Energy units and percentage calculator

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

£0.2611/kWh is Ofgem’s average capped electricity unit rate for standard variable tariffs paid by Direct Debit in England, Scotland and Wales, valid from 1 July to 30 September 2026. It includes 5% VAT and excludes the separate standing charge. It is not an average across all contracts and does not apply to Northern Ireland. Use your own tariff where available.

Last reviewed: 9 September 2026 · Ofgem · July–September 2026

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

Methodology · Report an error