Enter what an appliance draws and how long it runs. It converts that to kilowatt-hours and multiplies by your tariff to show what it actually costs to keep on.
Electricity is billed by energy, not power. Power is the rate an appliance draws, measured in watts. Energy is that rate multiplied by time, measured in kilowatt-hours, and the kilowatt-hour is the unit on your bill.
The arithmetic is watts ÷ 1000 × hours = kWh, then kWh × tariff = cost. A 1500 W heater running four hours a day uses 6 kWh a day. At 0.17 per kWh that is 1.02 a day, about 31 a month, and roughly 372 a year — for one appliance.
The monthly figure here uses 30.44 days, the average calendar month, so twelve months and one year agree rather than being 4% apart.
Most appliances carry a rating plate on the back or underside. If it gives volts and amps rather than watts, multiply them — a 230 V appliance drawing 6.5 A is about 1500 W. If it gives a VA rating, that is apparent power and will be somewhat higher than the real watts for an inductive load, so it overstates the cost slightly.
Rated wattage is the maximum, though, and for anything thermostat-controlled the real figure is much lower. A fridge is rated perhaps 150 W but only runs its compressor a third of the time, so its effective average is closer to 50 W continuous. A heater with a thermostat behaves the same way once the room reaches temperature. For those, either enter the rated watts and a realistic duty-cycle-adjusted number of hours, or use a plug-in energy monitor to measure what it actually consumes.
| Appliance | Typical draw | Notes |
|---|---|---|
| LED bulb | 8–12 W | Replaces a 60 W incandescent |
| Laptop | 30–65 W | Charging draws more than idling |
| Desktop PC | 150–500 W | Gaming machines sit at the top |
| Refrigerator | 100–200 W rated | Cycles — effective average is far lower |
| Ceiling fan | 50–75 W | Far cheaper than air conditioning |
| Window air conditioner | 900–1400 W | Also cycles with the thermostat |
| Space heater | 1000–2000 W | Resistive — runs at full rating |
| Electric kettle | 1500–3000 W | High draw, very short duration |
| Electric water heater | 3000–4500 W | Usually the largest single load in a home |
| Clothes dryer | 2000–5000 W | Heat pump models use far less |
Devices left plugged in but off do draw a little — typically 0.5 to 3 W each under modern efficiency rules. Twenty such devices at 2 W is 40 W continuous, which is about 350 kWh a year. Not nothing, but for most households it is a fraction of what a single water heater or air conditioner costs. Chasing standby draw before addressing heating, cooling and hot water is optimising the wrong end of the bill.
Most bills have a fixed standing or connection charge that you pay regardless of usage, so switching off an appliance never takes your bill to zero. Many have tiered rates, where the price per unit steps up after a threshold — common in India and parts of the US — which means an extra appliance may be charged at the top tier rather than your average rate. Time-of-use tariffs charge more at peak hours, so the same appliance genuinely costs more at 6 pm than at 2 am.
To get the number to enter here, divide the total on your last bill by the units consumed. That gives your effective all-in rate including standing charges and taxes, which is more honest than the headline unit price.
The useful move is comparison. Work out the annual cost of what you have, then the annual cost of the efficient replacement, and see how long the difference takes to pay back the purchase price. A 60 W incandescent running six hours a day costs about 22 a year at 0.17 per kWh; the 10 W LED equivalent costs under 4. The bulb pays for itself in months. A more efficient fridge, on figures like these, might take five years — worth doing when the old one dies, rarely worth doing early.
Take your most recent bill, divide the total amount by the number of units (kWh) consumed. That gives your effective rate including standing charges, taxes and any tiering, which is the honest number to use here. The headline unit rate quoted in a tariff comparison usually excludes fixed charges and will understate what an extra unit really costs you.
Three usual reasons. You are pricing one appliance, and the bill covers everything. Rated wattage is peak draw, but some appliances start at a much higher surge. And most bills carry a fixed standing charge plus taxes on top of the per-unit price. If you divide your bill total by units consumed and use that as the rate, the gap narrows considerably.
A small amount, generally 0.5 to 3 watts for modern equipment under standby efficiency regulations. Twenty devices at 2 W is roughly 350 kWh across a year. Worth switching off a power strip when convenient, but it is a rounding error compared with heating, cooling and hot water, which is where the money in a typical bill actually goes.
Estimate its duty cycle rather than its rated wattage. A fridge rated 150 W that runs its compressor about a third of the time behaves like 50 W continuous, so enter 150 W and 8 hours rather than 150 W and 24. A plug-in energy monitor removes the guesswork entirely and usually costs less than a month of the appliance it is measuring.
Only on a time-of-use tariff, where off-peak units are genuinely priced lower — common in the UK, parts of the US, and increasingly in India for larger connections. On a flat tariff the time makes no difference to your cost, though shifting load off peak still helps the grid. Check whether your meter and tariff actually support time-of-use before rearranging your laundry around it.
Related: Ohm’s Law Calculator · Voltage Drop Calculator · Fuel Cost Calculator · Budget Calculator Sizing the equipment rather than costing it? The BTU Calculator estimates the heating and cooling capacity a room needs.