4.6 The PV System

wiring diagram for solar PV system

The energy output from a single PV panel is typically in the range of 250-300 watts in bright sunshine. A photovoltaic system is normally built up from a number of panels (an array), to produce a more significant energy output.

The PV panels produce direct current (DC) electricity, whereas the electricity supply is alternating current (AC). An electrical device called an inverter is used to convert the DC to AC. The inverter is installed with switches on the AC and DC side to allow it to be isolated for maintenance.

The AC output from the solar installation is wired back to the main consumer unit in the building, where it should have a dedicated circuit breaker. A further switch gives the user a point of emergency isolation, and an energy meter is normally added to enable the performance of the system to be seen. The consumer unit is connected to the electricity grid, normally through an electricity meter that can measure export.

Such an installation is termed “grid-tied” because the electricity supply for the building is met by a combination of solar energy and grid electricity. In this type of installation, the inverter must have an “anti-islanding” capability, which means that in the event of a power cut, the inverter automatically disconnects the solar installation, thus preventing electrification of the grid and protecting people that may be working to restore the electricity supply.

For an unshaded PV system facing south at a tilt of 30-40 degrees in the UK, each kWp installed will produce between 750 and 1,100kWh per year depending on the location of the installation. For comparison the average electricity use for a household is 4,300kWh/year, but sizing a system is not as simple as matching the annual output to the annual demand.

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