Solar & the Wh budget
How to turn appliance power and running time into a daily energy budget — and how many panels you need.
A Wh budget for your appliances
Energy is measured in watt-hours (Wh). A 60 W cool box running half the day uses 60 × 12 = 720 Wh per day. A 1500 W induction hob for 20 minutes uses 1500 × 0.33 = 500 Wh. Add up every appliance and you have your daily need.
Wh/day = power (W) × time (h/day)
Working out the solar
A panel rated 100 W does not give 100 Wh per hour — it gives its rated power in ideal sun, perpendicular to the panel, at 25°C. In practice you count peak sun hours: how many hours a day the panel delivers full power on average.
Wh/day (solar) = panel W × peak sun hours × efficiency Sweden summer: ~5 peak sun hours Sweden winter: ~1 peak sun hour Norway summer: ~4–5 peak sun hours Norway winter: ~0.5–1 peak sun hour
- Efficiency around 0.75 after the charge controller, cables and battery losses
- Shade on a single cell drops the output of the whole array considerably
- An MPPT controller gives 15–30% more than PWM in weak conditions — Kraftur only lists MPPT controllers
Size for the worst day
If you want to be self-sufficient all year round, size for winter. For summer use, considerably less will do. A common approach is to rely on solar from April to October and top up with shore power or alternator charging for the rest of the year.