Energy is the category where the physics is settled and the price is not. A kilowatt hour is exactly 3.6 megajoules everywhere on earth, and it follows from the definitions of the watt and the hour rather than from any measurement. What a kilowatt hour costs is the opposite: it moves by country, by supplier, by the season and in some places by the hour of the day, and a standing charge sits on top of it that no per-unit figure can capture. So these tools carry the constants and ask you for the rate. Nothing here ships a price, because a price written into a page is wrong for most readers on the day it is written and wrong for all of them a year later — and the number on your own bill is the only one that answers the question you came with.
Tools in this category
Appliance running cost calculator
Turn the wattage on the label into what the thing costs you per day, month and year, at the price per kWh from your own bill.
kWh to joules
One kilowatt hour is exactly 3.6 megajoules. Convert it to joules, watt hours, Btu and kilocalories at once, or start from any of those instead.
Energy cost comparison
Put two appliances or two tariffs side by side and see what a year of the swap is worth, in money and in kilowatt hours.
Suggest a tool
Tell us what is missing and where its numbers should come from.
What should it convert, calculate or generate, and where would the authoritative numbers come from? A link to the standard or publication that defines them is the most useful thing you can send.
Opens your mail app with this page already filled in.
How these tools are built
Every tool in this category runs entirely in your browser: there is no upload, no account and no request to a server once the page has loaded. The numbers behind them are transcribed from the organisation that defines them — NIST for units, the W3C for web formats, the WHO for health measures — and each page names its source next to the formula. A tool whose reference values stop reproducing fails the build and does not ship, which is the whole reason the catalogue grows slowly.