The problem with a single number
Most solar savings calculators give you an annual kilowatt-hour figure and leave it there. "An 800W system in the south of England generates roughly 760 kWh per year." That's true as a rough average, but it hides an enormous amount of variation — between seasons, between cities, and between the angle and direction your panels face.
The sun is not a fixed lamp. It rises at a different point on the horizon every morning, climbs to a different maximum height at noon depending on the season, and sets at a different time each evening. In London in June, you might get nearly 16 hours of daylight. In Edinburgh in December, barely 7. The sun at noon in Brighton on a summer's day climbs to around 62 degrees above the horizon. In Glasgow in January it barely clears 12 degrees.
All of that variation has a direct, measurable effect on how much electricity a solar panel generates. So we decided to pull in real sun position data for each UK city — and here is how it works.
The open-source library doing the maths: SunCalc
We use a library called SunCalc, written by Vladimir Agafonkin and released freely under the BSD licence. It is the same library used by weather apps, agricultural planning tools, and astronomical software worldwide.
Feed it a date, a latitude, and a longitude, and it tells you the sun's precise altitude (height above the horizon, measured in degrees) and azimuth (compass direction the sun is pointing from, measured in degrees) at that exact moment. It also calculates sunrise, solar noon, sunset, civil twilight, and several other solar milestones for the day.
We map each city in our calculator to its coordinates — London sits at 51.51°N, 0.13°W. Edinburgh is at 55.95°N, 3.19°W. That four-degree difference in latitude is what gives London noticeably more useful winter solar hours than Edinburgh, even though both feel grey from October to March.
What "peak sun hours" actually means
This is one of the most misunderstood terms in solar, so it is worth being precise.
A peak sun hour is one hour during which your panel receives 1,000 watts of solar energy per square metre — equivalent to the sun being directly overhead in perfect conditions. That almost never literally happens in the UK (the sun is never overhead here; it always comes from the south at an angle). But it is a useful unit for comparing how much total energy is available on a given day.
To calculate peak sun hours, we sample the sun's altitude every 15 minutes from sunrise to sunset. For each sample we calculate sin(altitude) — a number between 0 and 1 that tells us how much of the maximum possible sunlight is hitting a horizontal surface at that moment. We multiply each value by 0.25 hours (one 15-minute slice) and add them all up.
☀ The maths in plain English
Imagine the sun is a spotlight being dragged across the sky. When it is low on the horizon (say 10°), it hits your panel at a shallow angle — like shining a torch sideways onto a table. You get very little useful light. When the sun is high up at noon (say 50°), the light hits your panels more directly, like shining the torch straight down.
sin(50°) ≈ 0.77. That means at solar noon in summer, your panel is receiving about 77% of what it would in theoretical perfect conditions. sin(10°) ≈ 0.17. Late afternoon in winter, you are getting about 17%.
Add those slices up across a full day and you get a number like 3.4 — meaning the day was equivalent to 3.4 hours of peak sunshine. That is your peak sun hours figure.
Typical peak sun hours across the UK
Because we are sampling real sun geometry rather than using a single lookup table, the figures update daily. But as a general guide, here is what to expect across UK cities at different times of year:
| City | Latitude | Summer avg PSH | Winter avg PSH | Annual avg PSH |
|---|---|---|---|---|
| Brighton | 50.8°N | 5.8h | 1.3h | 3.5h |
| London | 51.5°N | 5.6h | 1.2h | 3.4h |
| Southampton | 50.9°N | 5.7h | 1.3h | 3.5h |
| Bristol | 51.5°N | 5.5h | 1.2h | 3.3h |
| Birmingham | 52.5°N | 5.4h | 1.1h | 3.2h |
| Manchester | 53.5°N | 5.2h | 1.0h | 3.0h |
| Leeds | 53.8°N | 5.1h | 1.0h | 3.0h |
| Edinburgh | 56.0°N | 4.8h | 0.8h | 2.7h |
| Glasgow | 55.9°N | 4.8h | 0.8h | 2.7h |
The difference between Brighton and Glasgow is not dramatic in summer — a few panels in Glasgow on a clear July day will do almost as well as the same setup in Brighton. But in winter the gap widens considerably, and because winter days are so short at northern latitudes, the annual total diverges. Brighton gets around 30% more useful solar energy per year than Glasgow.
Solar noon and why it matters for panel placement
Solar noon is the moment each day when the sun reaches its highest point in the sky. It is not always at 12:00 — it shifts through the year and varies by your distance from the centre of your time zone. In the UK it typically falls between 12:10 and 13:10 depending on the time of year and where you are.
