a rate and the other a quantity.
• Using a nameplate rating as continuous draw, when most appliances cycle far below it.
• Ignoring a tariff's time-of-use bands, which can change the answer substantially.
• Forgetting VAT or sales tax, which the quoted rate may or may not include.