How the number is worked out
Three things decide how much a solar array produces in the UK: how big it is, which way it faces, and how much of the sky it can see. Everything else is detail.
The calculator starts from how much electricity you actually use, then works backwards. It finds the array size whose annual output roughly matches your annual consumption, and then caps that at whatever your roof can physically hold. Whichever of those two limits bites first is the answer you get.
Why bedrooms are a bad way to size solar
Plenty of sites will tell you a three-bedroom house needs ten panels. Bedrooms are a proxy for consumption, and a weak one. Two people in a four-bedroom house with a gas boiler may use less electricity than a family of five in a terrace with an electric car. Ofgem’s own typical figures changed on 1 July 2026 — medium usage fell from 2,700 kWh a year to 2,500 kWh — which tells you how much these averages drift.
Your annual electricity statement has the real number on it. Using that instead of a rule of thumb is the single biggest improvement you can make to an estimate like this one.
What the orientation figures mean
A south-facing roof is the benchmark. Turning the array away from south costs output in a fairly predictable way, and the factors used here follow the orientation data in SAP Appendix H: south-east or south-west keeps about 95%, east or west about 80%, and north drops to roughly 60%.
That last figure surprises people. A north-facing roof is not useless — it is just considerably worse value per panel, and it changes whether the job is worth doing at all. Many houses can split an array across two slopes, which improves matters and which no online calculator can model properly.
Generation is not the same as saving
A unit you generate and use yourself is worth the full price you would otherwise pay for it — 26.11p under the July to September 2026 price cap. A unit you export is worth whatever your Smart Export Guarantee tariff pays, and the best widely-available rate is currently around 15p, with some suppliers paying as little as 4p.
This is why self-consumption matters more than raw output. Without a battery a typical home uses only about a third of what it generates, because most of the generation happens in the middle of the day when nobody is home. That is the single biggest reason two identical arrays can save very different amounts.
Where this estimate stops
An MCS-certified design does not use three regions and four compass points. It uses irradiance tables covering 21 UK zones, with a value for every combination of roof pitch and orientation, and a shading assessment made on the roof rather than guessed from a dropdown. It also accounts for the things that decide whether a job is even possible: the state of your roof covering, your consumer unit, cable runs, and where an inverter can safely live.
So treat the number above as a sanity check. It is good enough to tell you whether solar is worth a conversation, and roughly what scale of system and cost you are talking about. It is not good enough to buy from — nobody’s online calculator is, including ours.
Where ByEco stands on this. We are going through MCS and TrustMark certification. Until that lands we can survey, design and quote, and we will tell you plainly what we can and cannot certify at the point you ask. Solar and heat pump installations are zero-rated for VAT until 31 March 2027.