Watt Hour Calculator
A watt-hour, written as Wh, is a unit of energy. It tells you how much energy a device uses or how much energy a battery stores over time. This makes watt-hours one of the most useful numbers for comparing appliances, batteries, solar systems, and daily energy use.
This calculator can work three ways: from watts and hours, from amp-hours and voltage, or from milliamp-hours and voltage. That makes it useful for both appliance energy planning and battery capacity conversions.
Short answer: Watt-hours are calculated by multiplying power by time. For batteries, watt-hours can also be found by multiplying amp-hours by voltage, or milliamp-hours by voltage and dividing by 1000.
Watt-Hour Energy Calculator
Estimate energy use for devices, batteries, and daily appliance loads
Watt-Hour Formula
The core watt-hour formula is simple:
Wh = Watts × Hours
For batteries, the same energy can be estimated in two other common ways:
Wh = Ah × Voltage
Wh = mAh × Voltage ÷ 1000
These formulas all describe the same basic idea: watt-hours measure total energy, not just power at a moment in time. That is why Wh is so useful for comparing device energy use and battery storage in one common unit.
Common Device Wh Examples
| Device | Example Use | Energy Use |
|---|---|---|
| LED TV | 50W × 4 hours | 200Wh/day |
| Fridge | 60W × 24 hours | 1440Wh/day |
| Laptop | 45W × 8 hours | 360Wh/day |
| Phone charger | 20W × 2 hours | 40Wh/day |
These are simplified examples based on straight watts-times-hours math. Real appliances such as fridges may cycle on and off, so actual daily energy use may differ from the simple nameplate wattage multiplied by 24 hours.
What a Watt-Hour Means
Energy Unit
A watt-hour is the energy used by a 1-watt device running for 1 hour. It combines power and time into one practical unit.
Appliance Planning
Watt-hours help estimate daily consumption for TVs, fridges, laptops, lighting, chargers, pumps, and other household loads.
Battery Comparison
Wh makes it easier to compare batteries across different voltages, because amp-hours alone do not show full energy unless voltage is included.
Solar and Backup
Off-grid systems, inverters, and battery banks are often sized in Wh or kWh because those units show total stored energy more clearly.
Daily Energy Use
To estimate daily energy use, calculate watt-hours for each device and add them together. That gives you a total daily Wh number, which is often the starting point for battery sizing, inverter planning, and solar system design.
For example, if a TV uses 200Wh/day, a laptop uses 360Wh/day, and a phone charger uses 40Wh/day, the total is 600Wh/day. Once you know that number, you can compare it directly with battery storage in Wh or kWh.
Practical warning: Nameplate wattage does not always equal real average energy use. Appliances with compressors, motors, or cycling behavior often use less or more energy than a simple constant-watt estimate suggests.
Why Wh Matters
Amp-hours are useful, but watt-hours are usually better for comparing different batteries and loads because voltage is already included. A 100Ah battery at 12V and a 100Ah battery at 24V do not hold the same energy, but watt-hours show the difference immediately.
This is why battery packs, power stations, laptops, and airline battery limits are often discussed in Wh. It is one of the clearest ways to describe total electrical energy in a form that works across devices, chargers, and storage systems.
Practical rule: Use watts for instant power, use watt-hours for total energy over time. If you are sizing a battery or comparing appliance use, Wh is usually the more useful number.