What is a PV System?
The main element of a PV system is the photovoltaic (PV) cell, which converts sunlight into electrical energy. A PV cell consists of layers of a semi-conducting material that absorb energy from sunshine to create an electric field, causing electricity to flow. The greater the intensity and duration of sunlight, the greater the flow and quantity of electricity. These individual cells combine to form panels.
Although PV panels can still generate some electricity on a cloudy day, they will only generate electricity whilst there is daylight, so the energy must either be consumed as it is being generated, stored in batteries, or exported to the National Grid (the grid).
Main components that typically make up a PV system
PV array
Several panels connected to one another are known as an array. Array sizes and arrangements often vary and can be designed to suit the available area by organising the panels in different ways.
Inverter
PV panels generate direct current (DC) electricity. Electricity used in buildings is alternating current (AC) so the energy from the PV system needs to be converted. The inverter is the electrical device that converts the DC generated from the PVs into AC that can either be used or exported to the grid.
Electrical Distribution System
The electrical network that transports the generated electricity around the building, comprising electrical cabling, cable containment such as trunking or conduits, distribution boards, metering, isolators, and switches.
Electrical energy storage system (EESS)
If the energy generated by the PV array is not used at the time, it can either be exported to the grid or stored in batteries by using an EESS for later use, as and when demand arises. An EESS consists of distribution and control equipment, and batteries. Our Installing Electrical Energy Storage Systems and Batteries in Historic Buildings page provides guidance on the types of systems available, as well as ongoing maintenance requirements and the issues to be considered in their design and installation within historic buildings.
PV tiles and slates
PV tiles and slates work in the same way as solar panels, but they are made to look like traditional roof tiles or slates. Where the roof covering is to be replaced with PV tiles or slates, you will need to consider how long they will last. This will be much less than traditional tiles or slates. See Installation Options for more information.
Energy yield
The maximum power generated under ideal conditions in full sun by a PV panel is expressed in kilowatt-peak (kWp) units. This assumes ideal conditions to generate solar energy and is derived from the kWp value when the sun is shining on the PV array at full strength and at an optimum angle. The energy yield (that is, the ratio between the energy generated and peak installed output (kWh/kWp)) is a good measure of how effective the installation is at generating energy. The energy yield will depend on the location, PV module selected, orientation and overall system efficiency.
A typical domestic installation which will cover a roof area of around 20m2 is around 3.5 kWp. This would generate enough electricity to provide almost half of an average family's annual requirement. On a church or a school, an array could be anywhere between 4 kWp and 50 kWp, and larger arrays, such as the one at King's Cross station (an area of 2,300m2), could be 240 kWp.
Energy yield (that is, the ratio between the energy generated and peak installed output (kWh/kWp)) is a good measure of how effective the installation is at generating energy. The energy yield will depend on the location, PV module selected, orientation and overall system efficiency.
The Microgeneration Certification Scheme provides ‘Irradiance Datasets’ to help estimate the annual output for different areas of the country, using the incline, how much the panel is tilted from the horizontal and the orientation of the array.
