Kilowatt hours: the unit behind your power bill
A kilowatt hour (kWh) is the amount of energy used by a 1,000-watt device running for one hour. Electricity bills, solar production estimates and battery sizes are all given in kWh, while the kilowatt (kW) measures the rate of power at a moment in time.
What is the difference between kW and kWh?
Think of a car. Speed in kilometres per hour is the rate, and distance in kilometres is the total. A kilowatt is the rate at which a device uses or produces energy, and a kilowatt hour is the total over time. A 2 kW heater running for 3 hours uses 6 kWh. A 5 kW solar array that produces at full power for 1 hour delivers 5 kWh, though in practice it only reaches full power briefly around midday.
The mix-up is common, and it matters when you read quotes. A battery rated in kWh holds energy, while an inverter rated in kW sets how much power it can deliver at once. See solar battery storage in Canada for how those two numbers interact.
How do you work out what an appliance uses?
Divide the watts by 1,000 to get kilowatts, then multiply by the hours. A few examples with assumed ratings:
- A 100-watt bulb for 10 hours: 0.1 kW times 10 h equals 1 kWh.
- A 1,500-watt space heater for 4 hours: 1.5 kW times 4 h equals 6 kWh.
- A 10 W phone charger over 24 hours: 0.01 kW times 24 h equals 0.24 kWh.
The rating plate on an appliance shows its power, and the real use depends on how often it cycles on and off. Your meter or utility app is the better guide for the whole house.
How does the kWh turn into the cost on your bill?
The energy charge is the kWh you used multiplied by your rate. As an illustration only, if a rate were 15 cents per kWh, a month with 800 kWh would carry 800 times 0.15, or 120 dollars of energy charges. Your real rate depends on your province, your retailer and the time of year, and bills usually add delivery, fixed and tax charges, so look at your own bill for the actual lines. How electricity costs compare with gas heating is covered in natural gas versus electric heating, and home heating costs in Alberta shows one provincial case.
Why do solar estimates use kWh?
Panels are rated in watts, but the benefit is the energy they produce over a year, in kWh. The relationship between system size, sunshine and output is explained in peak sun hours in Canada and how to read a solar production estimate. To size a system, compare your annual use in kWh from twelve months of bills with the expected output, as shown in how many solar panels to power a house. Quotes are often compared by price per watt, discussed in solar price per watt.
How do you convert kWh to gigajoules and BTU?
Natural gas in much of Canada is billed in gigajoules, so a common conversion is needed when you compare fuels. One kWh equals 3.6 megajoules, so one gigajoule is about 278 kWh. One kWh is also about 3,412 BTU, the unit often used for furnace and heat pump ratings, such as in what is HSPF.
As a worked example, a house that burns 100 GJ of gas in a winter used about 27,800 kWh worth of fuel energy. A furnace wastes part of it, so the heat delivered is lower. The Drake Landing project reported its solar heat in similar energy terms, and the performance results page shows how it was presented.
Quick answers
What is a kilowatt hour?
It is the energy used by a 1,000-watt device in one hour. Electricity is sold and measured in kWh.
What is the difference between a kW and a kWh?
A kW measures power at a moment, like speed, while a kWh measures energy over time, like distance. A 2 kW device used for 3 hours consumes 6 kWh.
How many kWh are in a gigajoule?
About 278. One kWh equals 3.6 megajoules, so 1,000 megajoules is roughly 278 kWh.
How do I find my kWh use?
Check your electricity bill or your utility's online account for monthly and annual kWh. Twelve months of data is best for sizing solar or a heat pump.