Power basics
Four units explain almost every battery and solar spec.
| Unit | What it measures | Think of it as |
|---|---|---|
| Volts (V) | Electrical pressure | 12V van system, 120V house outlet |
| Watts (W) | Power: the rate of use or production right now | A 60 W fridge, a 400 W solar array at full sun |
| Watt-hours (Wh) | Energy: power over time | The size of the tank |
| Amp-hours (Ah) | Charge capacity at a given voltage | Tank size, but only meaningful with volts |
The two formulas
Watts = volts × amps
Watt-hours = volts × amp-hours (or watts × hours)
This is why Ah alone can mislead: 100 Ah at 12V is 1,200 Wh, while 100 Ah at 24V is 2,400 Wh. Comparing in Wh avoids the confusion.
Sizing example
- List loads: a device that draws 60 W, run for 5 hours a day.
- Daily energy: 60 W × 5 h = 300 Wh.
- At 12V: 300 Wh ÷ 12 V = 25 Ah per day.
- Usable capacity: if a lead-acid battery should only be discharged about halfway for good life, you would want about 50 Ah of rated capacity for that day. A lithium battery whose maker allows most of its capacity to be used needs closer to 25 Ah, plus margin.
- Add margin for inverter losses (when running AC loads), cloudy days and battery aging.
Real loads vary with use. Check each device's label or measure it with a meter for accurate numbers.
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