Units
Before considering the amount of electricity that can be produced by photovoltaic panels, it is handy to understand the units of measurement used when discussing electricity production. The amount of electricity produced by a panel is measured in kilowatt hours, kWh. These units are the same as the units on an electricity bill and are therefore directly comparable. When purchasing a solar photovoltaic panel, an energy rating is applied to the performance of that panel, that is, the maximum amount of energy it is able to produce. This rating is termed the output rating. As an example, a 500W solar panel is able to produce up to 500W of power but this is very dependent on factors discussed below. This panel might be referred to as a 0.5 kilowatt power (kWp) panel. The other unit that is often referred to when discussing output performance is the insolation factor and this is the amount of sunlight that will hit the surface of the solar panel. It is measured in kilowatt hours per square metre per day, kWh/m2/day. It is an important unit to know, because as a measurement of actual sunlight hitting the panel, it is directly related to the amount of energy the panel will be able to produce. Peak sun hours are related to this, and are the number of hours in the day where the strength of the sunlight reaches 1000 Watts per square metre, W/m2.
Factors to Consider
The amount of energy a system will produce depends on a number of factors. The type of panel chosen (as some panels are more efficient than others), the size of the system, and the surface area of the panels are the first considerations. Related to the surface area, is the calculation of the insolation factor for the specific area that you live in, as this does vary across the UK; in other words, how much sunlight will be available for your panels in the area in which you live. The further north you live in the summer months, the greater the insolation factor will be (bearing in mind that the North Pole has 24 hours of sunlight on the longest day). In the winter, the more northern your location, the smaller the insolation factor, because the days get shorter the closer you are to the North Pole. Another factor is the peak sun hours; the closer you live to the equator, the greater the peak hours. The UK typically receives around 6 peak sun hours in the height of summer and 2 in the depths of winter. Panels need to be positioned to make the most of the sunlight, so the angle, direction, and amount of shade are important considerations.
Examples of Yields
There is no definitive answer to the question of yield, as each system will vary, but an example system output can be given. A typical 2kw solar system should produce about 1,500 to 1,600 kWh of electricity per annum. By looking at your yearly electricity usage you should get an idea of how much of a saving that should be; for a typical family a PV system should meet at least one third of their energy needs.
