Solar panel size calculator
A peak sun hour is one hour of 1,000 watts per square metre — the standard test condition panels are rated at. A location with 3.5 peak sun hours does not get 3.5 hours of daylight; it gets a whole day whose total energy adds up to 3.5 hours at full intensity. That is why a figure of 2.5 in Scotland and 5.5 in southern Spain is a realistic spread, and why the same array produces twice as much in one place as the other.
A planning estimate. A real design accounts for shading, roof orientation and pitch, local irradiance data and grid export rules.
Array size is daily energy divided by peak sun hours, divided again by one minus the system losses. Ten kilowatt-hours a day at 3.5 peak sun hours and 25% losses needs a 3.81 kWp array — nine panels of 430 W.
How to size a solar array
Peak sun hours vary far more by season than by location, which is what makes annual sizing and winter sizing two different problems. A UK array sized on an annual average of 2.8 peak sun hours will produce five times as much in June as in December. If the array feeds a grid-tied house, that seasonal swing is absorbed by the grid and the annual figure is the right basis. If it is charging an off-grid battery you have to size for the worst month, which typically triples the array — or accept a generator for the darkest weeks.
Questions
For 10 kWh a day at 3.5 peak sun hours, about 3.8 kWp — nine 430 W panels. Fewer in a sunnier climate, more in a cloudier one.