Metered, not modelled
Cardiff vs Bristol
Cardiff roofs made about 1% more than Bristol roofs last year.
800 against 789 kWh per kWp at the median, from 147 and 468 real systems.
Where the two actually overlap
Both areas, grouped by what each system made. The medians differ, but the shapes sit largely on top of one another — a good Bristol roof beats a poor Cardiff one comfortably. Location moves the odds; it does not decide the outcome.
Cardiff: the middle half of 147 systems made 736–870 kWh per kWp. Bristol: the middle half of 468 systems made 725–848 kWh per kWp.
Both figures are what real systems metered last year, so the gap mixes sunlight with system age, roof direction and shading. It is the difference in what people actually got, not a difference in sunshine alone.
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Output figures from openclimatefix/uk_pv (Hugging Face, CC-BY-4.0), collected by Sheffield Solar. How we measure.
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How sunlight falls on a UK roof, month by month.
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