The number of panels comes from three things: how much energy you use in a day, how much sun your site gets, and how much of the panels’ output actually reaches your meter. Get those, and the rest is arithmetic: array size in kilowatts, then divide by the wattage of one panel.
Step 1: find your daily energy use
Look at a power bill for your monthly kWh and divide by 30. A US home averaging about 900 kWh a month uses roughly 30 kWh a day; a smaller or more efficient home might be 10–15 kWh. Decide whether you want to cover all of that or just offset part of it — many people size to their daytime and shoulder loads and stay grid-connected for the rest.
Step 2: get your peak sun hours
Peak sun hours are the equivalent hours per day of full-strength (1 kW/m²) sun, averaged over the year. Most places land between 3.5 and 6 — check a local solar resource map. It is the single biggest factor in how many panels you need, so use a figure for your actual area, not a national average.
Step 3: size the array
The formula is array kW = daily kWh ÷ (sun hours × performance ratio). The performance ratio is 1 minus your losses — heat, dust, shading, wiring and inverter conversion take roughly 20%, so a well-installed roof system runs at about 0.8.
Example: 30 kWh a day at 4.5 sun hours and a 0.8 ratio needs 30 ÷ (4.5 × 0.8) = about 8.3 kW of panels. At 450 W per panel that is 8,300 ÷ 450 ≈ 19 panels.
PV System Sizing
Skip the arithmetic — enter your daily kWh, local sun hours and losses to get the array in kW, the number of panels and a matching inverter.
Panels by system size
Using 450 W panels, a rough guide:
| Array size | 450 W panels | Covers about |
|---|---|---|
| 3 kW | 7 | 10–13 kWh/day |
| 5 kW | 11 | 18–22 kWh/day |
| 6.6 kW | 15 | 24–29 kWh/day |
| 10 kW | 22 | 36–44 kWh/day |
Higher-wattage panels mean fewer of them for the same array — useful when roof space is tight. The kW figure is what matters; the panel count just follows from the wattage you choose.
What size inverter?
Panels are usually oversized relative to the inverter because they rarely hit their full rating — the DC:AC ratio. Around 1.2 (20% more panel power than inverter) is common and cost-effective, so an 8.3 kW array pairs with roughly a 7 kW inverter. Round to a model your supplier stocks.
Size up for winter
Peak sun hours are a yearly average, but winter days are shorter and cloudier. A system sized to the average falls short in the darkest months. If you need year-round self-supply rather than an annual offset, size to your winter sun hours — or accept pulling from the grid when the days are short.