A solar quote is usually a list of equipment, a drawing of your roof with rectangles on it, a savings figure in bold, and a payback period that sounds reasonable. The kit list is the least important part. These are the five numbers to find, and what each one should look like.
1. The system size, in kWp, against your annual usage, in kWh
The quote will state a size such as “4.4 kWp”. Your bills state what you use in a year, in kWh. In most of the UK a well-placed system produces roughly 800 to 1,000 kWh a year for every kWp installed, so 4.4 kWp is about 3,500 to 4,400 kWh.
If that is more than you use in a year, you will export most of it, and export is paid at a fraction of what you pay to buy electricity. A system sized to your roof rather than your usage is the most common error in a quote, and it is in the installer’s interest to make it: bigger systems cost more.
What good looks like: a size that is justified by your usage, with the reasoning written down, not just “we fitted as many panels as would go”.
2. The assumed yield, and where it came from
Somewhere, often in small print, the quote assumes how much the system will produce in a year. Look for “estimated annual generation” or similar. Check it against the rule of thumb above for your part of the country, and against the orientation of your roof. A north-facing or heavily shaded roof produces much less, and a quote that ignores that is a quote that will disappoint.
MCS-certified installers have to use a standard calculation method. Ask to see it. If the answer is a shrug, that tells you something.
3. The self-consumption percentage
This is the share of the generation you will use yourself rather than export, and it drives the savings more than anything else. Quotes routinely assume 50 per cent or more. For a household that is out all day, 25 to 35 per cent without a battery is more realistic. Every ten points of difference changes the savings figure substantially.
What good looks like: a self-consumption figure that reflects when you are at home and when you use electricity, and a savings calculation that shows it.
4. The price per kWp, with and without the battery
Divide the price of the solar part by the kWp. That gives you a figure you can compare across quotes and against published averages. Then look at the battery separately. A battery can add a large share of the total cost, and whether it pays for itself depends on your tariff and your evening usage, not on the solar. Ask for the quote with and without it. If the installer will not separate them, that is an answer too.
5. What is actually guaranteed, and by whom
Panels, inverter and battery each come with a manufacturer’s warranty. The installation itself has a workmanship warranty from the installer. The “performance guarantee” in the quote is usually the panel manufacturer’s degradation promise, not a promise that your system will produce the estimate on page one. Read who is promising what, for how long, and what you would need to show to claim.
What to do next
Take your last twelve months of bills, find your annual kWh, and compare it with the quote’s system size and estimated generation. If the generation is well above your usage and there is no battery, ask the installer why. If you would like an engineer to do the whole exercise on your actual usage and tariff, that is what the Second Opinion is for.