Solar Panels

How a domestic solar PV system works

Solar panels generate direct current when daylight falls on them — not heat, daylight, which is why they work on an overcast January afternoon, just less well. That DC electricity goes to an inverter, which converts it to the alternating current your house runs on and feeds it into the consumer unit.

From there it behaves like any other electricity in the house. Whatever is running at the time uses it first. Anything left over goes to a battery if you have one, and anything beyond that is exported to the grid.

The whole economic case rests on one distinction: electricity you use yourself is worth what you would otherwise have paid for it, and electricity you export is worth considerably less. Everything else — sizing, orientation, storage, tariffs — is really about shifting the balance between those two.

What a system is made of

Sizing: start with consumption, not the roof

System size is quoted in kWp — kilowatts peak — which is the output of the array under standard test conditions. It is a comparison figure rather than a promise; a 4kWp array does not produce 4kW for most of the day.

The common mistake is to size against roof area. Fill a large roof on a house that uses very little during the day and most of that generation is exported at a low rate. We size against your actual consumption pattern, which means asking what you use, when you use it, and whether anybody is home.

Orientation and pitch

South-facing gives the highest annual total. East and west produce less overall but spread it across the morning and evening, which frequently suits a household better and pairs well with storage. Pitch affects the seasonal curve: a steeper roof does relatively better in winter when the sun is low, a shallow roof relatively better in midsummer.

There is more detail in roof orientation and shading and solar on east-west roofs.

Shading

Shading is the factor most often assessed badly, because it changes through the day and through the year. A roof that is clear in June can lose two hours to a neighbour's chimney in December. Where shading is unavoidable, per-panel electronics limit the damage to the rest of the array — but they are a mitigation, not a cure, and on a heavily shaded roof the honest answer is sometimes that the roof is the wrong site.

The roof itself

Roof covering determines the fixing method. Concrete and clay tiles take a hook set into the rafter with the tile relieved so it sits back down flat. Slate is brittle and needs a different, more careful approach. Metal and profile sheet roofs use clamps or through-fixings depending on the seam. Flat roofs need a ballasted or mechanically fixed frame that raises the panels to an angle — a different job again.

Roof condition matters as much as covering. Putting a twenty-five year array on a roof with five years left in it creates an expensive problem later. If we think the roof should be dealt with first, we will say so — see removal and refit for roof works.

Property age is the other variable: homes built before 1920, 1920 to 1965 and post-1965 each bring different structures, coverings and electrical arrangements.

In-roof or on-roof

On-roof mounting sits the array above the tiles on rails. It is the standard approach, it is less expensive, and it can be retrofitted to almost any suitable roof.

In-roof mounting replaces a section of the covering with an integrated tray so the panels sit flush with the tile line. It costs more, it is far easier during a re-roof than as a retrofit, and in a conservation area or on a prominent elevation it is sometimes the difference between a scheme that suits the building and one that does not. Compared properly in in-roof versus on-roof solar.

The electrical side

Around the array sit the parts that make it safe and serviceable: DC isolation at the array and at the inverter, AC isolation, over-current protection, a generation meter and labelling that tells the next electrician what they are looking at.

The existing installation matters. On older properties it is common to find a consumer unit that is serviceable but full, or one due replacement anyway, and earthing arrangements that need attention. We assess this at survey and price it into the fixed quotation rather than raising it as a variation on the day.

Notification, certification and consent

DNO notification. Where total generation capacity is 16A or less per phase — about 3.68kW on a normal single-phase supply — the installation is notified to the network operator under G98 within 28 days of commissioning. Above that threshold a G99 application must be made and approved before installation, which takes time and has to be planned for. See G98 and G99 explained.

MCS. The installation is certified under MCS, which covers both the products and the installer. You need it to apply for an export tariff. See what MCS certification means.

Planning. Roof solar on residential buildings may be permitted development subject to limits and conditions. Listed buildings, conservation areas and World Heritage Sites carry additional restrictions and may require consent. See do you need planning permission and listed buildings and conservation areas.

Export, VAT and what the numbers depend on

The Smart Export Guarantee obliges larger licensed electricity suppliers to offer an export tariff. Suppliers set their own rate, contract length and terms — Ofgem does not set the rate — and rates must be above zero. Certification is required to qualify. See the Smart Export Guarantee explained.

VAT. Installation of qualifying energy-saving materials in residential accommodation in Great Britain is zero-rated from 1 May 2023 to 31 March 2027, reverting to the 5% reduced rate from 1 April 2027. Electrical storage batteries were added to the qualifying list from 1 February 2024. See VAT on solar panels.

Actual generation, savings and payback depend on the property, roof orientation and pitch, shading, system design, your electricity usage and tariff, export rates and future energy prices. Figures shown are estimates and are not guaranteed.

What that means in practice is set out in what affects the cost of solar panels and how payback is calculated.

Maintenance and lifespan

Panels need very little. The parts that need attention are the ones people forget: isolators, connectors, clamps and the inverter, which is the component most likely to need replacing within the life of the array. Birds nesting under panels are a common and avoidable problem.

Covered in maintenance and servicing, cleaning, bird proofing and inverter replacement.

How we install

Survey, remote design, site survey, final design and fixed quotation, permissions and DNO, installation, then commissioning and handover with the certification pack. The full sequence is on how Solar Fx works, and the standards we hold the work to are on our installation standards.

Many domestic installations are completed in one to three days once scaffolding is in place, although the actual duration depends on system size, roof type, access, electrical works and whether a battery is included.