Ranked roughly by how much difference each makes, so you know which to worry about.
1. Shading
The biggest single factor and the one most often glossed over in a quote.
On a conventional string system, one shaded panel affects the others wired with it. A chimney shadow moving across a roof through the afternoon can cost far more than the area it covers.
It is manageable with optimisers or microinverters, at extra cost. But it has to be identified first. See shading.
2. Orientation
South is the reference point. South-east and south-west lose a little. East and west typically lose around a fifth of annual output, though they spread generation across more of the day, which can suit your usage better than the raw total suggests.
North-facing is generally not worth doing.
3. System size
Obvious but worth stating: more panels generate more. Roughly 850 to 1,000 kWh per kW per year for a well-oriented unshaded roof in the UK. See what size do I need.
4. Time of year
British solar is strongly seasonal. The large majority of annual output falls between April and September, and a December day may produce a tenth of what a June day produces.
This is normal and not a fault. It is also why the first winter causes so many unnecessary calls to installers.
5. Roof pitch
Most UK roof pitches, somewhere around 30 to 40 degrees, are close to optimal. Very shallow and very steep both cost something. A flat roof needs a mounting frame to set the angle, which works fine but adds cost.
6. Location
Southern England receives more solar energy than northern Scotland, but the difference is smaller than people expect and it is not a reason not to install in the north.
7. Temperature
Counter-intuitively, panels produce slightly less when hot. A cool bright day can out-produce a hot one. The temperature coefficient on the datasheet describes this. It matters less in Britain than almost anywhere else.
8. Dirt
Rain does most of the cleaning on a normally pitched British roof. Worth attention if the pitch is shallow, if the roof is under trees, or if you are near something that produces a lot of dust. See cleaning panels.
9. Age
Output falls slowly each year. Over twenty-five years that adds up, but it is gradual. See degradation.
10. System losses
Cabling, inverter efficiency and connections all lose a little. A good quote accounts for these in the estimate rather than quoting the panels’ rated output.
What you can change
Most of this list is fixed by your house. What you can actually influence:
- Shading, by managing trees or specifying optimisers or microinverters
- System size, within the roof and grid connection limits
- Which roof faces are used
- Cleaning, occasionally
- Noticing faults early, by looking at the monitoring
And separately from output, the thing that most affects the value of what you generate is how much of it you use at home rather than export. That is usually worth more attention than squeezing another few per cent out of the panels. See how much you will export.