Technical Information

As the solar industry is still relativley new to the UK, we have compiled this page to answer some of the more technical questions that you may have about how a photovoltaic system works

The Feed-in Tariff pays a tax free income to domestic households and an index-linked payment for both commercial and domestic installations. On top of this, solar PV generated electricity also produces substantial savings on your standard import / supply bill.

Solar PV Efficiency

The efficiency of a PV panel is defined as its output power divided by the power of solar radiation that it is receiving (the solar radiation is referred to as insolation). Although the power output is directly affected by the level of insolation, it does not greatly affect the efficiency. In other words, reducing the insolation by 10% will lead to a drop in output power of approximately 10% so the ratio between input and output stays much the same. A common source of confusion concerns the difference between power and energy. Power is an instantaneous measurement expressed in Watts (abbreviated to W) or kilowatts (kW) where 1000W equals 1kW. Energy is a measurement of power over time usually expressed in kilowatt hours (abbreviated to kWh). For example a 100 Watt light bulb burning for 10 hours uses 100W x 10hrs = 1000 Watt hours or 1 kWh.

Types of solar cell

Thin film solar cells promise to make solar power more cost-effective in the future but currently they are only used on commercial properties due to their comparatively low efficiently of 9%, however they are exceptional in low light conditions. Most solar panels currently sold in the UK are Polycrystalline, made from multicrystalline silicon and have efficiencies of 11% to 14%. Monocrystalline silicon panels are slightly more expensive and efficient, around 15%. Hybrid panels consist of two types of cells, Monocrystalline and Amorphous (thin film). This makes these panels exceptional in all conditions and the most efficient MCS accredited panel at 19%. Efficiencies over 40% have been achieved using exotic materials and processes but the costs of such cells are prohibitively expensive.

Output losses

Each PV installation needs at least one inverter to manage the DC (direct current) output from the panels and to convert this into AC (alternating current) suitable for feeding into the national electricity grid. The percentage peak efficiencies quoted for quality inverters around 96, and in practice the energy loss is typically around 6%. Losses in the DC and AC wiring and other connections will amount to a further 2% or 3%. All losses are factored in to the SAP prediction. Using a single inverter, the maximum total output of all the panels is usually at least 2% less than the sum of the maximum outputs of the individual panels because the panels behave slightly differently. Dust and dirt will also reduce the output by a few percent and snow of course can have a considerable effect in winter. In heavily polluted arid areas overseas losses of up to 25% have been reported, but frequent showers in the UK keep our panels relatively clean, especially if they are installed at a reasonable tilt.

Temperature

Manufacturers measure the output of solar panels at 25 degrees centigrade under “full sun” isolation of 1000 Watts per square metre. However, in practice full sunlight will heat the panels to considerably higher temperatures. This is significant because the power output for most crystalline silicon solar cells falls approximately 0.5% per degree rise in temperature. Tests have shown that solar panels reach about 45 degrees centigrade under 800 Watts per square metre of insolation, so power outputs can be at least 10% less than expected when the sun is shining strongly in summer. (http://www.tuvptl.com/doc/abstract-NOCT.pdf) It is therefore advisable to have an air gap between solar panels and a roof or wall to aid air circulation. Consideration should also be given to leaving gaps between panels on larger installations to aid the dissipation of warm air. Hybrid panels lose the least efficiency with increasing temperatures.

Tilt and orientation of the panels

As most of the PV energy will be generated in the summer when the sun moves round in a wide arc, the direction of solar panels is not too critical (providing they don’t face northwards). Some people recommend that the tilt of the panel above the horizontal should be equal to the latitude where the panel is sited: this would be 52 degrees in the UK Midlands which lies around 52 degrees north of the equator. Others say panels should be tilted 20 degrees less than the latitude. NASA’s satellite data suggest the optimum tilt in the Midlands is 40 degrees whereas the EU gives a figure of approximately 36 degrees for the Midlands. In practice, most householders will want to fix their panels to their roof and fortunately most domestic roofs have a slope of around 40 degrees - which is just fine. As regards the compass direction of panels, turning a moderately tilted panel 30 degrees away from due south reduces the output by no more than 5%. Where panels are sited on the ground or on a large flat roof it is possible to mount them in devices which direct them toward the sun throughout the day, rotating them either vertically, horizontally or both. This can increase the output by around 30% compared with panels fixed in the optimum direction, but there are obviously cost, maintenance and planning implications.

Geographical location

The amount of solar radiation received annually does not vary greatly across the UK. In the Midlands approximately 1100 kilowatt hours are received a year on a one square metre of panel facing south and tilted at the optimum (fixed) angle. The figure is roughly 10% less for central Scotland and 10% more for south-west England.

Reduce your carbon footprint PV panels produce clean, emission free energy from a sustainable source, which will reduce your carbon footprint. Each kilowatt hour produced saves around 0.54 kg of carbon dioxide emissions.

Earn money from the electricity you produce Each unit of electricity that you generate from solar PV panels, (whether you use the power in your home or feed it back to the national grid) will earn you a tax free income, as part of the government's Feed-in Tariff.