SOLAR SIZING CALCULATOR
What size solar system do I need?
Answer five questions about your home and roof. You will get an indicative system size in kW, a panel count, and how much of your electricity it would cover. It takes about thirty seconds.
Ofgem’s typical values from 1 July 2026. Your annual statement shows your real figure — use that if you have it.
Not sure? Stand at your front door with a phone compass.
A panel is about 1.7m × 1.1m. A typical semi’s main roof slope takes 10–16.
RECOMMENDED SIZE
This is an estimate, not a design. A real system is sized from an on-site survey — measured roof, pitch, a proper shading assessment and your existing wiring. Treat this as a sanity check before you talk to anyone, including us.
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 for a household 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.
Common questions
How many solar panels do I need for a 3-bedroom house?
For a typical 3-bedroom home using around 2,500 kWh a year, roughly 8 to 12 panels (about 3.5 kWp to 5 kWp) will generate close to a year's worth of electricity. The exact number depends far more on your roof direction, shading and actual usage than on the number of bedrooms, which is why the calculator asks about those instead.
Is it worth fitting more panels than I need?
Sometimes. Panels beyond your own consumption still earn money through the Smart Export Guarantee, but export pays less than the electricity you avoid buying, so the return per panel drops once you pass your own usage. If your roof has the space and the cost per extra panel is low, it can still be worthwhile — especially if you later add a battery, an EV or a heat pump.
Does a north-facing roof rule solar out?
Not automatically, but it costs you a lot. Using the orientation factors in SAP Appendix H, a north-facing array produces roughly 60% of what the same array would make facing south. East or west roofs sit around 80%. Many homes split panels across two aspects, which the calculator cannot model — a survey can.
Will this number be the size you actually install?
No. This is an indicative estimate. A real system is sized from an on-site survey: measured roof dimensions, pitch, a proper shading assessment, the condition of your roof covering, and your consumer unit and cabling. MCS-certified design uses irradiance tables covering 21 UK zones for every combination of pitch and orientation, which is far finer than the three regions here.
Do I need a battery?
Only if you are exporting a lot of what you generate. Without a battery a typical home uses around a third of its own solar directly, because most generation happens while the house is empty. A battery lifts that considerably by storing daytime surplus for the evening. It also adds cost, so it improves your bill saving while usually lengthening payback.
Sources
- Ofgem — Review of typical domestic consumption values: decision, 27 May 2026 (electricity TDCV from 1 July 2026: low 1,600 kWh, medium 2,500 kWh, high 3,800 kWh).
- Ofgem — energy price cap, 1 July to 30 September 2026 (electricity 26.11p/kWh).
- SAP Appendix H — solar orientation factors.
- MCS MIS 3002 — solar PV design standard; irradiance tables covering 21 UK zones by pitch and orientation.
- Smart Export Guarantee tariff rates, current at September 2026.
Figures checked 1 September 2026. Estimates are indicative and do not constitute a system design or a quotation.