A watt-hour (Wh) is a unit of energy that tells you how much electricity a device uses or a battery stores over time. One watt-hour equals one watt of power running for one hour. Watt-hours matter because they let you match a battery or power station to your actual energy needs rather than guessing.
If you have ever compared portable power stations or solar batteries, you will have noticed that watt-hours appear everywhere. Understanding what they mean in practice is the key to choosing the right equipment and avoiding the frustration of running out of power at the wrong moment.
What Is a Watt-Hour?
A watt-hour measures energy, not power. The distinction matters:
- Power (watts): How fast energy is being used or produced at a given moment. A kettle uses 2000W.
- Energy (watt-hours): How much power has been used over a period of time. A kettle running for 30 minutes uses 1000Wh, or 1kWh.
Think of it like a water tank. Watts are the flow rate (litres per minute). Watt-hours are the total volume of water that has flowed.
How to Calculate Watt-Hours
The formula is simple:
Watt-hours = Watts x Hours
Examples:
- A 50W laptop running for 4 hours uses 50 x 4 = 200Wh
- A 10W LED lamp on for 8 hours uses 10 x 8 = 80Wh
- A 1500W heater running for 1 hour uses 1500 x 1 = 1500Wh (1.5kWh)
Watt-Hours and Battery Capacity
Portable power stations and solar batteries are rated in watt-hours. A 1024Wh power station can theoretically run a 100W device for just over 10 hours, or charge a 60Wh laptop battery around 17 times.
In practice, account for system efficiency (typically 85 to 90%) when making calculations. Inverter conversion losses, battery management system overhead and cable resistance all contribute to slightly lower real-world figures than the simple maths suggests.
Comparing to Amp-Hours
Some batteries, particularly 12V leisure batteries, are rated in amp-hours (Ah) rather than watt-hours. To convert:
Watt-hours = Amp-hours x Voltage
A 100Ah, 12V battery stores 100 x 12 = 1200Wh. A 200Ah, 24V battery stores 200 x 24 = 4800Wh. This is why watt-hours are a better comparison metric across different system voltages.
Why Watt-Hours Matter for Solar
Sizing Your Solar Array
To size a solar panel array, calculate your daily energy use in watt-hours, then work out how many panels you need to replace that energy each day. In the UK, a 100W solar panel typically generates 250 to 400Wh per day in summer. If you use 800Wh per day, you need at least 200 to 300W of panels to keep up, with some buffer for cloudy days.
Sizing Your Battery
Your battery needs to store enough energy to cover consumption during non-generating hours. For an off-grid setup with 800Wh daily use and 10 hours of evening and night consumption, you need at least 800Wh of usable battery capacity. Adding a 20 to 30% buffer to account for efficiency losses and cloudy days brings this to around 1000 to 1100Wh.
Common Watt-Hour Benchmarks
| Device | Typical Energy Use |
|---|---|
| Smartphone (one full charge) | 10 to 20Wh |
| Laptop (one full charge) | 40 to 100Wh |
| Mini fridge (per day) | 150 to 400Wh |
| LED television (4 hours) | 80 to 160Wh |
| CPAP machine (8 hours) | 240 to 480Wh |
| Kettle (one boil) | 100 to 150Wh |
Kilowatt-Hours: The Grid Scale
On your electricity bill, you pay per kilowatt-hour (kWh). One kWh equals 1000Wh. At a typical UK electricity rate of around 24p per kWh, using 1kWh costs 24p. This context helps you appreciate just how much energy a 5kWh home battery can store: roughly 20 hours of average UK household consumption.
