How many solar panels do I need?

Common questions

How do I calculate how many solar panels I need?

Size the system from your electricity use, then divide by the panel rating. The standard method is: system size in kilowatts = your daily use (kWh) ÷ (peak sun hours × performance ratio), and panels = system watts ÷ panel watts, rounded up. For example, a home using about 900 kWh a month (≈29.6 kWh a day) at 4 peak sun hours and an 80% performance ratio needs roughly a 9.2 kW system — about 24 panels at 400 W each. The calculator above does this for you; just enter the kWh from your power bill.

Does a 400W solar panel produce 400W?

Only briefly, and only in lab conditions. The 400 W is the panel's rating at Standard Test Conditions — 1,000 W/m² of light at 25 °C — which real roofs rarely hit. In practice heat, dust, wiring losses, inverter losses and any shading knock roughly 15–25% off, which is exactly what the performance ratio (about 0.80) in the formula accounts for. That is why sizing works from peak sun hours and a performance ratio rather than assuming every panel delivers its nameplate watts all day.

What is the 20% rule for solar?

It usually refers to the inverter loading ratio: installers commonly fit a solar array up to about 20% larger than the inverter's rating (a DC-to-AC ratio near 1.2), because panels seldom produce their full rating, so a slightly oversized array fills the inverter without much wasted output. Some people also use "20%" loosely for the amount you might oversize a system above current usage to allow for future demand. Both are rules of thumb, not code requirements — confirm the specifics with your installer.

How do I calculate what solar panel I need?

Work out the total system size first (daily use ÷ peak sun hours ÷ performance ratio), then choose a panel wattage and divide. A higher-wattage panel means fewer panels for the same kW, which helps on a small roof; a lower wattage means more panels but sometimes a lower price. Enter your panel's rating in the calculator and it returns both the system size in kW and the number of panels, plus an estimate of the roof area they take up.

What are peak sun hours and why do they matter?

Peak sun hours are the number of hours per day that the sun delivers a full 1,000 W/m² — a way of boiling a whole day's varying sunlight down to one figure. They run roughly 3 hours a day in cloudier northern regions up to 5–6 in sunny areas, and change with the season. They matter because system size is directly proportional to them: the same home needs a bigger array where there is less sun. Look up the figure for your own location and type it into the calculator rather than relying on a national average.

How many solar panels to power a house?

It depends almost entirely on how much electricity the house uses and how sunny the site is — there is no single number. A modest home on 500 kWh a month might need around 13 panels; a larger all-electric home on 1,500 kWh could need closer to 39 (both at 400 W, 4 sun hours, 80% efficiency). Start from your own bill rather than a generic figure, and remember a battery or grid connection covers the hours the panels are not producing.

Want the full walk-through? Read the solar panel guide →