kWh Converter (Wh, kWh, MWh, joules)

Understanding the kWh converter — concept and principle

The kilowatt-hour (kWh) is the unit on electricity bills, the size of an electric-car battery and the energy rating of appliances. Smaller devices use watt-hours (Wh), power plants and grids use megawatt-hours (MWh) and gigawatt-hours (GWh), and physics and fuel data use joules (J) and megajoules (MJ). This converter moves between all of them.

How it is calculated

Every unit is converted into kWh first, then into the target unit:

• 1 Wh = 0.001 kWh; 1 MWh = 1,000 kWh; 1 GWh = 1,000,000 kWh
• 1 kWh = 3,600,000 J = 3.6 MJ exactly
• 1 J = 1 ÷ 3,600,000 kWh ≈ 2.778 × 10⁻⁷ kWh

Result = value × (kWh in source unit) ÷ (kWh in target unit).

The principle

Energy is power multiplied by time. A watt is one joule per second, so running 1,000 W for one hour (3,600 seconds) uses 1,000 × 3,600 = 3,600,000 J. That product is defined as one kilowatt-hour. The prefixes are ordinary SI multiples of 1,000, so Wh, kWh, MWh and GWh differ by factors of a thousand. Because the joule is the SI unit and the hour is exactly 3,600 s, all these factors are exact. Do not confuse kWh (energy) with kW (power): kW is how fast energy is used, kWh is how much.

Worked example

1. Default inputs: value 1, from kWh, to Wh.
2. 1 kWh × 1,000 = 1,000 Wh.
3. Other units: 0.001 MWh, 0.000001 GWh, 1 × 3,600,000 = 3,600,000 J, which is 3.6 MJ.

How to read the result

A 2,000 W kettle running for 3 minutes uses 2,000 × 0.05 h = 100 Wh, or 0.1 kWh. A phone battery is often 15 to 20 Wh: 5,000 mAh at 3.85 V is 5 × 3.85 = 19.25 Wh. Electric-car batteries are commonly in the tens of kWh. To estimate cost, multiply kWh by your price per kWh, remembering that many tariffs, including Korea's residential tariff, charge higher rates as monthly use rises.

Common mistakes

• Writing kW when you mean kWh, or dividing kWh by hours and calling it energy.
• Converting battery mAh to Wh without multiplying by the voltage.
• Forgetting that 1 kWh is 3.6 MJ, not 3.6 J or 1,000 J.

Frequently asked questions

How many joules are in 1 kWh?

Exactly 3,600,000 J (3.6 MJ).

How do I convert mAh to Wh?

Wh = mAh × V ÷ 1,000. A 10,000 mAh power bank at 3.7 V holds 37 Wh.

What is the airline limit for power banks?

Under IATA guidance, power banks up to 100 Wh are generally allowed in cabin baggage, 100 to 160 Wh need airline approval and larger ones are not allowed; airlines may set stricter rules.

What is the difference between kW and kWh?

kW is power (a rate). kWh is energy (power × time). A 1 kW heater running 5 hours uses 5 kWh.

Sources: BIPM, The International System of Units (SI Brochure), 9th edition, 2019 · NIST Special Publication 811 (2008), Appendix B · IATA Dangerous Goods Regulations, lithium battery guidance for passengers