How to size a solar system: how many kW (and panels) do you need?
Published 21 June 2026
Divide your annual electricity use in kWh by your location's specific yield (kWh per kWp per year) to get the kWp to install, then divide kWp by your panel wattage for a panel count. A UK home using 4,000 kWh at a 900 yield needs roughly 4.4 kWp, about ten 440 W panels.
To size a solar system, divide your annual electricity use (kWh) by the specific yield for your location (kWh per kWp per year). That gives you the kilowatts-peak (kWp) to install. Divide by your panel wattage to get a panel count. The whole calculation takes five minutes.
Below you will find each step, a roof-area check, the oversizing trap most guides skip, and a full worked example for a UK home.
Start with your annual kWh
Find your yearly electricity consumption on your annual statement, your smart meter app, or your online account. Add up four quarterly bills if that is all you have.
A flat for one or two people might use 1,800 kWh a year. A family house with teenagers and a dishwasher running daily sits closer to 4,500 kWh. Add an electric car or a heat pump and you can pass 7,000 kWh. Use your real number. The national average misleads in both directions.
Convert kWh to the kWp you need
Every 1 kWp of installed panels produces a yearly amount of electricity that depends on your location. Engineers call this the specific yield, measured in kWh per kWp per year. It varies with latitude, roof angle, and orientation.
Approximate specific yields for a well-oriented south-facing roof (PVGIS, European Commission):
| Region | Specific yield (kWh/kWp/year) |
|---|---|
| Southern Spain (Andalusia, Seville area) | 1,600–1,800 |
| Portugal, Southern Italy, Greece | 1,300–1,600 |
| Central Spain, Northern Italy | 1,200–1,400 |
| Germany, Netherlands, Northern France | 950–1,050 |
| United Kingdom and Ireland | 850–1,050 |
The Germany and Netherlands range of 950–1,050 kWh/kWp reflects regional variation within each country: southern Germany reaches the upper end while the Netherlands clusters toward 875–950 (PVGIS data download, JRC European Commission, 2025).
The UK range spans roughly 750 kWh/kWp in northern Scotland to 1,050 kWh/kWp on the south coast (Cornwall), with a national average near 900 kWh/kWp (Payaca UK Solar Yield Calculator, MCS irradiance data MIS 3002, 2026; PVGIS). England and Wales typically land between 850 and 950 kWh/kWp.
To size for full annual coverage, divide your usage by your local yield:
kWp needed = annual kWh ÷ specific yield
A London household using 4,000 kWh a year, at a yield near 900, needs roughly 4.4 kWp. The same household in Seville needs about 2.5 kWp for an identical bill, because the sun does more of the lifting. A UK-default or US-default sizing rule fails you the moment you live somewhere else. Our calculator pulls yield figures directly from PVGIS and NASA climate data, so the estimate reflects your actual coordinates rather than a national guess.
Turn kWp into a panel count
Residential panels today are rated between 400 W and 450 W each. The current sweet spot for UK rooftops is around 410–440 W (108-half-cut TOPCon format). Pick a wattage and divide:
panels = (kWp × 1,000) ÷ panel wattage
The 4.4 kWp London system, built with 440 W panels, needs ten panels. Round up to fit your roof and your inverter. The final count shifts once an installer measures your actual obstructions.
Will it fit? The roof-area rule of thumb
A modern 400–440 W panel covers roughly 1.7–2.0 m², depending on the model. Installers budget around 2 m² of roof footprint per panel once they allow for the small gaps required by mounting clamps and edge setbacks (confirmed by MCS installation guidance and UK installer practice).
That works out to approximately 5–6 m² per kWp for a typical modern array. Ten panels want around 20–22 m² of clear, well-angled roof.
Walk outside and look at your largest roof face. A chimney, a vent stack, a satellite dish, or the shadow of a neighbour's tree will each carve out usable space. A south-facing slope is the prize in the northern hemisphere. East and west faces still work: expect to lose 15–20% of yield compared to due south, which you can offset by adding a panel or two.
The oversizing question: read your export rules first
You could install a larger system than your usage suggests. Whether you should depends on what your grid pays you for the surplus you export.
You consume only part of your generation as it happens. The rest goes back to the grid. In the UK, the Smart Export Guarantee (SEG) pays a per-kWh rate set by each supplier: best tied rates run 20–25p, a typical flat rate is 12–15p, and the worst tariffs pay 1–2p (Which?, April 2026). Australia's feed-in tariffs have been falling for the same reason, now mostly 3–8c/kWh by state. Germany and the Netherlands still offer statutory feed-in rates, but these are also declining.
When your export rate is well below your import rate, every exported kWh earns you far less than a self-consumed one saves you. So the sweet spot is a system sized close to what you actually use during daylight, not one that maximises raw generation. A battery shifts more output into self-consumption and can justify a larger array, though it adds cost you should test before committing. See are solar batteries worth it before you assume storage closes the gap.
