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About this tool
Find the panel count, module efficiency and roof area needed for a target solar capacity, plus expected daily generation.
A solar panel area calculator turns a target kilowatt capacity into the number of panels and the square feet of roof they will occupy. It derives module efficiency straight from the panel's own spec sheet using the Standard Test Conditions definition — rated watts divided by module area times 1000 W/m² — then multiplies the glass area by a mounting factor for row spacing, walkways and edge setbacks. Daily and annual output come from peak sun hours and a system performance ratio.
Open Solar Panel Area Calculator on AltFTool — it loads instantly in your browser.
Enter the values you already know.
Fine-tune the options to match your scenario.
Read the result and use it in your planning or reporting.
Flags a panel whose claimed watts and dimensions imply an impossible efficiency.
Tilted flat-roof rows use about 2.2× the glass area; flush pitched mounting only about 1.15×.
Tells you if the array fits and, if not, the largest system your roof will actually hold.
About 28–32 m² (300–345 sq ft) if the panels lie flush on a pitched roof, and roughly double that for tilted rows on a flat terrace. Ten 550 Wp panels of 2278 × 1134 mm give 25.8 m² of glass, which becomes about 29.7 m² once edge setbacks and a service gap are allowed.
Two 550 Wp panels give 1.1 kW, so 1 kW takes two modern panels — but panel counts must round up, since you cannot install a fraction of a panel. Older 330 Wp panels needed three, which is why the same kilowatt rating took more roof a few years ago.
Mass-market monocrystalline modules sit at 20–23% in 2026, with the best commercial panels near 24.5%. A single-junction silicon cell cannot exceed the Shockley-Queisser limit of about 33%, so any panel advertised above that is either a data-entry error or a multi-junction laboratory cell.
Multiply installed kW by your site's peak sun hours per day and by the performance ratio: a 5.5 kW array at 5 peak sun hours and a 0.78 performance ratio makes about 21 kWh a day, or roughly 1,400 kWh per kWp per year. Actual output varies with shading, soiling, panel temperature and grid availability, so treat the figure as a planning estimate and ask your installer for a site-specific shade study.
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