The most common objection to solar in the UK is the weather. It is a fair question. But the data on how much electricity solar panels actually generate in the UK might surprise you, and the answer has a bigger effect on plug-in solar payback times than most people realise.
How solar output is measured
Solar generation potential is measured in peak sun hours, which represents the number of hours per day when sunlight intensity is enough to produce maximum rated output from a panel. A location with four peak sun hours per day would generate 4kWh of electricity from a 1kW system on an average day.
The UK averages between 2.5 and 4 peak sun hours per day depending on location and season. That is significantly less than southern Spain or Italy, but comparable to Germany, which has over a million plug-in solar installations and growing.
Regional differences across the UK
The south of England gets noticeably more solar radiation than Scotland or Northern Ireland. Cornwall, Devon and the south coast receive around 1,200 to 1,400 kWh of solar energy per square metre per year. Central England typically sees 1,000 to 1,100 kWh. Scotland averages around 900 to 1,000 kWh in most areas.
For an 800W system, this translates to annual generation of roughly 500 to 700 kWh in the south of England, 400 to 600 kWh in the Midlands and north of England, and 350 to 500 kWh in Scotland. Even at the lower end of that range, the savings are meaningful relative to the cost of a kit.
Seasonal variation
UK solar output is highly seasonal. In June and July a well-positioned system can generate three to four times as much electricity per day as it does in December. This is worth bearing in mind when estimating your annual saving. The peak generation months of April through September account for the majority of your annual output, and summer is when the payback clock really starts to run.
Does it still make sense in the north?
Yes, though the payback period is slightly longer. At 400 kWh per year and 27.69p per kWh, a system in a less sunny part of the UK saves around £110 annually. At a kit cost of £500, that is still a payback of around four to five years, after which the electricity is essentially free. The system will typically last 20 years or more with no maintenance beyond occasional cleaning.
The orientation factor
Where you live matters less than which way your panels face. A south-facing balcony in Manchester will outperform an east-facing balcony in Brighton. If you have a choice of where to mount your panels, prioritise south-facing over any other variable.