Energy Calculator
Calculate energy step by step: E (kWh) = P (W) × t (h) ÷ 1,000.
In short
Formula: E (kWh) = P (W) × t (h) ÷ 1,000. 1 kWh = 3.6 MJ.
Energy
135 kWh
- Per day
- 4.5 kWh
- Watt-hours
- 135,000 Wh
- Megajoules
- 486 MJ
What this calculator does
Calculate energy step by step: E (kWh) = P (W) × t (h) ÷ 1,000. Worked example, questions and limitations included.
Use it to turn Power, Running time per day, Number of days into a checked result you can compare, copy, or rerun with different assumptions.
The page shows the formula, a numeric worked example, and the assumptions that affect this electrical calculation.
Inputs and what they mean
- Power
- — number value.
- Running time per day
- — number value.
- Number of days
- — number value.
How to use it
- Enter each value in the unit shown next to the box (use the unit converter first if your numbers are in other units).
- Check the breakdown to see every intermediate step.
- Read the limitations before relying on the result.
Formula
Total time = hours per day × days.
Hours per day are limited to 24; negative values are rejected.
Uses the device's average draw — for cycling devices use an average, not the nameplate peak.
E (kWh) = P (W) × t (h) ÷ 1,000. 1 kWh = 3.6 MJ.
Inputs used: Power, Running time per day, Number of days.
Worked example
1,500 W heater, 3 h a day for 30 days
- Daily: 1,500 × 3 = 4,500 Wh = 4.5 kWh.
- Monthly: 4.5 × 30 = 135 kWh.
Reading the result
The headline figure is the main answer. Any breakdown underneath shows the parts that make it up, so you can check the working and see what changes when you adjust an input.
Limitations and assumptions
- Assumes constant average power.
- No tariff or cost is applied.
- Standby and start-up surges are not included unless you add them to the average.
- Results are estimates for planning and learning. Real designs, installations and bids must be checked by a qualified, licensed professional against the codes that apply where you work.
- The result depends on the values you enter for this energy calculator; it does not supply missing rates, rules, prices, dates, or assumptions for you.
Common questions
What is the difference between kW and kWh?
kW is the rate power is used at a moment; kWh is the amount used over time. A 1 kW device running for one hour uses 1 kWh.
Why is my bill different?
Thermostats, compressors and standby modes mean devices rarely run at full nameplate power. Measure with a plug-in meter for a better average.
How do I turn this into cost?
Use the equipment energy cost calculator and enter your own tariff — no electricity price is assumed here. Check the formula, example, and limitations on this page before using the result for a real electrical decision.
How do I use the Energy Calculator?
Enter the required values for Power, Running time per day, Number of days. The calculator applies the formula on this page and shows the main result with any supporting breakdown so you can check the arithmetic.
What formula does the Energy Calculator use?
E (kWh) = P (W) × t (h) ÷ 1,000. 1 kWh = 3.6 MJ. The visible formula section above lists the calculation path and the edge cases the page handles, so the result can be checked without relying on the form alone.
Can the Energy Calculator be used for exact decisions?
Use it as a calculation aid, not as a substitute for checking the underlying rule, contract, policy, or professional advice that applies to your situation. When a result depends on local rules, personal details, prices, or dates, enter those values yourself and confirm them before acting.
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