Will solar panels work on my roof?
Almost certainly yes — and the two things everyone worries about, pitch and bearing, matter far less than the one thing most people never check.
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The optimum is a very flat peak
There is a best tilt and bearing for any latitude, and installers will quote it at you. What they less often mention is how gentle the fall-off around it is. Move a long way from the optimum and you lose surprisingly little.
The reason is that a panel does not only collect the direct beam. It also collects diffuse light from the whole sky dome and light reflected off the ground, and those two components care very little which way the panel points. On an overcast day a panel's orientation is nearly irrelevant — and in a cloudy climate, a large share of the annual total arrives on days like that.
The practical rule. Anything within about 45° of due south (or due north in the southern hemisphere), at any pitch between roughly 15° and 50°, will land above 90% of the theoretical maximum. Almost every ordinary pitched roof facing anywhere in the southern half of the compass qualifies.
What bearing costs you
Typical figures for a mid-latitude northern site with panels at a standard roof pitch, expressed as a percentage of what a due-south array would produce over a year:
| Panels face | Roughly | Verdict |
|---|---|---|
| South | 100% | The reference |
| South-east / south-west | 95–98% | No practical difference |
| East / west | 80–85% | Perfectly viable; often the cheaper option per panel |
| North-east / north-west | 60–70% | Marginal; needs cheap panels or high electricity prices |
| North | 50–60% | Last resort in the northern hemisphere |
Note how small the penalty is for a south-east or south-west roof. A 3% difference on a typical domestic system is a few weeks of generation across a twenty-five year life — nowhere near enough to justify mounting frames that tilt panels away from the roof plane, which add cost, wind load and visual objection for almost nothing.
What pitch costs you
The optimal tilt is roughly three-quarters of your latitude, plus a few degrees — around 35° to 40° for the UK and northern Europe, around 30° for the southern US, less nearer the equator. UK roofs are commonly pitched between 30° and 45°, which is why almost every British roof is already close to ideal.
A flat roof is the one case worth thinking about. Panels laid flat lose 10–15% against the optimum at mid latitudes, and they collect dirt because rain does not run off cleanly. Tilting them to even 10–15° recovers most of the gap and solves the soiling.
Very steep tilts have one niche use: they favour winter sun, when the sun is low. A near-vertical array on a south wall generates poorly in June and unusually well in December, which suits anyone whose demand peaks in winter.
The case for east–west roofs
An east–west roof can take panels on both slopes, so you can fit roughly twice as many as on the south slope alone. Two arrays at 82% beat one at 100%, and the combined output is spread across the day — a morning peak and an evening peak instead of one midday spike.
That shape matters more than it used to. If you are exporting to the grid for a low tariff and buying back at a high one, electricity you generate at breakfast and dinner time is worth considerably more than electricity you generate at noon and immediately sell. A flatter, wider generation curve also lets a smaller battery do more work.
Shade is the thing that actually kills output
Here is the asymmetry worth internalising. Turning your array from south to west costs you perhaps 15%. A single tree, chimney or neighbouring gable that shades part of the array for a few hours a day can cost you far more than that — and unlike orientation, it can get worse over time as trees grow.
Two reasons shade hurts disproportionately:
- Winter shade lasts far longer than you expect. A 10 m tree 15 m to the south of your house casts a shadow past your roofline for most of a December day at UK latitudes, while barely touching it in June. Assessing shade on a summer afternoon tells you almost nothing about the months when you need every watt.
- Panels are wired in series. Shading a fraction of one panel can drag down a whole string. Modern systems mitigate this with optimisers or microinverters, but mitigation costs money and never fully recovers the loss.
The right question is therefore not "which way does my roof face?" but "what is the skyline around my roof, and where does the sun sit relative to it in December?" That is exactly the sort of question a sun path diagram answers at a glance: plot your obstructions as a horizon profile, overlay the winter solstice arc, and see whether the sun clears them.
Check your own roof
The solar tool computes the true optimum for your latitude, shows what your actual pitch and bearing give against it, and lets you set a skyline obstruction angle to see what nearby buildings and trees cost you. It reports a percentage rather than a headline kilowatt-hour figure, because the percentage is the part that can be computed honestly without weather data.