Appliance Energy Calculator
How much electricity an appliance uses in a day, a month and a year — from its wattage and how long it runs, including standby.
Energy is power multiplied by time: a 2,000 W heater running an hour uses 2 kWh.
How the appliance energy calculator works
Energy is power multiplied by time: a 2,000 W heater running an hour uses 2 kWh. The arithmetic is trivial and the surprises come from the two ends of the range. High-power appliances used briefly — kettles, toasters — matter far less than people assume, while low-power devices left on permanently add up: a 5 W device on standby all year uses 44 kWh, more than a kettle boiled twice a day.
That is why standby is worth its own row. It is invisible individually and substantial across a household.
Formula: kWh = watts × hours ÷ 1000
Worked examples
| Inputs | Energy per year | Note |
|---|---|---|
| A 2 kW heater, 2 hours a day all year | 1,460 kWh | 1,460 kWh a year |
| A 5 W device left on standby permanently | 43.8 kWh | 43.8 kWh — more than a kettle used twice daily |
| A television: 150 W for 4 hours, 1 W standby | 226.3 kWh | 226 kWh, of which 7 is standby |
FAQFrequently asked questions
How do I calculate an appliance's energy use?
Watts × hours ÷ 1000 gives kilowatt-hours. A 2,000 W heater for two hours uses 4 kWh.
Does standby really matter?
Across a household, yes. A 5 W device left on all year uses 44 kWh — more than a kettle boiled twice a day.
Which appliances use the most?
Anything that heats or cools for long periods: heating, hot water, air conditioning, tumble dryers. Brief high-power use matters far less.
Where do I find the wattage?
On the rating plate or in the manual. A plug-in energy monitor is more reliable, since many appliances rarely draw their maximum.
Why 0.5 kg of CO₂ per kWh?
It is a middle figure between grids. The UK is now well below it, much of Australia above, and US states vary enormously.
Where these figures come from
- UK DESNZ / DEFRA — Greenhouse gas reporting: conversion factors — the flight, fuel and waste emission factors used here
- Scarborough et al. (2014), Climatic Change — Dietary greenhouse gas emissions — the per-diet daily emission figures
- US EPA — Technical Assistance Document for the Reporting of Daily Air Quality (AQI) — the PM2.5 breakpoints the air quality calculator uses
- UK Department for Energy Security and Net Zero — conversion factors — the official UK emission factors
Last checked: September 2026. Factors are taken from published government and peer-reviewed sources, each named on the page with its year.