Irradiance is sunlight power per square metre (W/m²); irradiation is that summed over time (kWh/m²). The annual figure at your site is the foundation of any yield estimate.
Angling panels toward the midday sun captures more. The best fixed tilt is below your latitude, facing the equator; off-axis trades annual yield for a different seasonal balance.
Heat, wiring, inverter conversion, soiling and downtime bundled into one factor, typically 0.75–0.85. A lossless system would be 1.0.
Annual energy per kW installed, stripping out system size so locations compare fairly: a sunny site might yield 1,800, a cloudy northern one closer to 800.
Climate data gives global horizontal irradiation. A panel is neither flat nor pointed at the sun, so the number must be moved onto its plane, and that is where estimators go wrong. The horizontal total is split into beam and diffuse using the Erbs correlation on the clearness index (London is roughly half diffuse, why British roofs are more forgiving about orientation than Spanish ones). The beam is projected onto the tilted plane by a ratio integrated across the whole day. Then the sky is treated as uniformly bright (the isotropic assumption), so a tilted panel sees (1 + cos β)/2 of it, plus a little from the ground.
It compared the sun’s angle at solar noon only and scaled by orientation with an expression identical for every direction from east round to north. A north-facing array came out at 91% of south and above a flat roof, which is impossible: pointing a panel away from the sun cannot beat pointing it at the sky. Integrating across the day fixes it, because a surface facing the wrong way spends most of the day with the sun behind it, and noon is the one moment that hides this.
It under-reads a clear-sky site by a few per cent, because real skies are brighter near the sun. For a tool someone is using to decide whether to spend money, that is the right direction to be wrong in.
The familiar rule is “set it to your latitude”. Diffuse light arrives from the whole sky rather than the sun’s direction, and the more of it the flatter you want the panel: the optimum comes out at 41° for London at 51.5°N and 31° for Phoenix at 33.4°N, below latitude in every location on the list.
Enormously, and more than the old version of this page suggested. Facing the equator at a good tilt, London collects about 19% more than a flat roof. Turn that same array to face the pole and it collects 59% of the south-facing figure — and less than if you had left it flat, because it loses the direct beam and half the sky at once. East or west costs about 18%, which is often worth accepting for a flatter output curve across the day.
On tilt, the familiar rule is “set it to your latitude”. That is too steep. Diffuse light arrives from the whole sky rather than from the sun’s direction, and the more of it there is the flatter you want the panel: the optimum here comes out at 41° for London at 51.5°N and 31° for Phoenix at 33.4°N. Below latitude in every location on the list.
The fraction of a system’s theoretical output it actually delivers once every real-world loss is accounted for — heat derating, wiring resistance, inverter conversion, soiling, and small mismatches between panels. A well-installed system typically lands around 75–85%.
This tool uses baked-in long-run climate averages, not live weather, satellite imagery, or site-specific shading. A tree, a chimney, or an unusually cloudy year can shift real output meaningfully above or below the estimate — which is exactly why installers run a proper site survey before quoting.
Mostly through irradiation — how much sunlight a location receives on average — and secondarily through temperature, since panel efficiency drops slightly as they heat up. Two identically sized systems in different climates can have meaningfully different annual yields.
Releases in which this page changed, newest last. Derived from the archived copy of every release, not from notes written afterwards — so it reflects what actually shipped. Site-wide passes are left out; they are in the full changelog.