Five minutes here saves a wasted appointment. Work through the four questions below and you will know whether to go ahead, and what to raise with installers if you do.
1. The roof
Direction South is best. South-east and south-west are close behind. East and west produce less in total, perhaps a fifth less than south, but they spread generation across more of the day, which suits a household using electricity morning and evening. A north-facing roof is generally not worth it.
Pitch Most UK roof pitches work well. Very shallow and very steep both cost a little output. A flat roof needs a mounting frame, which adds cost but is entirely normal.
Space As a rough guide, a 4kW system needs somewhere around 16 to 20 square metres of clear roof. Chimneys, vents, roof windows and valleys all eat into the usable area more than people expect.
Condition and age This is the one to be firm about. If the roof will need replacing within about ten years, do the roof first. Taking a system off and putting it back costs well over a thousand pounds and no installer will do it for free.
Covering Concrete tile is straightforward. Slate takes longer and is easier to break. Some coverings, including certain older asbestos-cement sheets, need specific handling. Mention what you have.
2. Shading
Shade matters more than most people realise, because on a conventional string system one shaded panel can drag down others connected with it.
Look at the roof at three points in the day. Note anything that casts a shadow: chimneys, dormers, aerials, neighbouring buildings, and trees, including trees that will be taller in five years.
Shading is not a deal-breaker. Optimisers or microinverters manage it at additional cost. But an installer who has not asked about shading has not produced a reliable output estimate. See shading.
3. How you use electricity
This matters as much as the roof and hardly anybody asks about it.
A unit of solar you use saves you the full import price, about 26.32p. A unit you export earns you roughly half of it.
So ask yourself:
- Is anyone home during the day?
- Could you move the washing machine, dishwasher or immersion heater into daylight hours?
- Do you have, or are you getting, an electric car or a heat pump? Both use a great deal of electricity and transform the sums.
- Would you consider a battery and a time-of-use tariff?
If the house is empty from eight until six and you will not add a battery, solar still works, it just works less well. That is worth knowing before somebody presents you with an optimistic savings figure.
4. Permission
Roof-mounted solar on a house is normally permitted development. The exceptions catch a lot of British housing:
- Listed buildings need listed building consent.
- Conservation areas have restrictions, often on panels visible from the road.
- Article 4 Directions remove permitted development rights in specific areas.
- Flats and maisonettes usually need freeholder consent, and the roof may not be yours.
- Scheduled monuments.
Rules differ across the UK. See can I install panels.
A quick verdict
Good candidate unshaded south, east or west roof in sound condition, someone home during the day or willing to add a battery, no planning restriction, staying put for years.
Worth doing but manage expectations partial shading, smaller roof, house empty during the day, no battery planned.
Probably not, for now heavy shading, north-facing only, roof needing replacement, moving soon, or a listed building where consent is unlikely. Consider plug-in solar or a battery on its own instead, both of which work without a suitable roof.
What to do next
Get quotes from up to 3 installers. Tell each one the same things, in particular about shading and your daytime usage, so you are comparing like with like. Take the questions list with you.