Solar panels

A photovoltaic system works from panels mounted on the roof: the panels turn sunlight into direct current, and the inverter converts it into alternating current at the voltage and frequency of the grid and feeds it straight into the building's distribution board. The building consumes its own generation first and draws only the shortfall from the grid. One detail is worth settling at the outset: a grid-connected inverter shuts down when the mains fails, so a solar system is not in itself a backup power supply.

The roof determines what system can be installed on a site. Orientation, pitch, usable area, the load the structure can carry, the condition of the waterproofing and shade from parapets, flues, ventilation units or a neighbouring building all count — and shade falling on a single panel pulls down the output of the whole series-connected group. The same survey settles where the inverter goes: it has to stand in shade, ventilated and within reach. The route of the DC cable from the roof down to the distribution board is drawn at that point too, along with the roof penetrations and the detail that makes the waterproofing good again, the earthing and the surge protection. Capacity is then matched against the real consumption profile, because the hours of the day at which the site uses power are what matter: a wrongly sized array exports a large share of its generation at exactly the times the building does not need it.

Years after commissioning the system still runs in the same mode, but its condition cannot be seen from the ground — it shows in numbers: in the monthly generation, the currents of each group and the inverter's error log. Several things repay attention: dust and snow on the panels; new shade that was not there at the start, thrown by a tree that has grown or by a building put up next door; the roof penetrations and the cable clamps; and the inverter, as the hardest-working part of the system. This is exactly why the documentation handed over at completion is needed — the schematic, the panel layout and the commissioning records: they state what generation was expected and how the system was put together. Without monitoring, an inverter that has tripped or a damaged group goes unnoticed for months, while whatever is miscalculated or badly mounted at the start can only be corrected by taking it down again.