kWh Calculator
Watts and hours into kilowatt-hours, and into BTU.
Work out kWh. Watts and hours into kilowatt-hours, and into BTU. Shows the annual figure, which is the one that surprises people.
Kilowatt-hours per day
4.500
135.0 kWh over 30 days
Kilowatts are the rate; kilowatt-hours are the amount. A 1,500 W load draws 1.500 kW whenever it is on and accumulates 4.500 kWh over 3.0 hours. The BTU figure is exact rather than approximate — a kilowatt-hour is 3.6 megajoules and a BTU is defined as 1055.05585262 joules.
How the kWh Calculator works
A kilowatt-hour is a unit of energy: one kilowatt sustained for one hour. Power is the rate, energy is the amount, and confusing the two is why a 2,000 W heater and a 2,000 W kettle produce wildly different bills. The conversion to BTU is exact — a kilowatt-hour is 3,412.14 BTU by definition — which makes it the honest way to compare an electric appliance against a gas one.
Also known as: kilowatt hour calculator · watts to kWh · energy usage calculator · kWh usage calculator
Power against energy
Kilowatts are a rate; kilowatt-hours are an amount. A 2 kW heater draws 2 kW the whole time it is on, and accumulates 2 kWh for each hour it stays on.
That distinction is why a 2,000 W kettle and a 2,000 W heater produce completely different bills. Identical power, wildly different duration.
It is also why amps alone cannot give you energy. Current is a rate too, and it needs voltage to become power and hours to become energy.
In practice
A 1,500 W load run three hours a day is 4.5 kWh daily and 135 kWh over a month.
In heat terms that is 15,354 BTU a day, using the exact conversion of 3,412.14 BTU per kilowatt-hour.
That conversion is definitional, not measured: a kilowatt-hour is exactly 3.6 megajoules, and a BTU is defined as exactly 1055.05585262 joules. Dividing one by the other gives the figure with no rounding anywhere in it.
Why the BTU conversion matters
Because it is the only honest way to compare an electric appliance against a gas one. Gas appliances are rated in BTU, electric in watts, and until both are in the same energy unit the comparison is meaningless.
Efficiency then has to be layered on top. A gas appliance delivers less heat than its input rating; a heat pump delivers considerably more heat than its electrical input, because it moves heat rather than generating it.
Which is why a heat pump can appear to be several hundred percent efficient without violating anything. The coefficient of performance is a ratio of heat moved to work done, not a claim about creating energy.
Acting on it
Convert everything to kilowatt-hours before comparing anything, then apply efficiencies separately.
Read your meter a week apart to get a real household figure rather than relying on an estimate or an average.
When comparing fuels, compare delivered heat per unit of money — not energy per unit of money.
Frequently asked questions
What is a kilowatt-hour?
The energy used by a one-kilowatt load running for one hour. It is what your meter counts and what you are billed for — a unit of energy, not of power.
What is the difference between kW and kWh?
Kilowatts are the rate of use at any instant; kilowatt-hours are the total accumulated. A 2 kW heater draws 2 kW whenever it is on, and uses 2 kWh for every hour it stays on.
How many BTU in a kWh?
3,412.14, exactly, since a kilowatt-hour is 3.6 megajoules and a BTU is defined as 1055.05585262 joules. It is a definitional conversion with no rounding in it.
How do I convert amps to kWh?
Amps alone are not enough — multiply by volts for watts first, then by hours and divide by a thousand. Amps measure current, not energy.
How many kWh does a house use?
It varies enormously with climate, size, heating type and occupancy, so any average is close to meaningless for a specific house. Your own bill over twelve months is the only figure worth using.
Put this calculator on your own site
Free to use, on any site, commercial or not. Paste this where you want it to appear. It is a plain iframe, so it works in WordPress, Squarespace, Wix, Webflow, Ghost and anything else that accepts HTML.
<iframe src="https://www.thecalclibrary.com/embed/kwh-calculator" width="100%" height="640" style="border:1px solid #e2e8f0;border-radius:12px" loading="lazy" title="kWh Calculator"></iframe>The only condition is that the credit line stays visible. It sits inside the frame, so you do not have to do anything to keep it.
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