PV panels contain silicon cells (or other semiconductor materials) that when exposed to light produce a voltage between their terminals. These cells are wired together within a panel to produce a useful amount of power/voltage. Normal panel sizes are 80W and 180W. Panels have a positive and negative wire out of a junction box on the back. These are normally connected in series with other panels in an array to produce a high DC voltage. This power is then passed through an inverter that converts the DC voltage into grid synchronised useable AC mains electricity. This power is fed back into your fuse board where it is first used by your household appliances before any surplus power is exported to the grid (see ‘additional information’ below for information on exporting to the grid).

You can also get stand-alone PV systems that are not connected to the grid but instead charge batteries for use in low voltage applications. However, these types of systems are most commonly used in rural areas where grid power supplies are unavailable or difficult to connect to.

PV panels work with any light but work best and hardest when exposed to direct sunlight. The ideal orientation is 30° from the horizontal facing south. Any deviation from this will start to compromise the amount of power they will harvest. A slope between 0° and 60° and an orientation of 20° either side of south may only mean a 10% drop in the rated output. This can be overcome by increasing the size of the array (a set of solar panels is referred to as an array), but this also increases its cost and the work it will have to do to pay for itself in generated electricity.

PV arrays do not like any part to be in shade as the shaded section acts as a bottle neck for electricity flow in the whole array. Badly overshaded sites will have very compromised harvest rates that will increase payback times.

There are several types of PV cells, with the more efficient cells being more expensive. The following types of PV cells are listed in order from most to least efficient (and expensive): monocrystalline silicon cells, multicrystalline silicon cells, thin cell technologies (e.g. cadmium telluride (CdTe) and copper indium diselenide (CIS)) and amorphous silicon. Most PV cells used for land-based applications are based on one of the silicon technologies.

 

Solar PV products and arrays are rated by the power they generate at Standard Test Conditions (STC - 25 °C, light intensity of 1000W/m2, air mass = 1.5), defined as kilowatt 'peak' (kWp) power. The following table compares the features and UK performance of these four technologies for stand-alone modules and for building integrated systems:

 

	Thin Film	Polycrystalline	Monocrystalline	Hybrid
Cell Efficiency at STC	8 - 12%	14 - 15%	16 - 17%	18 - 19%
Module Efficiency	5 - 7%	12 - 14%	13 - 15%	16 - 17%
Area needed per kWp - for modules	~16m2	8m2	7m2	~6m2
Area needed per kWp - for BIPV (variation due to tile spacing)	Solar metal roofing - 24m2 Glass-glass laminate - 25m2	Glass-glass laminates - 10 - 30m2	Sunslate - 10m2 Glass-glass laminates - 8 - 30m2	n/a
Annual energy generated per kWp (south-facing , 30° tilt)	800 kWh/kWp	750 kWh/kWp	750 kWh/kWp	900 kWh/kWp
Annual energy generated per m2 (south-facing, 30° tilt)	~51 kWh/m2	100 kWh/m2	107 kWh/m2	~146 kWh/m2
Annual CO2 savings per kWp	344 kg/kWp	323 kg/kWp	323 kg/kWp	387 kg/kWp
Annual CO2 savings per m2	22 kg/m2	40 kg/m2	46 kg/m2	60 - 65 kg/m2

 

In general, thin film cells perform better in low light conditions but require considerably more surface area than crystalline modules for the same output. Crystalline cells require a smaller surface area and make up 85% of the market. The costs for modules made from these different technologies are comparable.

Although there are small efficiency and cost differences between mono- and multi- (or poly-) crystalline the relatively small size of domestic installations is such that the efficiences are not significant.