# Energy Calculator

> Calculate energy step by step: E (kWh) = P (W) × t (h) ÷ 1,000.

Page: https://freecalculator.tech/tools/energy-calculator
Last reviewed: 2026-11-03

## Formula

E (kWh) = P (W) × t (h) ÷ 1,000. 1 kWh = 3.6 MJ.

- 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.

## Inputs

- Power
- Running time per day
- Number of days

## Worked example: 1,500 W heater, 3 h a day for 30 days

Monthly energy use.
1. Daily: 1,500 × 3 = 4,500 Wh = 4.5 kWh.
2. Monthly: 4.5 × 30 = 135 kWh.

## 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.

### 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.

## 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.

## Sources

- OpenStax — https://openstax.org/books/college-physics-2e/pages/7-7-power

## Cite this page

- APA: Free Calculator. (2026, November 3). Energy Calculator. https://freecalculator.tech/tools/energy-calculator
- MLA: "Energy Calculator." Free Calculator, 3 Nov. 2026, https://freecalculator.tech/tools/energy-calculator.
- Link: Energy Calculator — Free Calculator, https://freecalculator.tech/tools/energy-calculator