kWh Calculator

Convert between watts, kilowatts, kilowatt-hours and energy cost.

Please note: For planning and educational use only. Electrical installation is governed by wiring regulations and is dangerous when done incorrectly. Always have work designed, installed and inspected by a qualified electrician.

What the kWh Calculator does

A kilowatt-hour measures energy: power multiplied by time. Any one of power, time or energy follows from the other two, which is the basis of every electricity bill calculation.

Formula

kWh = (Watts ÷ 1,000) × Hours
Watts = (kWh ÷ Hours) × 1,000
Hours = kWh ÷ (Watts ÷ 1,000)
1 kWh = 3.6 MJ

Inputs explained

Input	Unit	Required	Notes
Calculate	one of 3 options	Yes	—
Power	W	In some modes	Accepts more than 0. Shown Calculate is Energy used from power and time, or Calculate is Run time from energy and power.
Time	hours	In some modes	Accepts more than 0. Shown Calculate is Energy used from power and time, or Calculate is Power draw from energy and time.
Energy	kWh	In some modes	Accepts more than 0. Shown Calculate is Power draw from energy and time, or Calculate is Run time from energy and power.
Rate per kWh	selected currency	Optional	Accepts 0 or more.
Currency	one of 10 options	Optional	—

How to use it

Choose Calculate and Currency.
Fill in the remaining inputs the form shows for your choice.
Optionally add Rate per kWh.
Select Calculate.

Worked example

A 2,000 W heater running for 3 hours.

Mode: Energy
Watts: 2000
Hours: 3

2 kW × 3 h = 6 kWh, which is 21.6 MJ. At $0.28/kWh that costs $1.68.

Frequently asked questions

What is the difference between kW and kWh?

kW is a rate of use — how fast you consume. kWh is a quantity — how much you consumed. A 2 kW heater for 30 minutes uses 1 kWh.

Method and sources

Method. Kilowatt-hours as power in kilowatts multiplied by hours of operation.

Applies to. None — a unit definition, not a regulated procedure

Assumptions

Power draw is constant over the period.

Limitations

A cycling or variable load makes the rated figure an overestimate; a metered reading is the only accurate measure.
This is energy, not cost — pricing depends on a tariff structure not modelled here.

Sources

The kilowatt-hour as a unit of energy — Derived from SI units; 1 kWh = 3.6 megajoules. The definition underlying the conversion.