Sizing an off-grid solar system starts with one number: how much energy you use per day in watt-hours (Wh). Without it, you’re guessing at panel and battery size. This guide shows you how to calculate daily energy use so your system is built on real numbers.
Why Watt-Hours (Wh)?
Energy is power × time. A 100 W bulb running for 5 hours uses 100 × 5 = 500 Wh. Solar and batteries are sized in Wh (or kWh = 1,000 Wh) because we care about how much energy we need over a day, not just how many watts a device draws at one moment.
The Formula for One Appliance
Daily energy (Wh) = Watts × Hours used per day
- Watts: Look at the appliance label, manual, or use a plug-in meter. Use running watts (not surge) for continuous loads.
- Hours per day: Estimate how many hours it runs on a typical day.
Examples:
- Fridge 50 W, 24 h → 50 × 24 = 1,200 Wh
- LED lights 20 W, 4 h → 20 × 4 = 80 Wh
- Laptop 45 W, 3 h → 45 × 3 = 135 Wh
Build a Simple Load List
List every load you’ll run from the system, then add them up:
| Appliance | Watts | Hours/day | Wh/day |
|---|---|---|---|
| Fridge | 50 | 24 | 1,200 |
| Lights | 20 | 4 | 80 |
| Laptop | 45 | 3 | 135 |
| Phone charging | 10 | 2 | 20 |
| Total | 1,435 Wh/day |
That total is your daily energy use. Use it in our peak sun hours formula to size panels and with days of autonomy to size batteries. For a full walkthrough, see how to do a load list for off-grid solar.
Don’t Forget These
- Inverter loss: If you use an inverter, add ~10–15% to the total (e.g. 1,435 × 1.1 ≈ 1,579 Wh).
- Surge vs running: Motors (fridges, pumps) have a brief surge; size the inverter for surge and use running watts for daily Wh.
- Seasonal changes: Heating, cooling, or longer nights change use. Size for your highest typical day or worst month if you want year-round reliability.
Quick Check
- Small setup (cabin, van): Often 500–2,000 Wh/day.
- Family home: Can be 5,000–15,000+ Wh/day.
If your total seems way off, re-check a few items: fridge and lighting are usually the biggest. Use the WattSizing calculator to enter your loads and get daily Wh plus recommended panel and battery size.
