What is the difference between kW and kWh?
What is the difference between kW and kWh?
kW measures power; kWh measures energy. A kilowatt describes how quickly energy is used or produced. A kilowatt-hour describes the amount of energy used, generated or stored. “What is its power in kW?” and “How many kWh did it use today?” therefore ask different questions.
At constant power: energy (kWh) = power (kW) × time (hours). A device drawing 2 kW for half an hour uses 1 kWh. The “h” stands for hours; kWh does not mean kW per hour.
Three different powers, the same energy
Each example below uses 1 kWh. All three plots use the same scale: height represents power, width represents time, and the shaded area represents energy.
Higher power can use energy over a shorter period. Comparing kW ratings alone does not establish total consumption; operating time also matters. When comparing devices performing the same task, efficiency and control behaviour affect the result too.
Energy use and cost calculator
Use a constant power value or the average electrical input power over the entered duration. The same usage is assumed each day. Use a decimal point or comma without thousands separators.
Try an example:
2 kW × 3 h/day × 30 days = 180 kWh
Enter an optional unit price to estimate the cost.
Cost equals energy use × your entered unit price. Tariff tiers, time-of-use rates, taxes and fixed charges are not added automatically.
How do W, kW, Wh and kWh differ?
| Unit | Quantity | Conversion |
|---|---|---|
| W / kW | Power | 1 kW = 1000 W |
| Wh / kWh | Energy | 1 kWh = 1000 Wh |
| kWh / hours | Average power over that period | 6 kWh ÷ 3 hours = 2 kW |
Divide minutes by 60 to obtain hours: 15 minutes = 0.25 hours; 30 minutes = 0.5 hours; 90 minutes = 1.5 hours. Entering 0.30 hours for 30 minutes gives the wrong result.
What if power changes during operation?
Suppose a machine draws 2 kW for the first 30 minutes and 0.2 kW for the next 90 minutes. Calculate the energy of each interval and add the results:
| Operating interval | Calculation | Energy |
|---|---|---|
| First 30 minutes | 2 kW × 0.5 hours | 1 kWh |
| Next 90 minutes | 0.2 kW × 1.5 hours | 0.3 kWh |
| Total: 2 hours | 1 + 0.3 | 1.3 kWh |
The average power over this operation is 1.3 ÷ 2 = 0.65 kW. Enter 0.65 kW, 2 hours and 1 day in the calculator to obtain the same result. For thermostatically controlled heaters, refrigerators and variable-load motors, multiplying the rated power by the entire time switched on may misrepresent actual energy use.
Time matters in monthly energy estimates: a continuous 10 W load uses 0.01 × 24 × 30 = 7.2 kWh over 30 days. Even a small standby load can therefore accumulate energy use over a long period.
What do the meter and electricity bill show?
An electricity meter's active-energy register records accumulated energy in kWh. If two readings of the same register are 12,540 kWh and 12,720 kWh, the difference is 180 kWh. This simple example has no additional meter multiplier; transformer-operated metering must account for any applicable multiplier.
To estimate the energy charge, multiply consumption by the applicable unit price. An electricity bill may include other items depending on the tariff. The calculator estimates the amount covered by your entered price; it does not automatically recreate the complete bill.
Actual consumption can be recorded with an energy meter suitable for the load's current and connection type. Energy-monitoring smart plugs can track kWh for suitable plug-in loads, while building automation can help relate consumption to time and areas of use.
Why does motor nameplate power differ from consumption?
An electric motor's kW or HP rating usually describes shaft output power. A motor delivering 7.5 kW at the shaft with 90% efficiency at that point requires approximately 7.5 ÷ 0.90 = 8.33 kW of electrical input. If that condition lasts eight hours, energy at the motor input is approximately 66.67 kWh.
This is an example operating point; consumption changes at a different load. Variable frequency drive losses also affect the total drawn from the supply. Do not apply efficiency a second time when using measured electrical input power. Power factor and current calculations are separate from converting power and time into energy.
kW and kWh in batteries and solar systems
A battery's kWh rating describes energy capacity, while the system's kW rating describes available power. In an ideal loss-free calculation, 1 kWh of usable energy supplies a 0.5 kW load for two hours. Real UPS runtime depends on usable capacity, conversion losses and output power limits; Ah and Wh are different units too.
Solar panel capacity is stated in kW, or kWp for peak power under specified test conditions; energy generated during the day is stated in kWh. A 5 kWp system does not guarantee 5 kWh every hour. Irradiance, temperature, shading and panel angle affect daily production.