Percentages & Everyday Math
Electricity Cost Calculator
Work out what an appliance costs to run per day, month, and year from its power rating and your tariff.
Result
Enter your numbers and press Calculate to see the result.
Calculating…
The electricity cost formula
Every electricity bill comes from one calculation. The appliance's power rating in watts is divided by 1,000 to get kilowatts, multiplied by the hours it runs, and multiplied by your tariff:
cost = watts ÷ 1000 × hours × price per kWh
A 100 W lamp running 5 hours a day uses 0.5 kWh daily. At $0.17 per kWh that is 8.5 cents a day, $2.55 a month, and $31 a year — from one bulb.
kW and kWh are not the same thing
This is the distinction the whole subject turns on. Kilowatts measure rate — how fast something draws power right now. Kilowatt-hours measure quantity — how much energy it consumed over time.
Speed and distance is the exact analogy: kW is the speedometer, kWh is the odometer. A 2 kW heater running for 30 minutes uses 1 kWh; the same heater running 3 hours uses 6 kWh. The kW never changed. You are billed for kWh, which is why runtime matters more than the rating on the label.
On a bill, a "unit" is one kilowatt-hour — the two words mean the same thing.
What common appliances cost to run
Typical ratings, at $0.17 per kWh. Check your own appliance's plate — ratings vary widely by model and age.
| Appliance | Power | Typical use | Per year |
|---|---|---|---|
| LED TV (55") | 90 W | 5 h a day | ≈ $28 |
| Ceiling or pedestal fan | 60 W | 8 h a day | ≈ $30 |
| Fridge-freezer | 150 W average | Always on, cycling | ≈ $110 |
| Electric heater | 2,000 W | 4 h a day, 4 months | ≈ $163 |
| Dishwasher | 1,400 W | 1 h, 5 times a week | ≈ $62 |
| Desktop PC | 200 W | 8 h a day | ≈ $99 |
The pattern is consistent: anything that makes heat dominates the bill. A heater costs more in four hours than a television does in a day, because heating is measured in kilowatts while electronics are measured in watts.
What a typical home uses
A US household averages roughly 10,500 kWh a year, around 875 kWh a month. UK homes run far lower at about 2,700 kWh a year, largely because heating and hot water are usually gas rather than electric. Rates differ just as much — US state averages range from about 11 cents per kWh to over 40 cents in Hawaii, with California and New York well above the national middle.
Because both the usage and the rate vary by a factor of three or more, a national average bill tells you very little. Your own tariff is on your bill, and it is the only number worth putting into the calculator.
Converting kWh to other energy units
| Unit | 1 kWh equals | Where it is used |
|---|---|---|
| Megajoules (MJ) | 3.6 MJ | Gas bills in Australia and scientific work |
| Joules (J) | 3,600,000 J | The SI base unit of energy |
| Megawatt-hours (MWh) | 0.001 MWh | Industrial and grid-scale metering |
| Gigawatt-hours (GWh) | 0.000001 GWh | Power station output |
| Amp-hours (Ah) | 83.3 Ah at 12 V | Battery capacity — divide watt-hours by voltage |
Amp-hours need a voltage. A kWh is energy; an amp-hour is charge. They only convert once you know the system voltage, which is why a 100 Ah battery is 1.2 kWh at 12 V but 4.8 kWh at 48 V.
Gas in kWh and m³ works the other way: gas meters read cubic metres, and suppliers convert to kWh using a calorific value, roughly 1 m³ ≈ 10.5 kWh for UK natural gas.
kVA, kVAh, and three-phase supplies
On commercial supplies you will meet kVA (apparent power) alongside kW (real power). The ratio between them is the power factor:
kW = kVA × power factor
Domestic customers pay for kWh only. Commercial tariffs often bill kVAh as well, penalising a poor power factor, which is why industrial sites install correction equipment. For sizing a supply, the formulas are:
| Supply | Formula |
|---|---|
| Single phase | kW = V × A × power factor ÷ 1000 |
| Three phase | kW = √3 × V × A × power factor ÷ 1000 |
The √3 (about 1.732) is what makes three-phase supplies carry substantially more power over the same conductors.
Comparing running costs against a car journey? The fuel cost calculator works out trip costs the same way. To see what a rate rise means in percentage terms, use the percentage change calculator.
Electricity Cost Calculator — frequently asked questions
What is the difference between kW and kWh?
Kilowatts measure how fast electricity is being drawn; kilowatt-hours measure how much was used over time. kW is the speedometer, kWh is the odometer — and your bill charges for kWh.
Is a unit on my bill the same as a kWh?
Yes. One unit is one kilowatt-hour. Suppliers use the words interchangeably on statements.
How much electricity does a TV use?
A 55-inch LED TV draws around 90 W, so five hours a day is about 164 kWh a year — roughly $28 at $0.17 per kWh. Older plasma sets can use three times as much.
How many kWh does a house use per year?
A US home averages around 10,500 kWh a year. UK homes average about 2,700 kWh, mostly because heating and hot water there are usually gas rather than electric.
How do I convert kWh to amp-hours?
You need the voltage. Divide watt-hours by volts: 1 kWh is 1,000 Wh, which is 83.3 Ah at 12 V but only 20.8 Ah at 48 V.
How do I calculate kW on a three-phase supply?
kW = 1.732 x volts x amps x power factor / 1000. The square root of 3 is what lets three-phase carry more power through the same conductors than single phase.