Electricity Cost Calculator

How an electricity bill is actually built

Every appliance costs money the same way: power multiplied by time, priced by the kilowatt-hour. Take the wattage from the rating label, multiply by the hours it runs, divide by 1000, and you have kilowatt-hours. Multiply that by your tariff and you have money. A 1500 W space heater running for one hour uses 1.5 kWh; at the US average of roughly 17 cents that is about 25 cents an hour. Everything else this calculator does is scaling that number to a month (30.44 days, the average) and a year (365.25 days), adjusted for how many days a week the thing actually runs.

Worked example: the space heater that doubles a winter bill

A 1500 W plug-in heater in a cold bedroom, eight hours a night, every night. Daily energy is 1500 x 8 / 1000 = 12 kWh. At 0.17 per kWh that is 2.04 a day, about 62 a month and 745 over a full year of that habit. In the UK at 0.27 per kWh the same heater costs 3.24 a day, close to 99 a month. This is the single most common cause of winter bill shock. Resistance heating turns one unit of electricity into exactly one unit of heat, while a heat pump moves three or four — which is why running one portable heater in an unused room is often more expensive than heating the whole house better.

Worked example: the fridge and the duty cycle

A modern fridge-freezer has a compressor rated at perhaps 150 W, but it never runs continuously — the thermostat cuts it in and out. Over 24 hours the compressor is typically working around a third of the time, so instead of 150 x 24 / 1000 = 3.6 kWh a day, the real figure lands closer to 1.1 to 1.3 kWh, which lines up with the 400 to 500 kWh a year printed on most energy labels. For anything thermostatic — fridge, freezer, air conditioner, heat pump, electric radiator with a stat — enter equivalent full-power hours rather than wall-clock hours. Eight hours of window AC at 1000 W with the compressor cycling half the time behaves like four hours of full-power use.

Worked example: charging an electric car

A Level 2 charger drawing 7200 W for four hours adds 28.8 kWh, roughly 100 miles of range in a typical EV. At 0.17 per kWh that is 4.90, or about 5 cents a mile — close to a third of the fuel cost of a 30 mpg petrol car at 4.50 a gallon. Charging is also the biggest load most households can deliberately shift into an off-peak window, which is where a time-of-use tariff finally pays for the hassle of watching the clock.

Reading the preset list

The presets are typical mid-range figures, not promises. Notice the shape of the list: the things people worry about (LED bulbs at 10 W, laptops at 60 W) sit at the bottom, and the things nobody thinks about (dryer at 3000 W, dishwasher at 1800 W, EV charger at 7200 W) sit at the top. Anything that makes heat or moves heat is a kilowatt-class load. Anything that only pushes electrons around a circuit board is a tens-of-watts load. That single distinction predicts most of a bill.

Where the savings actually are, ranked

In rough order of impact: space heating and cooling first, because they draw kilowatts for hours at a time; then water heating and the tumble dryer; then cooking; then everything else. Three dryer loads a week at 3 kWh each is about 39 kWh a month — more than many fridges use. By contrast, swapping ten 60 W incandescent bulbs for 10 W LEDs saves 500 W while they are lit, or roughly 60 kWh a month at four hours a day: worthwhile, but it took ten devices to get there. Unplugging phone chargers achieves nothing measurable. If you want a shorter bill, work top-down from the heat.

Limits of this estimate

The calculator assumes one flat rate, and many bills are not flat. Tiered rates raise your marginal price once you pass a monthly threshold; time-of-use plans can charge two or three times more at peak; and nearly every supplier adds a fixed daily standing charge you pay regardless of use. It also assumes nameplate wattage equals actual draw, which is a ceiling rather than an average for anything with a motor, a compressor or a variable heat setting. For a device you genuinely care about, leave a plug-in energy meter on it for a week, read the measured kilowatt-hours, then come back here and price them against your tariff.