Why filling the roof first time is cheaper than adding later

The fixed costs are the whole argument

A solar installation has two kinds of cost. There is the kit that scales with how much you buy, and there is everything else: scaffolding, the crew for the day, the inverter, the isolators, the cable runs, the grid application, the certification. That second group barely moves whether six panels go up or fourteen.

Labour alone is £2,051 on every job we price, identical across all three standard system sizes. Model the whole matrix and it collapses to one line: installed cost is roughly £5,227 plus £439 per kWp. On a small system, about two thirds of the price is fixed cost.

Solar only, from the OVO regional partner pricing matrix. Generation at the UK average of 998 kWh per kWp.

Now imagine coming back in three years

Say you fit six panels, the bills stay high, and you decide to add four more. The panels are the cheap part. What you pay for again is the scaffolding, another crew day, another set of electrical works, and in many cases a fresh application to your Distribution Network Operator. None of that gets cheaper the second time.

None of that is a reason to panic buy. It is a reason to design the whole system once, on your free design call, even if you phase the spend. Our G98 and G99 guide explains the threshold that decides which route your application takes.

But do I really need that much electricity?

This is the fair objection, and the answer changed when export rates did. On a legacy 4p export rate, generation you cannot use is close to worthless and matching the array to the household makes sense. At 20p, surplus stops being waste and starts being income.

A 14 panel system exports roughly 3,500 kWh a year. At 20p that is about £700 back to you annually, from panels that cost a few hundred pounds each to add once the scaffold and the crew are already paid for. The full conditions are in our export rate guide.

A battery changes the picture again. Without storage, most homes self consume roughly a quarter to a third of what they generate. With it, that typically rises to somewhere between 70 and 85%, which is why the case for a bigger array strengthens once a battery is in the design. That sequencing is set out in the solar and battery strategy.

The one thing that can stop you

Roof space is usually not the ceiling. The grid connection rules can be. On a single phase home the inverter is capped at 3.68kW without prior approval from your network operator, and above that you need permission before the work happens. The threshold applies to the inverter rating rather than the panel capacity, so a well designed system can carry meaningfully more panels than the number suggests.

Whether your home is single or three phase changes that limit substantially, and it takes about a minute to check. Here is how to tell.

The order that works

1. Work out your daily electricity use from a bill rather than a guess. Most UK homes get through 8 to 12 kWh a day.
2. Size the battery to the part of that day the sun cannot cover, which for most homes is the evening peak.
3. Fill the roof, because the expensive parts of the job are already paid for.
4. Take the best export rate you qualify for and treat the surplus as income rather than spillage.
5. Do it in one visit, because scaffolding and crew days do not get cheaper with time.

Every area has a named OVO expert who does exactly this sizing exercise on the call. Find yours on the coverage page, or see the full price matrix in our cost guide.

OVO SEG Install Exclusive pays 20p per kWh where OVO installs both solar and battery, or 15p per kWh for solar only. The exclusive rates require you to take your electricity supply from OVO and are subject to system size limits. OVO's standard SEG rate of 4p per kWh is open to customers of any supplier. Rates correct at July 2026 and subject to change.