How much electricity solar panels actually produce

Rated output is measured in a laboratory. Real output is that figure minus a series of losses, each of which is knowable.

A 400 W panel does not produce 400 W in normal conditions. That rating comes from Standard Test Conditions: 1000 W/m² of irradiance at a cell temperature of 25°C. Real roofs are hotter and usually less brightly lit.

Step-by-step

1. Enter array size, orientation and tilt.
2. Enter your location's solar resource.
3. Read the monthly output, with each loss itemised.

Where the output goes

• Temperature. Panels lose roughly 0.3–0.4% per degree above 25°C. On a hot roof cells reach 60°C or more, costing 10–15%. This is why hot climates produce less per installed kilowatt than their sunshine suggests.
• Inverter conversion. Around 2–4%.
• Wiring and mismatch. A few per cent.
• Soiling. Dust and pollen, 2–5% depending on rainfall.
• Shading. Highly site-specific and potentially the largest of all.

Together these usually total 15–25%. A tool quoting a single "performance ratio" is bundling all of this into one number you cannot check, which is why they are itemised here.

Orientation and tilt

South in the northern hemisphere, tilted near your latitude, maximises annual yield. The penalty for being off-optimal is smaller than people expect — east or west facing typically costs 15–20%, not half.

And maximum annual yield is not always the goal. West-facing panels produce later in the day, which matches evening consumption and may be worth more than a larger total exported at midday for a poor rate.

Shading deserves separate attention

Shade is not proportional. With panels wired in series, one shaded panel can drag down a whole string far beyond its share. A chimney shadow crossing two panels can cost more than removing those two panels entirely — which is what optimisers and microinverters exist to address.

Frequently asked questions

Why is real output below the rated figure?

The rating is measured at 25°C cell temperature and full irradiance. Real panels run much hotter and lose roughly 0.3–0.4% per degree above that, before inverter, wiring, soiling and shading losses.

How much does facing east or west cost?

Typically 15–20% of annual yield rather than anything catastrophic. West-facing output arrives later in the day, which can be worth more if exports are poorly paid.

Does a little shade matter much?

Far more than its area suggests. Panels in series are limited by the weakest, so one shaded panel can reduce a whole string. Optimisers or microinverters limit the damage.