Free Solar Panel Count calculator with clear step-by-step results.
Sizes an array from consumption rather than roof space. Specific yield - the kilowatt-hours a kilowatt-peak produces annually in your location - is derated for shading and orientation, and the required capacity is divided by panel rating and rounded up to whole modules.
Required capacity
kWp = consumption x offset target / (specific yield x (1 - shading loss))
Panels required
Panels = ceil(required kWp x 1000 / panel watts)
Roughly 750-950 kWh/kWp in northern Europe, 1,100-1,400 in southern Europe and the northern US, and 1,500-1,800 in desert climates. National solar irradiance tools give a site-specific figure.
Because generation and consumption are out of phase. Without storage or a generous export tariff, oversizing simply exports more energy at a low price.