Location
Your roof
System (optional)
Without this figure we can only show clear-sky geometry. Look yours up in the EU's PVGIS or NREL's NSRDB and paste it in for a real-world estimate. Typical: UK 950, Spain 1750, Texas 1750, Sydney 1650.
Solar panel angle & yield
How much sun a panel collects depends on only two things you control — which way it faces and how steeply it leans. Here is what your roof gives up against the best possible orientation at your latitude.
Yield against every orientation
% of the best possible at this locationRows are roof pitch, columns are the bearing the panels face. Your current choice is highlighted. Anything above about 90% is, in practice, not worth re-engineering a roof for.
Collected energy by month
Best tilt for each season
| Optimising for | Best tilt | Best bearing | Gain over your roof |
|---|
What is and isn't modelled
The sun's position, the angle it strikes your panels at, and therefore the relative merit of one orientation against another, are pure geometry — those numbers are solid. The atmosphere is handled with a standard clear-sky model, so absolute irradiance figures describe a cloudless year, which no real place has.
That is why the headline number here is a percentage of the optimum rather than a kilowatt-hour figure. The percentage barely moves with cloud cover, so it survives the thing we cannot know. If you supply your local annual solar resource, the tool scales the geometry onto it and gives an absolute estimate — still ignoring panel temperature, soiling, inverter clipping and any shading finer than the skyline angle you set.
One consequence worth knowing: because the model gives every month a cloudless sky, it values winter sun more highly than reality does, and so returns an optimal tilt a few degrees steeper than published figures for cloudy maritime climates. In northern Europe, read the suggested tilt as an upper bound and expect the real optimum to sit around five degrees shallower. The flatness of the peak means this barely matters — anywhere within about fifteen degrees of the optimum loses under two per cent.
Common questions
What tilt should my panels be?
A reasonable rule is about three-quarters of your latitude plus a few degrees — roughly 35–40° for the UK and northern Europe, near 30° for the southern United States, and flatter still near the equator. Most pitched roofs already sit in that band, which is why re-angling panels off the roof plane rarely pays for itself. The tilt table on this page shows how little you lose either side of the peak.
Is an east–west roof worth it?
Usually yes. Each slope collects around 80–85% of what a south-facing array would, but you can fit panels on both, so total output beats a single south-facing array. The generation curve is also flatter — a morning peak and an evening peak instead of one at noon — which is worth more when you are buying back expensive grid power at breakfast and dinner rather than exporting cheaply at midday.
Where do I find my local annual solar resource?
Look up your coordinates in the European Commission's PVGIS tool or, in the United States, NREL's NSRDB. Both give annual global horizontal irradiation in kWh/m². Typical figures are around 950 for the UK, 1,300 for northern Italy, 1,750 for southern Spain or Texas, and 1,650 for Sydney. Enter it in the panel and this page converts its geometry into a kilowatt-hour estimate.
What is a performance ratio?
It is everything that stands between the light hitting your panels and the electricity reaching your meter: panel temperature, inverter losses, wiring, dust and dirt, and mismatch between modules. A well-installed modern domestic system typically runs at 0.75–0.85. Lower it if your roof gets hot and has little airflow behind the panels, or if the array is partly shaded.
How much does shade actually cost me?
Far more than a poor bearing does, and disproportionately, because panels wired in series are dragged down by the worst-lit module. A single chimney or tree can cost more than turning the whole array from south to west. Winter is when it bites: the low sun means a tree fifteen metres away can shadow a roof for most of a December day while barely touching it in June.