kWh Calculator
Enter a power, how long it runs and your price per kWh to get the energy used in kWh, Wh and MJ.
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Formula
kWh = power (kW) × time (h)Wh = kWh × 1000MJ = kWh × 3.6(exact)cost = kWh × price per kWh
Units: 1 kW = 1000 W, 1 MW = 1,000,000 W; 1 minute = 1/60 h, 1 second = 1/3600 h, 1 day = 24 h.
Worked example
2.5 kW for 90 minutes at 0.20 per kWh:
- Time: 90 ÷ 60 = 1.5 h
- Energy: 2.5 × 1.5 = 3.75 kWh = 3750 Wh = 13.5 MJ
- Cost: 3.75 × 0.20 = 0.75
Frequently asked questions
How do I calculate kWh?
Energy in kWh is power in kilowatts times time in hours. A 2.5 kW load running 90 minutes (1.5 hours) uses 2.5 × 1.5 = 3.75 kWh.
How many MJ is a kWh?
Exactly 3.6 MJ, because 1 kW × 3600 s = 3600 kJ. So 3.75 kWh is 3.75 × 3.6 = 13.5 MJ, and 1 kWh is 1000 Wh.
What is the difference between kW and kWh?
kW is a rate of using energy (power); kWh is the amount of energy used over time. A 1 kW heater running for one hour uses 1 kWh.
How do I find the cost of that energy?
Multiply kWh by your price per kWh. 3.75 kWh at 0.20 costs 0.75. This calculator is for learning and design checks only. It is not a safety certification: wiring, breaker and cable sizing must follow your local electrical code and be checked by a qualified electrician.