How to calculate the cost of running an appliance

Three steps: find the watts, multiply by the hours to get kilowatt-hours, and multiply that by your price per kWh. Everything else — cycling appliances, amps-only labels, monthly estimates — is a variation on those three steps.

The formula

Worked example 1: a desk fan

A fan rated 60 W, run 5 hours a day: 60 × 5 ÷ 1,000 = 0.3 kWh a day. Over 30 days that is 9.00 kWh, and at $0.17/kWh it costs $1.53 a month — about $18.36 a year if the habit runs all year.

Worked example 2: a heater for the evening

A 1,500 W space heater run 2 hours an evening: 1,500 × 2 ÷ 1,000 = 3 kWh a day, 90.0 kWh a month, $15.30 a month. Same formula; the heater's wattage is 25× the fan's, and it shows.

When the label gives amps, not watts

Multiply amps by volts: watts ≈ amps × volts. North American outlets are 120 V (240 V for large appliances like dryers and ovens on dedicated circuits); most of Europe is 230 V. A nameplate reading "6 A, 120 V" means a maximum draw of about 720 W. Note that this is a maximum — see the next section before using it as your estimate.

Cycling appliances: the one real complication

Fridges, air conditioners, water heaters and ovens do not run at full draw continuously — a thermostat switches them on and off. For these, the honest figure is an average wattage across a full day, multiplied by 24 hours (or by the hours the appliance is active). That is the convention this site uses for its fridge and AC figures. If the appliance has an energy label stating kWh per year, skip the watts entirely: divide by 365 for a daily figure, or by 12 for a monthly one, and multiply by your rate.

From a day to a month (and why estimates drift)

Multiply the daily figure by the days the appliance actually runs in a month — 30 is the convention here, but a washer might run 12 days and a heater, in a cold snap, all 31. Seasonal appliances make yearly figures slippery: an air conditioner run hard for four months and never for eight does not cost "monthly × 12" of anything meaningful. Use the monthly figure for the months the appliance is actually in service.

Finally, a bill is more than energy: fixed service charges, delivery tiers and taxes sit on top, which is why a calculator total rarely equals a real invoice even with perfect inputs. The calculator's job is ranking and planning — which loads matter, what a change saves — and for that, the three steps above are all you need. Try them on the calculator.

Questions

What is a kilowatt-hour, in plain terms?

The amount of electricity used by a 1,000 W appliance in one hour — or any combination that multiplies to the same: 500 W for two hours, 100 W for ten. It is the unit your bill counts.

How do I calculate cost per hour directly?

Cost per hour = watts ÷ 1,000 × rate. A 2,000 W appliance at $0.17/kWh costs $0.34 an hour. Per minute, divide by 60 again — handy for kettles and microwaves.

My smart meter shows kW, not kWh. What's the difference?

kW is the rate right now (the watts ÷ 1,000 at this instant); kWh is the accumulated amount. Watch the kW reading while switching things on and off and you can read each appliance's draw from the jumps — a free version of a plug-in meter.

Can I trust the wattage printed on the box?

As a maximum or a typical figure, usually. As your exact draw, no: settings, load and age all move it. For a number you plan money on, measure with a plug-in energy monitor over a typical day — especially for cycling appliances.