What a kilowatt-hour is
A kilowatt-hour, written kWh, is the standard billing unit for electricity and, in many places, for gas. It is an amount of energy: the energy used by a device drawing one kilowatt (1,000 watts) of power for one hour. Power is how fast energy is used at a moment, and energy is the total over time. The difference between them is explained further in the article on energy units, but the short version is that your meter counts energy, and that is what you are billed for.
Some other ways to reach the same amount: a 100-watt device left on for ten hours uses one kWh, and a 2,000-watt heater running for 30 minutes also uses one kWh. Because the unit combines power and time, lowering either one lowers the total.
How the meter turns use into a number
An electricity meter adds up the energy flowing through it, and the register increases as you use more. Your bill takes the difference between two readings, the earlier and the later, and that difference is your usage for the period. If last month’s reading was 41,200 and this month’s is 41,830, the usage is 630 kWh. The accuracy of that figure depends on the readings being real, which is why estimated vs actual meter readings matter.
Gas works similarly but often adds a step. Many gas meters measure volume, such as cubic feet or cubic meters. The supplier then converts volume to energy using a calorific value, which reflects how much energy the gas contains, and sometimes a correction for temperature and pressure. The conversion is generally shown on the bill. Some regions bill gas in therms or similar units instead of kWh.
From kWh to a charge
The core calculation is simple:
energy charge = kWh used x price per kWh
Using the 630 kWh example at a rate of 18 cents per kWh gives 630 x 0.18 = $113.40. That is the energy part only. Other lines are then added, such as a fixed charge, delivery or network costs, taxes and any credits.
Some bills use more than one rate. Examples include tiered or block pricing, where the first block of use costs one rate and the next block costs another, and time-based pricing, where the rate depends on when energy is used. If your usage is split into bands on the bill, the same formula applies to each band, and the results are added together. For example, if 400 kWh is billed at 15 cents and the remaining 230 kWh at 20 cents, the energy charge is 400 x 0.15 = $60.00 plus 230 x 0.20 = $46.00, for $106.00.
Delivery and supply may be separate
In some markets the bill separates the cost of the energy itself from the cost of delivering it. Both may be charged per kWh, so you see two rates for the same usage. The delivery rate pays for the wires, pipes and related services, and the supply rate covers the energy purchased. Together, they make up the price you pay for each unit. The next layer of items, such as taxes and network charges, is covered in a separate article.
What things cost in kWh
Seeing appliances in kWh makes the bill less abstract. The figures below are illustrative, and real devices vary:
| Example | Power | Time | Energy |
|---|---|---|---|
| LED bulb | 10 W | 5 hours | 0.05 kWh |
| Laptop charging and use | 50 W | 8 hours | 0.4 kWh |
| Space heater | 1,500 W | 3 hours | 4.5 kWh |
| Clothes dryer cycle | roughly 3,000 W | 1 hour | roughly 3 kWh |
The heater row works out as 1.5 kW x 3 hours = 4.5 kWh. At 18 cents per kWh, that is about 81 cents. Heating and cooling, water heating and cooking are typically the big movers in a household, while small gadgets add up more slowly. Standby power can also add a steady background amount that you may not notice. Check the nameplate or manual of your own devices for real wattages.
Checking your own bill
A few quick checks:
- Subtract the previous reading from the current reading and compare with the usage on the bill.
- Multiply the usage by the stated rate, and compare with the energy charge line.
- Confirm the units (kWh, cubic meters, therms) and any conversion factor for gas.
- Look at whether the usage fits the period length. A 60-day bill should show roughly twice the usage of a 30-day bill, other things equal.
- Compare with the same season last year, since weather affects heating and cooling.
If a figure seems wrong and the readings are accurate, ask the supplier for a breakdown. If a meter fault is possible, ask the utility how to request a test. Do not open or modify a meter yourself.
Why the kWh figure matters
The kWh number is the part of your bill most connected to what you actually do at home. Changing it is the route to a lower energy charge no matter what the rate is, and that is where energy efficiency measures come in: better insulation, efficient appliances, and changed habits all reduce kWh. The price per kWh is mostly outside your control in the short term, though tariff choice can influence it.
This article is general information, and the exact units, rates and layouts on your bill depend on your supplier and location.