Self-consumption without a battery
Without a battery, a typical home self-consumes 30–40% of generation (midpoint ~35%). The rest exports. Working from home pushes that figure toward 45–55%; being out all day drops it toward 15–25%. That split (not the headline generation figure) drives your actual payback, because self-consumed units save you the full import rate while exported units earn the (usually lower) export rate. The 30–40% range is consistent with a UK peer-reviewed field study of 302 households that found average self-consumption of around 45% of generation, meeting roughly 24% of an average household's annual demand (McKenna, Pless and Darby, Energy Policy, 118, 2018).
The UK export cap: G98 and G99
Your grid operator caps how much you can export without extra approval. In the UK, ER G98 (Issue 2, March 2025) covers systems at or below 3.68 kW per phase, equivalent to 16 A at 230 V. Systems within this limit use a fit-and-inform process: your installer notifies the DNO within 28 days of commissioning. Above 3.68 kW per phase, you need a G99 application, which can add months to the process.
Most domestic systems sit comfortably under the G98 threshold. A larger array paired with export limitation (capping the inverter's export at 3.68 kW) lets you install 5 kW or more while staying on the simpler G98 route. Worth discussing with your installer.
A worked example: Manchester, 3-bedroom house
Take a three-bedroom house near Manchester. The annual statement reads 4,200 kWh. The roof faces south-east with about 28 m² clear.
| Step | Calculation | Result |
|---|---|---|
| 1. Specific yield (Manchester, near-optimal south-facing roof) | PVGIS data | ~880 kWh/kWp |
| 2. kWp needed | 4,200 ÷ 880 | 4.8 kWp |
| 3. Panel count at 440 W | (4,800 ÷ 440) → round up | 11 panels |
| 4. Roof area check | 11 × 2 m² = 22 m² | Fits in 28 m² |
| 5. G98 check | 4.8 kWp, single phase | Under 3.68 kW export limit in practice; no G99 needed |
This household lands on an 11-panel, 4.8 kWp system. Without a battery, roughly 30–40% of the 4,200 kWh generated is used directly; the rest exports at the SEG rate. That split (self-consumption saving roughly 28p/kWh in the UK versus export earning roughly 15p/kWh) is what drives the real payback. Use the calculator for your country's tariff figures. We model both streams separately, then show payback, NPV, and IRR rather than a single rosy year-one figure. See solar payback, NPV and IRR explained for how those numbers work.
Frequently asked questions
How do I find my annual electricity use in kWh?
Check your annual statement from your supplier, open your smart meter app, or log into your energy account online. If you only have quarterly bills, add all four together. Use your actual figure. National averages (around 2,700 kWh/year for a UK home, falling to 2,500 from July 2026) can be off by a factor of two depending on household size and habits.
What is specific yield and why does it matter?
Specific yield is the annual output per kilowatt-peak of installed capacity, measured in kWh/kWp/year. It captures your local climate: latitude, cloud cover, and typical roof angle. A system in Seville produces nearly twice as much per kWp as one in Edinburgh — about 1.8 times. Use the wrong yield figure and your sizing estimate is wrong from the first calculation.
How many solar panels fit on a typical UK roof?
A standard semi-detached house with a clear south-facing slope of around 20–25 m² can typically fit 10–12 modern panels (400–440 W each), giving a system of around 4–5 kWp. That covers the annual use of a household consuming 3,600–4,500 kWh/year, assuming a specific yield near 900 kWh/kWp for central England.
Do I need planning permission or DNO approval for solar in the UK?
Most residential solar installations in England and Wales are permitted development, so no planning permission is needed. You do need to notify your Distribution Network Operator (DNO). Systems at or below 3.68 kW per phase fall under ER G98 (fit-and-inform, 28-day notification). Larger systems need a G99 application, which can take several months.
Should I oversize my solar system?
Oversizing makes sense when your export rate is close to your import rate, you plan to add an EV charger or heat pump, or you can add export-limiting hardware to stay within G98. It makes less sense when your export rate is well below your import rate and you have no storage, because every extra kWp then generates units that earn you far less than they would save. Run the numbers for your tariff before committing to a larger array.
Conclusion: sizing your solar system
Sizing a solar system comes down to one calculation: annual kWh divided by your local specific yield, then a panel count and a roof-area check. The two figures that matter most are your actual consumption and your location's yield, not a generic rule of thumb. Get those right and the rest of the arithmetic follows.
The economics (whether the right-sized system actually pays) turn on your import tariff, your export rate, and how much you use while the sun is up. You can model all three for your own roof in under a minute.
Estimate only, not a quote. Figures use local climate data and average tariffs; your installer's quote will be exact.
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