Solar noon matters because it is the moment your south-facing panels generate the most power. A panel that is shaded at solar noon — by a chimney, a neighbouring wall, an overhanging roof — loses more output than a panel shaded at 9am or 4pm. Understanding when solar noon falls for your city helps you work out whether your planned mounting location will be clear at the critical hour.
Our calculator displays the solar noon time for your city so you can think about this before you buy.
The azimuth: which direction should panels face?
The azimuth is the compass direction the sun is pointing from — measured in degrees, where 0° is north and 180° is due south. In the UK, the sun always travels through the southern sky. It rises roughly in the south-east, reaches maximum height due south at solar noon, and sets in the south-west.
This is why south-facing panels are the gold standard. They catch the sun throughout the day at the angle closest to perpendicular. Panels facing east get good morning output but miss the afternoon. West-facing panels are the reverse. North-facing panels in the UK are genuinely poor — they spend most of the day facing away from the sun.
South-east and south-west orientations lose only around 4% of annual output compared to due south. If your balcony or wall faces SE or SW, you are in excellent shape. East or west loses around 20%. If you can prop panels at an angle to face more south, even a partial rotation makes a difference.
Optimal tilt: the angle from flat to vertical
Lay a panel flat on the ground and it points straight up. Stand it upright and it faces the horizon. Somewhere in between is the angle that maximises annual energy capture for your location.
For the UK, the standard formula (from Duffie and Beckman's Solar Engineering of Thermal Processes) gives an optimal fixed tilt of roughly latitude × 0.76 + 3.1 degrees. For London at 51.5°N, that works out to about 42°. For Edinburgh at 56°N, it is around 46°.
Those figures assume you want to maximise output across the whole year. If you want to skew generation towards summer — when solar panels on a smart tariff can save the most — you would flatten slightly. If you want to catch more winter sun for heating or battery charging, you would steepen.
In practice, most plug-in solar kits use a fixed bracket at around 30–35° — slightly shallower than the theoretical optimum but fine for balcony and wall installations where a steeper angle would catch too much wind load.
How all of this feeds into your savings estimate
The sun position data does two things in our calculator. First, it provides a live reference showing you today's solar conditions for your city — sunrise, sunset, solar noon, peak sun hours, and the ideal tilt — so you can see how today compares to the annual average.
Second, it validates and contextualises the annual generation figure. The core savings calculation uses PVGIS data (the EU's Photovoltaic Geographical Information System, based on years of satellite observations) for a calibrated annual yield per city. The SunCalc layer adds the daily picture and confirms that the orientation factor you have selected is sensible for your actual location.
Together they give you a number you can trust — not a generic "typical UK household" figure, but an estimate grounded in the actual geometry of the sun's path over your city, your panel orientation, and the price you currently pay for electricity.
London's PVGIS base yield for an 800W south-facing system at 35° tilt is 760 kWh/year. At the current Ofgem rate of 27.69p/kWh, self-consuming 75% of that generation (570 kWh) saves £158 per year. The SunCalc panel shows that today's peak sun hours in London are around 3.5h — on track with the annual average. Payback on a £499 kit: approximately 3.2 years.
See your city's solar profile now
Enter your city, panel orientation, and electricity rate. Our calculator shows today's peak sun hours alongside your personalised savings estimate.
Open the CalculatorFree. No account needed. Takes about 30 seconds.
What it cannot tell you
Sun position data tells you how much solar energy is geometrically available at your location. It does not tell you whether that energy reaches your panel on any given day — clouds, haze, and pollution all reduce actual irradiance below the theoretical maximum. The UK has variable cloud cover throughout the year, which is why PVGIS satellite data (which averages real measurements over many years) is more reliable than pure astronomical calculation for predicting annual output.
Sun position also does not account for shading. A panel in full sun for six hours but shaded during solar noon will perform worse than a panel with partial cloud that is never completely shaded. If you have any doubt about shading on your planned installation spot, the solar noon time displayed in our calculator tells you exactly when it is most critical for your panel to be in direct sun.
The bottom line
Tracking the sun is not just a technical flourish. It is what allows our calculator to show you that a south-facing panel in Brighton genuinely earns its money back faster than a west-facing panel in Edinburgh — and to explain precisely why. Solar panels are a long-term financial decision. The more accurately you can estimate your return before you buy, the more confident you can be in the purchase.
All the sun data in our calculator updates in real time from your browser. No account, no data sharing. Just your city, today's sun, and what it means for your bill.