Watts is how hard. Watt-hours is how long. Mix them up and nothing adds up.
Watts and watt-hours sound alike but measure different things. Getting them straight is the key to sizing any battery or solar system.
1. Watts: power
Watts (W) tell you how fast energy is being used right now. A 60W light uses energy at a rate of 60 watts.
2. Watt-hours: energy
Watt-hours (Wh) tell you how much energy is used over time. Run that 60W light for 3 hours and it uses 180Wh. A kilowatt-hour (kWh) is 1,000Wh, the unit on your power bill.
3. The sum
Energy (Wh) = power (W) × time (hours).
4. Batteries
A battery stores energy. A 100Ah 12.8V battery holds about 100 × 12.8 = 1,280Wh. You can only use part of that without harming it, depending on the chemistry.
5. Worked example
| Device | Watts | Hours | Wh |
|---|---|---|---|
| Fridge (average) | 45 | 24 | 1,080 |
| Lights | 10 | 4 | 40 |
| Phone charging | 10 | 2 | 20 |
| Total | 1,140 |
6. Quick summary
- W is the rate. Wh is the amount.
- Wh = W × hours.
- Size batteries and solar in Wh per day.
- Try the Energy (Wh and kWh) Calculator.
240V AC mains can kill. In Australia, work on household mains wiring must be carried out by a licensed electrician. Low-voltage DC can still deliver very high current, so treat batteries with respect.
