Solar PV Sizing Calculator
Sizing a solar array starts from one number: how much energy you use per day. Divide that by your peak sun hours, allow for system losses, and you have the array size — then divide by your panel wattage for the panel count. This calculator does both, with a battery hint from the same figures.
Formulas: Array (kW) = daily kWh ÷ (peak sun hours × (1 − losses)) · Panels = array watts ÷ panel watts (rounded up)
Worked example: a home using 10 kWh/day with 5 peak sun hours and 20% losses: array = 10 ÷ (5 × 0.80) = 2.5 kW, and with 400 W panels that is 2500 ÷ 400 = 6.25, rounded up to 7 panels. Peak sun hours are not daylight hours — they are the equivalent hours at full 1000 W/m² sunshine, typically 3–6 depending on location and season; use your worst month, not your best, if the system must work year-round.
For the battery to store that energy overnight, use the battery runtime calculator in reverse: decide the load and hours, then read the capacity you need.
Frequently asked questions
What are peak sun hours?
The number of hours per day at a standard 1000 W/m² irradiance that would deliver the same total solar energy as your real, varying daylight. A location with 5 peak sun hours might have 12 hours of daylight — most of it weak morning and evening sun.
Why include system losses?
Inverter losses, temperature derating, soiling, wiring and mismatch typically cost 15–25% of the panels' rated output. Ignoring them is the most common cause of undersized systems.
How many panels do I really need?
This gives the energy answer. Roof space, shading, orientation and local rules may change it — a south-facing unshaded roof in the northern hemisphere (or north-facing in the southern) performs closest to the calculation.