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Watt to Watt-Hour Calculator

Convert watts and watt-hours using duration

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Method and source check

Last checked:

Automated source and implementation review, including worked examples and documented numerical limits. This review does not provide professional advice or specialist approval.

Method

At constant power, multiply watts by elapsed hours for watt-hours. Divide watt-hours by a positive duration for watts. An optional energy charge is kWh × the user-entered price per kWh.

Wh = W × hours

W = Wh / hours

kWh = Wh / 1000

energy-only charge = kWh × entered price per kWh

Variables and units

W
constant power in watts
Wh
energy in watt-hours
hours
duration converted from entered unit
rate
optional price per kWh in the user’s chosen currency

Worked example

At constant 100 W for five hours, energy is 500 Wh = 0.5 kWh. At an entered price of 0.12 per kWh, the energy-only charge is 0.06 in the entered currency.

Limitations

Energy, power, duration, and the optional entered-currency energy charge retain floating-point calculation precision; the UI displays up to six significant digits.

  • Wh-to-W requires a positive duration. Device examples are hypothetical constant-power cases; energy-only cost excludes all other bill charges.
  • The form schema rejects derived overflow and unsupported numerical combinations before showing results; the calculation helper expects schema-validated input.
  • 100W×5h → 500Wh
  • 500Wh/5h → 100W
  • 500Wh/0h rejected
  • 100W×1min → about 1.667Wh before display rounding
  • 1e308 W × 24 hours is rejected on power; 500 Wh ÷ 1e-320 seconds is rejected on energy; an unrepresentable converted duration is rejected on time.

Sources

Table of Contents

How to Use

  1. Choose watts to watt-hours or watt-hours to watts.
  2. Enter power or energy, then enter the duration and its unit. A watt-hours-to-watts calculation needs a duration greater than zero.
  3. Optionally enter a price per kWh for a simple energy-charge estimate.
  4. Read watt-hours and kilowatt-hours together; the optional charge uses only the rate you entered.

Choose watts to watt-hours or watt-hours to watts

Enter power or energy, then enter the duration and its unit. A watt-hours-to-watts calculation needs a duration greater than zero. Watts (W) measure power - the rate at which energy is used or produced at any given moment. Watt-hours (Wh) measure energy - the total energy used or produced over time. The relationship is: Energy (Wh) = Power (W) × Time (hours).

For example, a 100W light bulb running for 10 hours consumes 1,000 Wh (or 1 kWh) of energy. Understanding this conversion is essential for calculating electricity costs, sizing batteries, and managing energy consumption.

This formula assumes constant power over the entered duration.

Method source: U.S. Energy Information Administration. https://www.eia.gov/energyexplained/electricity/measuring-electricity.php

Conversion Formulas

The conversion between watts and watt-hours depends on time:

  • Energy (Wh) = Power (W) × Time (hours)
  • Power (W) = Energy (Wh) ÷ Time (hours)
  • 1 kWh = 1,000 Wh
  • 1 Wh = 3,600 joules

Worked example

At constant 100 W for 2 hours, energy is 200 Wh or 0.2 kWh. Multiply kWh by an entered price per kWh for an energy-only charge, excluding other bill items.

Hypothetical constant-power examples

These are hypothetical constant-power examples. A device whose power changes or cycles on and off will use a different amount of energy; use measured energy for a real bill estimate.

A constant 100 W for 2 hours gives 200 Wh; at constant 150 W for 24 hours, the arithmetic gives 3,600 Wh. Neither example describes a device with changing power.

Frequently Asked Questions

What is the difference between watts and watt-hours?
Watts measure power (rate of energy use), while watt-hours measure energy (total amount used over time). Think of it like speed vs. distance: watts tell you how fast you're using energy, watt-hours tell you how much total energy you've used.
How do I calculate watt-hours from watts?
Multiply the power in watts by the time in hours: Watt-hours = Watts × Hours. For example, a 100W device running for 5 hours uses 500 Wh of energy.
How do I convert watt-hours to kilowatt-hours?
Divide watt-hours by 1,000 to get kilowatt-hours (kWh). For example, 2,500 Wh = 2.5 kWh. Electricity bills typically use kWh for larger energy amounts.
How can I estimate my electricity cost from watt-hours?
Multiply energy in kWh by the entered price per kWh. For example, 2 kWh at a rate of 0.12 per kWh gives 0.24 in the entered currency. Other bill charges are excluded.
Why is understanding watt-hours important for solar and battery systems?
This calculation gives energy from constant power and time. Battery and solar sizing also depends on losses, usable capacity, varying loads, and system requirements.

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