Sizing Your System

Start with self-consumption

Every unit you use in the house is worth the full price you pay for electricity. Every unit you export is worth the export rate, which is typically a fraction of it. So the question that decides your return is not “how much will this generate?” but “how much of it will I actually keep?”

Without storage, a typical household keeps somewhere between a quarter and forty per cent of what it generates. With a battery sized against the evening load, that figure rises substantially. This is the whole argument for storage, and it is why we ask for your bill before we count panels.

How big an array?

Roof area sets the ceiling; your consumption sets the sensible answer. As a rough guide, a well-oriented unshaded roof in this region generates in the order of 900 kWh per kWp per year — so a 4 kWp array is modelling around 3,600 kWh annually. Compare that against your own annual figure and the shape of the decision becomes obvious.

Bigger is not automatically better. Beyond the point where you can use or store what you make, extra panels are earning export rate only, and the payback on that last increment is much longer than the payback on the first.

How big a battery?

Battery sizing is a question about your evenings. We are looking for how much electricity you use between roughly four in the afternoon and midnight, and how consistent that is across the year.

Oversized, and you have paid for capacity that sits empty. Undersized, and you are back on the grid by eight o’clock in winter. Where a time-of-use tariff is available the calculation changes again, because the battery earns its keep charging cheaply overnight regardless of the weather.

Orientation and pitch

South-facing at around 35° produces the highest annual total. East–west arrays produce less in total but spread it across morning and evening, which frequently suits a household better and lifts self-consumption. We model your actual roof rather than applying a rule of thumb, because the rule of thumb is answering a different question from the one you care about.

Things that change the right answer

• An EV, now or soon. The single biggest factor. A car is a large flexible daytime load and it converts surplus generation into fuel.
• A heat pump, now or soon. Moves a large electrical load onto your own generation and strengthens the case for a bigger array.
• Your tariff. A time-of-use tariff plus a battery can beat a bigger array on a flat rate.
• Who is home during the day. Shift patterns, home working and retirement all change self-consumption before any hardware does.
• Roof condition. Panels last decades. If the covering does not, deal with that first.

What we will not do

We will not quote a system size before we have seen your consumption, and we will not present a single confident payback figure as though the assumptions behind it were certainties. You get a range, and you get the assumptions, so you can challenge them — ours and anyone else’s.

Try the sizing model on the home page to see how array size, battery capacity and tariff move the numbers, then send us your annual kWh figure and we will model it properly.

Send us your annual kWh figure and a photo of the roof and we will tell you what is realistic before anyone visits.

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