Aphotovoltaic cell is a very simple thing: a square piece of silicon typically 182 millimetres on each side and about a fifth of a millimetre thick, with thin wires on the front and an electrical contact on the back. Shine light on it, and an electric potential – a voltage – will build up across the silicon: hence “photovoltaic”, or PV. Run a circuit between the front and the back, and in direct sunlight that potential can provide about seven watts of electric power.

This year, the world will make something like 70 billion of these solar cells – the vast majority of them in China – and sandwich them between sheets of glass to make what the industry calls modules, but most other people call panels: 60 to 72 cells at a time, typically, for most of the modules which end up on residential roofs, and more for those destined for a commercial plant.

The Economist