Choose a conversion within the same physical quantity. Watts and kilowatts describe power: how quickly energy is used or delivered. Watt-hours and kilowatt-hours describe energy accumulated over time. A percentage change separately compares an old value with a new value.
Formula
1 kW = 1,000 W; 1 kWh = 1,000 Wh. Divide by 1,000 to convert to the kilo-unit and multiply by 1,000 to convert back. Percentage change = (new value − old value) ÷ old value × 100.
Worked example
A 2,000 W appliance has a power of 2 kW. Running it for half an hour uses 1 kWh, equivalent to 1,000 Wh. Reducing annual use from 500 to 400 kWh produces a −20% change, meaning a 20% reduction.
What the result means
A conversion changes the unit, not the physical amount. A negative percentage means a decrease; a positive percentage means an increase. Use the electricity calculator when you also need operating time and costs.
Electricity price · United Kingdom
Enter the unit rate including any applicable tax. Check your bill rather than adding a tax percentage automatically. Standing charges, export payments and subscription fees are separate. Ofgem information applies to Great Britain; Northern Ireland has its own regulator and market arrangements.
Assumptions and limits
Power cannot be converted directly to energy without a duration. Percentage change has no defined finite result when the old value is zero.
Questions answered
Is kW/h the same as kWh?
No. kWh is energy; kW/h describes a change in power over time.
Does a 20% reduction reverse a 20% increase?
No. The second percentage applies to a different starting value.
Data sources
- Ofgem · July–September 2026
- Ofgem: Understanding electricity and gas bills
- Ofgem: Price cap and standing charges explained
- Utility Regulator: Consumer information for Northern Ireland
Last reviewed: 2026-09-08