Solar PV Basics

A photovoltaic cell converts light into electricity through the photovoltaic effect: photons from daylight knock electrons loose in a semiconductor, usually silicon, and the cell structure directs those electrons into a current. Cells are wired into modules, the familiar panels, and modules are wired into strings that feed an inverter. No fuel is burned, no moving parts are involved in generation itself, and panels produce electricity in diffuse daylight as well as in direct sun, though output is far higher in bright conditions.

That daylight point matters for cloudy climates. A panel in northern Europe still generates on overcast days, at a lower rate, and annual output depends on total light across the year, not on counting only sunny days. Snow, dust and bird droppings reduce light reaching the cells, which is why cleaning and siting still matter.

Panel types differ mainly in cell technology and efficiency: monocrystalline panels are generally more efficient per square metre, polycrystalline panels are less common in new homes now, and thin-film panels suit some large or curved surfaces but need more area for the same output. Efficiency figures describe output per square metre under standard test conditions, so a more efficient panel helps most where roof area is limited.

Strings and inverters shape the system too. Panels in a single string share a current, so a shaded or faulty panel can drag down its string mates on some designs, while microinverters or optimisers let each panel work more independently. The choice depends on shading, roof shape and budget, and should be explained in the proposal rather than assumed.

Check the roof area and shape, the direction and pitch, and the shading pattern across the day and across seasons, since a tree that is bare in winter can shade heavily in summer. Ask for the panel model, its rated peak power, its efficiency and its temperature coefficient, which shows how much output falls as cells heat up on a hot roof.

Also ask whether the proposal uses a string inverter, optimisers or microinverters, and why that choice suits this roof. A proposal that names no panel model and no inverter type has not given enough detail to compare.

In the EU, PV products must meet product safety and labelling rules for the single market, while installation, grid connection and support are national matters. Member states differ on whether installers need specific certification, on connection notification thresholds and on how small rooftop systems are treated in building regulations.

In the UK, MCS certification covers PV installations for supported schemes, building regulations cover roof structure, fire safety and electrical work, and the local distribution network operator must be notified or give approval depending on system size. Those layers together, rather than any single rule, govern a UK rooftop installation.

A frequent mistake is judging panels only by peak wattage, without checking efficiency, temperature behaviour or warranty terms, so a cheap high-wattage panel on a hot, small roof disappoints. Another is assuming panels need direct sun and dismissing a good east-west roof that actually produces well across a longer day.

A third mistake is ignoring the inverter in the budget: it is the component most likely to need replacement during the panels lifetime, and its warranty, location and ventilation deserve as much attention as the panels themselves. It is also useful to ask about panel degradation and soiling in the local climate, because coastal salt, urban dust, pollen and bird activity all affect long-term output differently, and a production estimate that ignores the local soiling pattern will look tidier on paper than it performs on the roof.

Keep the panel and inverter data sheets, the string layout drawing, the production estimate, the warranties for panels, inverter and workmanship, and the commissioning record. Those papers let a later installer or buyer see exactly what is on the roof.

Ask a qualified installer: which panel and inverter models are proposed and why, how does shading affect this string design, what is the temperature coefficient, and what warranties cover each part. Model-specific answers beat generic claims about solar.