Solar Panel Count Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/5/2026
The number of solar panels needed is calculated with (annual consumption in Wh) ÷ (one panel's wattage × useful sunlight hours per day × 365 × system efficiency). For a household using 4,000 kWh/year with 400 W panels and 4 useful sunlight hours per day, you'll need about 9 panels.
Explanation
This calculation follows the basic sizing method used by residential solar installers: divide the annual energy to produce by the energy a single panel can actually produce over a year. A 400 W panel only produces its rated wattage under optimal sunlight (perpendicular radiation, clear sky) — the rest of the time, its actual output is lower. That's why the calculation uses "useful sunlight hours" (also called peak sun hours), an average that condenses a day's full variability into a fictional number of hours during which the panel would produce its rated wattage continuously; this average varies a lot by region (higher in southern France than in the north) and by the roof's orientation and tilt. System efficiency, the second loss factor, covers everything that dissipates energy between the panel and the electrical outlet: converting DC to AC through the inverter, cable resistance, and the panels heating up, which slightly reduces their efficiency in hot weather. This calculator gives a useful ballpark figure for a first estimate; a professional installer will refine this result with a precise shading and orientation study of the roof. To find your starting annual consumption, see our electricity consumption calculator.
Example: a household using 4,000 kWh per year
Inputs
Consumption: 4,000 kWh/year. Panels: 400 W. Useful sunlight: 4 h/day. System efficiency: 0.8.
Calculation
Number of panels = (4,000 × 1,000) ÷ (400 × 4 × 365 × 0.8) = 4,000,000 ÷ 467,200 ≈ 8.56.
Result
About 9 panels (8.56 rounded up) are needed to cover this consumption.
Frequently asked questions
Should I round the result up or down?
Always round up: a sizing of 8.56 panels means 8 panels wouldn't quite be enough to cover the target consumption over a full year. In practice, installers also often round based on how many panels fit neatly on the available roof space, not just the theoretical calculation.
Why is the number of useful sunlight hours so different from the actual length of the day?
Because it doesn't measure the time between sunrise and sunset, but a full-power equivalent average: in the morning and evening, sunlight is weaker and more oblique, so less efficient, and clouds reduce output the rest of the time. This average condenses all that daily and seasonal variability into a single figure representative of the year.
Does this installation make me fully self-sufficient for electricity?
Not necessarily: this calculation sizes total annual production, but solar output is concentrated during the day and in summer, while a household's consumption is spread across the whole day and year (even more so in winter, with heating). Without a battery storage system — see our battery runtime calculator for that kind of sizing — an installation generally stays grid-connected for periods of low production.