Put a savings calculator on a solar website and you have made a promise. Not a marketing promise — a specific number, which the homeowner will write down, and which you will be asked about at the survey.
Most of those calculators are wrong in the same direction, for the same two reasons, and both of them are choices rather than mistakes.
Reason one: pretending every unit is worth the same
A unit of electricity used in the house saves the import price — what the household would otherwise have paid to buy it. A unit exported to the grid earns the export rate, which is a good deal less. The spread between the two is roughly 2.2 times.
A calculator that blends them into one average is guessing which side of that spread its user falls on. And the answer varies enormously: a retired couple at home all day consume most of what they generate, while a household that leaves at seven and returns at six exports most of it. Same roof, same panels, materially different saving.
Blend the two rates and you are not modelling a house. You are modelling an average of houses, then quoting it to one.
Reason two: assuming energy prices keep climbing
The other lever is inflation. Assume energy prices rise a few per cent a year for twenty-five years and the lifetime figure inflates dramatically — on an assumption nobody can check, about a market nobody has successfully predicted.
Put both levers together and a lifetime number can roughly double. That is how two calculators, fed the same roof, produce figures that do not look like they belong to the same planet.
The bill arrives either way
Here is the part that is genuinely commercial rather than pedantic. An inflated figure does not lose you the enquiry. It wins you the enquiry. Then it loses you the job, later, in front of the customer, after you have spent the survey time.
Because the homeowner will check. They will put the same address into two other calculators, they will read a forum, and somewhere in that process your number becomes the outlier. At that point you are not discussing solar any more. You are defending your own website, and every other figure you have given them is suddenly in question too.
A lower, defensible number does something an inflated one cannot: it survives contact with the customer’s own arithmetic, and it makes everything else you say more credible.
What we do instead
SolarCheck is built to be reproducible at the kitchen table rather than impressive on the page.
- It measures the roof rather than guessing it. Usable area, pitch and orientation come from survey imagery for that address, and the array is sized to what the roof actually takes.
- The saving is split in two. What comes off the bill, and what the export tariff pays, shown separately — because they are not the same money.
- Self-consumption comes from the MCS tables. MGD 003: the same lookup an MCS-certified installer has to use for a performance estimate. Your website figure and your paperwork then agree instead of arguing.
- No invented energy inflation and no invented payback. It assumes zero price inflation, includes panel degradation, and will not quote a payback period from a system price it made up.
- Batteries priced separately. Storage is the most overstated upsell in the trade, so the uplift is shown on its own rather than folded into the headline.
It is still an estimate and it says so on screen. MGD 003 is explicit that it gives an average across similar properties rather than a prediction for one house. What it is accurate enough for is a conversation that does not fall apart when you turn up.
Why the honest number wins more work
Every solar enquiry is really one question: how much would I actually save? If your site cannot answer it, the visitor goes and finds someone who will — which is the argument for having a checker at all. But answering it badly is worse than not answering, because it costs you the survey as well as the job.
The installers who escape the price war are the ones whose numbers hold up. You cannot compete on being cheapest against a firm that has decided to be cheapest. You can absolutely compete on being the only quote the homeowner was able to verify for themselves.
And an enquiry that arrives through your own checker is yours alone. It is not the same homeowner sold to three other firms that morning, which is the practical difference between generating your own and buying shared leads.
See what this looks like for solar installers, or book fifteen minutes and we will run it against a real roof on your patch.
Grant figures on this page were checked against gov.uk on 30 August 2026 and are indicative — the binding figure is the one from the official application. Spotted something out of date? Tell us and we will fix it.