PV System Sizing

It sizes a solar array to cover a given amount of daily energy use — telling you the array size in kWp, roughly how many panels that is, and a matching inverter size. Enter your usage and a few local figures and it does the rest.

Array size (kWp) = daily energy use (kWh) ÷ (peak sun hours × performance ratio). The performance ratio is 1 minus your system losses. For example, 10 kWh a day in a location with 4.5 peak sun hours and 20% losses (a 0.8 performance ratio) needs about 10 ÷ (4.5 × 0.8) = 2.8 kWp of panels.

Peak sun hours are the equivalent number of hours per day of full-strength (1 kW/m²) sunlight your site receives, averaged over the year. Most locations fall between 3.5 and 6 — check a local solar resource map or your national weather service. It is the biggest single factor in how many panels you need, so use a figure for your actual area.

Not all the energy the panels produce reaches your meter. Heat, dust and shading, wiring resistance, and inverter conversion all take a share. A well-installed rooftop system typically loses about 20%, which is a performance ratio of roughly 0.8. Poor shading or a hot roof means higher losses and a bigger array.

The calculator divides the array size by your panel wattage and rounds up to a whole panel. A 2.8 kWp array built from 450 W panels is 2,800 ÷ 450 ≈ 7 panels. Higher-wattage panels mean fewer of them for the same array size — useful when roof space is tight.

Panels are usually oversized relative to the inverter because they rarely hit their full rating. This is the DC:AC ratio — around 1.2 (20% more panel power than inverter) is common and cost-effective. The calculator divides the installed array by that ratio to suggest an inverter size; round to the nearest model your supplier stocks.

Peak sun hours are a yearly average, but winter days are much shorter and cloudier, so a system sized to the average will fall short in the darkest months. If you need to cover your usage year-round — rather than just offset it annually — size the array to your winter sun hours, or accept pulling from the grid in winter.