Most beginners ask how many solar panels they need. The books reframe it: start with how much electricity you actually use in a day, then build a system that carries that load through the hardest month of the year. Sizing works backwards from your consumption, not forwards from a panel count.
Start by measuring your daily load
Every appliance has a wattage. Multiply it by the hours you run it and divide by 1,000 to get kilowatt-hours — the unit the whole system is sized in. Add these up across everything you'll run and you have your daily load. Refrigeration is usually the largest continuous draw, and anything that makes heat costs far more than it seems.
Understand peak sun hours
A panel's rating is a laboratory figure, not an all-day output. What matters is peak sun hours: the hours per day equivalent to full-strength sun at your location and season. This varies dramatically between summer and winter, and your local winter figure is the one that really sizes the array.
Allow for losses
Wiring, the charge controller, battery inefficiency, dust, heat and imperfect angles all take a cut. Planning around roughly 80 per cent of theoretical output is a sensible starting assumption rather than a guarantee.
Plan for winter, not the average
A system sized for the yearly average will run beautifully in summer and leave you short in the darkest weeks. Common approaches are to oversize the array, pair solar with another source such as wind, or keep a generator for the hardest stretch — and treat that fuel as a planned cost, not a failure.
Batteries: usable capacity is what counts
You can't safely drain a battery bank flat; how deep you can go depends on the chemistry. Decide how many days you want to run without strong sun, then size the bank so those days sit inside the safe range rather than at the edge of it.
A rough sizing sketch
- Add up daily kWh across every appliance.
- Find your worst-month peak sun hours locally.
- Array (kW) ≈ daily kWh ÷ (sun hours × 0.8).
- Size batteries around usable capacity and your desired days of backup.
- Leave headroom — people almost always add loads later.