What is the difference between a kilowatt and a kilowatt-hour?

A kilowatt is a rate, a kilowatt-hour is an amount. A 2 kW heater running for 3 hours uses 6 kWh — and it is kWh that the electricity bill charges for.

How many watts is one horsepower?

745.7 watts for mechanical (imperial) horsepower; the metric horsepower used in European car specifications is 735.5 W.

Why do mAh ratings matter for batteries?

Milliamp-hours measure charge: a 5000 mAh power bank can in theory supply 500 mA for 10 hours. Real output is lower because voltage conversion loses 10–25%.

6 free tools

Digital & Electrical Converters

Convert energy, power, frequency and electric charge, and solve Ohm's law from any two values, with the SI prefixes handled properly. Free and no sign-up.

NewAnalog-to-Digital ConverterAnalog-to-Digital ConverterOpen NewEnergy ConverterConvert joules, kilojoules, calories, kilocalories, watt-hours, kilowatt-hours and BTU.Open NewPower ConverterConvert watts, kilowatts, megawatts and horsepower, metric or mechanical.Open NewFrequency ConverterConvert hertz, kilohertz, megahertz and gigahertz.Open NewElectrical Charge ConverterConvert coulombs, millicoulombs, microcoulombs, amp-hours and milliamp-hours.Open NewOhm's Law CalculatorWork out voltage, current, resistance or power from any two of them.Open

In short

Electronics and digital conversions, starting with an analog-to-digital converter that computes the output code, step size and quantization error for any resolution.

Overview

Conversions for electronics and computing: energy, power, frequency, charge and signal values, with the SI prefixes that cause most of the confusion.

Which one do you need?

Energy — joules, calories, watt-hours and BTU
Power — watts, kilowatts and horsepower
Frequency — hertz through gigahertz, and RPM
Electric charge — coulombs, amp-hours and milliamp-hours
Circuits — voltage, current, resistance and power from any two
Reading a clock face — analogue dial to digital time

All 6 tools in this section are free, work on a phone, and keep what you type on your own device. Nothing is stored and there is no sign-up.

Digital & Electrical Converters: frequently asked questions

What is the difference between a kilowatt and a kilowatt-hour?: A kilowatt is a rate, a kilowatt-hour is an amount. A 2 kW heater running for 3 hours uses 6 kWh — and it is kWh that the electricity bill charges for.
How many watts is one horsepower?: 745.7 watts for mechanical (imperial) horsepower; the metric horsepower used in European car specifications is 735.5 W.
Why do mAh ratings matter for batteries?: Milliamp-hours measure charge: a 5000 mAh power bank can in theory supply 500 mA for 10 hours. Real output is lower because voltage conversion loses 10–25%.