Solar Panel Calculator
Peak sun hours folds latitude, season and cloud into one number, which is why the rest is simple.
The formula
daily kWh = kWp * peak sun hours * performance ratio
Peak sun hours does the hard work
A location's daily solar energy is expressed as the number of hours it would take to deliver the same total at exactly 1 kW per square metre — the standard test condition panels are rated at. That single number folds in latitude, season, cloud and day length, which is why the rest of the arithmetic is as simple as it looks.
The performance ratio is where the losses hide
A panel rated at 400 W produces that at 25 °C in a laboratory. In a roof in summer it runs at 50 °C and loses roughly 10 %, and then there is wiring, the inverter, dust and any shading. A performance ratio of 0.75 to 0.85 covers all of it, and a system quoted without one is quoting a laboratory number.
Shading is worse than it looks
Panels in a string behave somewhat like a single pipe: shade one and the whole string drops toward its output. A chimney shadow crossing one panel can cost far more than that panel's share, which is why optimisers and microinverters exist and why a survey matters more than the panel brand.
How much to trust a footprint
Life-cycle figures for the same product routinely differ by a factor of two between studies, because deciding what to count — the farm, the packaging, the shop, the drive home, the disposal — is a methodological choice rather than a measurement. The numbers here are central estimates from published meta-analyses. They are reliable for comparing one option against another, which is what people actually use them for, and not reliable to the second significant figure.
The other thing worth keeping in view is scale. Individual choices matter and they are not where the emissions are: the difference between the best and worst quartile of beef producers is larger than most people's entire dietary footprint, and that is a question about agriculture policy rather than about shopping.