May to August delivers 60–70% of the UK's annual solar energy. Here's a month-by-month guide to UK solar generation and what it means for battery storage and energy bills.
The UK solar calendar
Solar generation in the UK is heavily front-loaded into summer months. The further north you are, the more extreme this seasonality becomes. Here's a rough guide to monthly output as a percentage of annual total for a south-facing UK system:
| Month | Typical share of annual output |
|---|---|
| January | 2–3% |
| February | 4–5% |
| March | 7–8% |
| April | 9–11% |
| May | 12–14% |
| June | 13–15% |
| July | 12–14% |
| August | 10–12% |
| September | 7–9% |
| October | 4–6% |
| November | 2–3% |
| December | 1–2% |
May, June, July and August together typically account for 47–55% of annual generation. Add April and September and you're looking at 60–70% of annual output in just six months.
Why summer is so dominant
It's not just about longer days. The angle of the sun above the horizon matters enormously. In June, the sun reaches a maximum elevation of around 62° in southern England — close to optimal for a 35° tilted roof. In December, that maximum elevation drops to just 15–17°, meaning the sun is barely above the horizon and solar panels receive a fraction of the energy even during the short available daylight.
Implications for battery storage
This seasonal pattern has significant implications for how you use a solar battery. In June and July, a 5 kWh battery might fill by mid-morning and spend the rest of the day diverting excess to the grid. In December, the same battery might only charge to 20–30% from solar alone.
If you're on a time-of-use tariff like Octopus Go or similar, winter is when overnight charging becomes most valuable. Summer is when solar self-sufficiency is achievable. The Sun Hours app's 7-day forecast helps you plan accordingly — for example, seeing a low-generation week ahead might prompt you to schedule overnight charging before prices rise.
Best months to maximise savings
For most UK households, the solar "golden period" runs from mid-April to mid-September. During this window:
- Solar generation typically exceeds household daytime consumption on sunny days
- Battery storage has the best chance of covering evening demand
- Export to the grid (if you have SEG tariff) is at its highest
- Hot water diversion (if you have an immersion diverter) is most effective
Outside this window, the contribution of solar shifts from "covers a lot of usage" to "reduces the bill meaningfully but doesn't dominate."
Tracking your seasonal performance
The Sun Hours app shows you both your 7-day forward forecast and historical data for your location, so you can see how solar irradiance has varied over the past year and anticipate what's coming. Checking the app in early spring is a good way to see when your "solar season" is about to kick off — so you can make sure your battery is set up to capture it.
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