Sizing your solar system
Getting the right size solar system is one of the most important decisions you’ll make. Too small and you’re leaving generation potential on the table. Too large and you’ve overspent on capacity you can’t use effectively. Here’s how to think through the sizing question properly.
Start with your electricity consumption
Your annual electricity usage is the starting point for everything. Check your bills or your Octopus app dashboard. Ofgem’s current typical domestic consumption value is 2,700 kWh per year, but that number varies enormously. A small flat with two people might use 1,500 kWh. A large family home with an EV, heat pump and home office could easily hit 6,000-8,000 kWh.
You don’t necessarily need to match your solar generation to your consumption (export payments mean surplus isn’t wasted), but understanding your baseline helps you make sensible decisions about system size. Our payback calculations page walks through the financial side in detail.
How much do panels generate?
In the UK, each kWp (kilowatt peak) of solar panels generates roughly 850-1,000 kWh per year. The exact figure depends on several factors:
- Location: Southern England gets more sunlight hours than northern Scotland. A system in Devon might produce 950 kWh per kWp, while the same system in Aberdeen might manage 800 kWh per kWp.
- Roof orientation: South-facing roofs are ideal and achieve the reference figures above. East or west-facing roofs still work well, typically producing 80-85% of what a south-facing roof achieves. North-facing roofs are generally not worth installing panels on.
- Roof pitch: The optimum angle in the UK is around 30-35 degrees from horizontal. Most pitched roofs are close enough that it makes little practical difference.
- Shading: Trees, chimneys, neighbouring buildings and dormer windows all cast shadows. Even partial shading on one panel can reduce output across a whole string if your system uses traditional string inverters (microinverters or optimisers handle shading much better).
Panel sizes and roof space
A modern solar panel is typically rated at 400-420W and measures roughly 1.7m by 1.1m (about 1.7 square metres). So for a 4kW system, you need approximately 10 panels, taking up around 17 square metres of roof space.
Most standard UK semi-detached houses can fit 8-12 panels on a single roof face. Detached houses often have room for more. Terraced houses with smaller roofs might be limited to 6-8 panels.
Here’s a quick reference:
| System size | Approximate panels | Roof space needed | Annual generation (UK avg) |
|---|---|---|---|
| 2 kW | 5 panels | ~8.5 m² | 1,700-2,000 kWh |
| 3 kW | 7-8 panels | ~13 m² | 2,550-3,000 kWh |
| 4 kW | 10 panels | ~17 m² | 3,400-4,000 kWh |
| 5 kW | 12 panels | ~20 m² | 4,250-5,000 kWh |
| 6 kW | 14-15 panels | ~25 m² | 5,100-6,000 kWh |
Self-consumption: the number that matters
How much of your solar generation you actually use in your home directly affects your financial return. Self-consumed electricity saves you the full import rate (around 27.7p/kWh on a standard tariff at the Q1 2026 price cap). Exported electricity earns you the export rate (15p/kWh on Outgoing Octopus, dropping to 12p from March 2026). The gap between those figures means every extra unit of self-consumption is worth roughly 16p more to you.
Without a battery, most households self-consume 30-50% of their generation. If you work from home or have high daytime electricity use (EV charging, heat pump), you’ll be at the higher end.
With a battery, self-consumption rises to 60-80%. The battery captures surplus daytime generation and releases it in the evening when you actually need it.
This is why an oversized solar system without a battery can have diminishing returns. Past a certain point, you’re just exporting more and more at the lower export rate rather than using it yourself.
Sizing a battery
If you’re adding a battery, the general rule of thumb is to match it to your evening and overnight electricity consumption. Think about how much electricity your household uses between 4pm and 7am the next morning. That’s roughly what your battery needs to cover.
A 5 kWh battery stores enough for a few hours of typical household consumption. It’ll take the edge off the evening peak and maybe get you through to bedtime, but you’ll still be importing overnight.
A 10 kWh battery is the sweet spot for most households. It comfortably covers an evening and most of the overnight period. On a summer day, your solar panels will often fill a 10 kWh battery completely.
A 13.5 kWh battery (like the Tesla Powerwall 3) gives you a bigger buffer and more flexibility, particularly in winter when solar generation drops off. It also means you can store more cheap overnight grid electricity if you’re on a time-of-use tariff like Flux.
Going above 13.5 kWh is possible with modular systems but the additional cost rarely pays back quickly enough to justify it. The first 10 kWh of battery capacity does the heavy lifting. Each additional kWh above that has diminishing returns because there’s less surplus energy to capture. For more on charge and discharge approaches, see battery storage strategies.
MCS certification
MCS (Microgeneration Certification Scheme) certification is not optional if you want to receive export payments. The Smart Export Guarantee requires that your installation is MCS certified, meaning an MCS-accredited installer must carry out the work using approved equipment.
Beyond enabling export payments, MCS certification also means your installation meets recognised quality standards. It covers system design, equipment specification, installation quality and post-installation documentation. Any reputable solar installer will be MCS accredited, so this is mainly something to verify rather than worry about.
Your installer will register the system with MCS after installation and provide you with an MCS certificate. You’ll need this when applying for an export tariff from Octopus.
Planning permission
Good news: most domestic solar panel installations in England and Wales are classified as permitted development, meaning you don’t need planning permission. There are a few exceptions:
- Listed buildings require listed building consent
- Conservation areas may have restrictions, particularly for panels visible from the road
- Flat roofs where panels would be mounted on a frame that projects more than 0.2m above the roof surface
- Panels that protrude more than 0.2m beyond the roof plane
In Scotland and Northern Ireland, the rules are slightly different but broadly similar. If in doubt, check with your local planning authority before committing. Your installer should also flag any potential issues during the site survey.