Sizing your solar system

Getting the right size solar system is less about chasing the biggest panel count and more about matching your roof, daytime use, battery plan and export tariff. A good quote should explain those trade-offs clearly rather than just showing a headline annual saving.

Start with your electricity consumption

Your annual electricity use is the starting point. Check your bills, Octopus app data or smart-meter history. Ofgem’s typical domestic electricity consumption value is 2,700 kWh a year, but that is only a benchmark. A small flat can use far less. A larger home with an EV, heat pump, home office or battery can use much more.

You do not have to match annual solar generation to annual consumption exactly. Summer export, winter import and battery behaviour all matter. The useful question is: how much of the generation can you use well, store sensibly or export on a decent tariff? Our payback calculations page covers the financial side in more detail.

Estimate generation by kWp, not panel count

Solar systems are normally sized in kWp, or kilowatt peak. In UK planning estimates, a well-sited system often produces roughly 850 to 1,000 kWh a year for each kWp installed, with better results on unshaded south-facing roofs in sunnier areas and weaker results where orientation, pitch or shading work against the system.

The main factors are:

• Location: homes in southern England usually have more solar yield than homes in northern Scotland.
• Roof orientation: south-facing roofs are usually best. East and west-facing roofs can still be useful, especially if they spread generation across morning and afternoon. North-facing roofs need a more careful installer case.
• Roof pitch: many pitched roofs are close enough to useful angles that pitch is rarely the only deciding factor.
• Shading: trees, chimneys, neighbouring buildings and dormers can materially reduce output. Optimisers or microinverters can help where shading affects only part of the array, but they do not turn a poor roof into a perfect one.

Ask installers for the annual generation estimate, the assumptions behind it and whether the design uses MCS-style postcode, pitch, orientation and shading factors.

Panel sizes and roof space

Modern domestic panels are often around 400W to 460W, so the same roof can now carry more capacity than older rule-of-thumb guides suggest. Energy Saving Trust says a typical home system is around 3.5 kWp and often needs about 10 to 20m² of roof space, usually six to twelve panels.

Use this as a rough planning guide only:

A small, unshaded roof can beat a larger but awkward roof. A split east-west system can also make practical sense if it lines up better with your household use.

Self-consumption: the number that affects the return

Self-consumption is the share of solar electricity you use in the home before it goes to the grid. It matters because each self-consumed unit avoids buying electricity at your import rate, while exported units earn the export rate instead.

At the time of writing, a standard capped electricity unit is around 24.7p/kWh under the Q2 2026 cap, while Octopus says fixed Outgoing Octopus export fell to 12p/kWh from 1 March 2026. Exact import rates vary by region, payment method and tariff. For the latest export options, check Octopus’s current export tariff pages as well as this guide.

That means every extra unit you use at home instead of exporting can be worth roughly 13p more, before allowing for your exact tariff. This is why the right system size depends on when you use electricity, not just how much you use in a year.

Without a battery, many homes export a meaningful share of summer generation. With a battery, a larger share can be shifted into the evening, but the battery still has to be big enough, efficient enough and cycled often enough to justify its cost. For a deeper explanation, see solar import, export and generation.

Sizing a battery

If you add a battery, size it around usable daily behaviour rather than the largest capacity the installer can quote. Think about:

• how much electricity your home uses from late afternoon to the next morning
• whether you want the battery mainly for solar self-consumption, cheap overnight charging, backup resilience or a mix of those
• whether the inverter can charge and discharge quickly enough for your tariff strategy
• whether your winter solar generation will actually fill the battery often enough

A small battery can take the edge off evening imports. A mid-sized battery can cover more of the evening and overnight period for many homes. A larger battery can suit high-use homes, EV households, heat-pump homes or people deliberately charging from cheap off-peak electricity, but extra capacity has diminishing returns if it often sits partly empty or partly unused.

MCS guidance also treats battery storage as a certified technology in its own right. A certified installer should explain the battery’s usable capacity, charge and discharge power, warranty, expected cycles, handover pack and whether you will receive an MCS certificate. More detail is in battery storage strategies.

Do not size solar and export separately

Export can rescue surplus generation, but it should not be an afterthought. GOV.UK says Smart Export Guarantee applicants normally need a meter capable of half-hourly export readings, typically a smart meter, and an installation certified through MCS or an equivalent scheme such as Flexi-Orb. You apply directly to an export supplier, which does not have to be your import supplier.

For an Octopus household, this means checking:

• whether your smart meter and export MPAN setup are ready
• whether fixed Outgoing Octopus, Agile Outgoing or another export option suits your pattern
• whether any battery export strategy conflicts with your import tariff
• whether older Flux-style references still apply, as Flux is no longer a normal new-switcher recommendation

If export is a big part of the quote, ask the installer to show the export-rate assumption and how sensitive the payback is if that rate changes.

MCS, DNO and paperwork

MCS or an equivalent certification scheme matters because it is commonly needed for export payments and gives you a recognised installation record. Your installer should explain the design, register the system, provide the certificate and give you handover documents.

Your installation also needs the right Distribution Network Operator paperwork. Smaller domestic systems are often handled through standard notification or approval routes, but larger systems, batteries and export-capable setups may need more formal approval before or after installation. Do not treat this as a paperwork detail after the panels are fitted.

Planning permission

Most domestic rooftop solar installations are still treated as permitted development, but the conditions matter. In England, panels normally need to sit as close as practicable to the roof and avoid projecting too far beyond the roof plane. Flat-roof systems, listed buildings, conservation areas, national parks and properties with special restrictions need a more careful check.

Rules can differ across England, Wales, Scotland and Northern Ireland, and local constraints can override the usual answer. A good installer should flag this during survey, but you can also check with your local planning authority before committing.

A sensible sizing process

1. Pull your annual and half-hourly electricity use if you have it.
2. Separate daytime use, evening use, EV charging, heat-pump load and any future changes.
3. Ask for generation estimates by kWp using your actual roof orientation, pitch, postcode and shading.
4. Compare a modest, medium and roof-maxed design rather than only one quote.
5. Model solar-only first, then add the battery and export assumptions.
6. Check MCS, DNO, planning and export-tariff requirements before signing.
7. Keep the final decision practical: the best system is usually the one you can use well, not the one with the biggest headline output.

If you are comparing Octopus tariffs alongside a solar quote, start with solar with Octopus, export rates explained and the tariff comparison tool.