Solar Panel Power Output

A photovoltaic module's power is irradiance times area times efficiency, $P = GA\eta$, and the nameplate on the back is nothing more than this equation evaluated at standard test conditions. IEC 61215 defines those as 1000 W/m² of irradiance, a cell temperature of 25 °C, and an air mass of 1.5 — the spectrum sunlight has after a slanted path through the atmosphere.

All three are laboratory conditions, and the field seldom supplies any of them at once, let alone together. Taking nameplate wattage as real-world output is the commonest mistake in domestic solar arithmetic, and it runs 20–30% optimistic before the sun has even moved across the sky. A panel on a roof in full summer sun receives roughly the right irradiance but runs at 55–65 °C, which alone costs 10–15%. Then come soiling, module mismatch, wiring losses, inverter conversion, and the plain geometric fact that the sun is square to a fixed panel for only part of a day.

Efficiency is worth reading carefully. Mass-market monocrystalline silicon sits at 19–23%, polycrystalline a little below, thin film 10–16%, and laboratory multi-junction cells under concentration exceed 45%. Datasheets sometimes quote cell efficiency, which excludes the frame and the gaps between cells; the honest figure for a roof is module efficiency, over the full outside dimensions, and it runs a point or two lower. The Shockley–Queisser limit caps a single-junction cell near 33% whatever the material, because photons below the band gap pass straight through and the excess energy of photons above it is lost as heat.

Efficiency also matters less than people expect. Two arrays of the same rated wattage produce the same energy whether they are 18% or 22% efficient — the more efficient one simply occupies less roof. Efficiency buys area, not electricity, and it only becomes decisive when the roof runs out before the target does.