Do not start with the product.
Almost every disappointing installation we hear about started with a decision to buy a thing rather than a decision to solve a problem. Work out what you are trying to fix first, because it changes the answer.
Do not size a system to your roof.
Size it to what your household actually uses and when it uses it. A large array over a house that is empty all day exports most of what it makes, and export is worth considerably less than the electricity you avoid buying.
Do not sign in the room.
A quotation that expires tonight is a sales technique, not a price. Anything genuinely worth doing is still worth doing next week, and nothing on this site will ever ask you to decide on the day.
Do not fix the heating before you fix the building.
If a property loses heat quickly, a new heating system inherits that problem. That applies to heat pumps most of all, and it is why our heat pump guide starts with the building rather than the machine.
Do not treat a headline payback figure as a forecast.
A payback number is only meaningful against a specific property, a specific pattern of use, a specific tariff and a specific date. We do not publish averages and call them estimates, and neither should anyone quoting you.