25 questions to ask your solar installer

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What is on it

1. Your roof — measure, do not guess. Pitch, orientation in degrees from south, covering, condition, whether it has been altered, and a grid for recording each shading obstruction with its direction and height. These are the inputs every generation figure depends on, and the ones most often assumed rather than measured.

2. The quote’s own numbers, laid out in the order MIS 3002 prescribes — capacity, orientation, inclination, postcode region, the kWh/kWp figure, the shade factor, the annual output, and the price per kWp.

3. Self-consumption, with the disclosures MGD 003 requires: occupancy archetype, assumed consumption, expected self-consumption in both per cent and kWh, and grid independence — plus the import and export rates assumed.

4. What you must be given before you sign. A checklist of the eight items MIS 3002 requires in writing at or before the point the contract is awarded.

5. The 25 questions, grouped by company, design, money and paperwork, and after the sale.

Reference tables. Annual output per kWp for all 25 of our UK locations at 35° facing south, and the percentage for every other pitch and orientation.

The one calculation that matters

Everything else on the sheet supports this:

Annual output ÷ kWp ÷ shade factor = implied kWh/kWp

Compare that against the reference table for your location, pitch and orientation. At 35° facing south the UK range is 692 to 1,042 kWh per kWp — so a quote implying more than about 1,050 is claiming a better result than anywhere in our dataset achieves.

That single division catches optimistic modelling faster than any amount of reading, and it is why the tables are printed on the sheet rather than linked from it.

Using it before you have quotes

Fill in section 1 and take it up to the loft. Measure the pitch against a rafter with a phone spirit level; check the orientation properly, because a roof most people call “south-facing” is frequently 20–30° off.

Then hand the same measured figures to every installer you approach. Three quotes built on three different assumed orientations are not comparable, and giving them the real numbers removes that variable before it appears.

Using it after

Fill in sections 2 and 3 from each quote and run the division. Then work down section 4 and mark what you were actually given.

Missing items are the useful finding. A quote with no shade factor on a visibly shaded roof, or no module layout drawing, or no sunpath diagram where the shade factor is below 1.00, was not modelled from your roof — and that is a specific, checkable thing to raise rather than a vague dissatisfaction.

Where it goes further than a conversation, see comparing quotes properly and, if a figure turns out to be unachievable, mis-selling.