Solar FAQs
PV systems: frequently asked questions
Solar Panels
- Grid connected solar PV system an overview
- What is a photovoltaic cell and how does it work?
- What are monocrystalline, polycrystalline and amorphous cells?
- What is a PV module
- How is the solar electricity used?
- What happens if the system generates more than the building is using?
- What happens during a power cut?
- Do installations need batteries?
- Can the system be used to run a dedicated load?
- How is a system sized?
- How is the size of a PV system expressed (what is kWp)?
- Will a 2kW system provide 2kW in the UK?
- Do PV systems work in the UK?
- How much energy will a PV system provide in the UK?
- What is the difference between kW, kWp and kWh?
- How much CO2 emissions will a PV system prevent?
- What is the best location for an array
- Does a solar PV array have to be exactly south facing?
- What range of array pitch is acceptable?
- What are the effects of shade?
- What range of systems is available?
- Do systems need planning permission?
- What is the life expectancy of a PV system?
- What maintenance do PV systems need?
- Do PV arrays need regular cleaning?
Solar Panels
Q. Grid connected solar PV system an overview
A solar photovoltaic (PV) system provides electricity from sunlight. A grid connected system is connected directly into (and synchronised with) a building’s existing mains supply. On a sunny day, the power generated by the PV system reduces the amount of electricity that needs to be purchased from the electricity company. On very sunny days, or at times when not many loads are turned on, electricity production may be greater than the demand in the building - and the excess will be exported out into the grid.
The system consists of three elements: An array of photovoltaic modules (the part that generates the electricity); an Inverter (an electronic device that safely converts the output of the solar array into AC “mains”); Cables, display(s) and switchgear
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Photovoltaic or PV cells are semiconductor devices that convert sunlight directly into electricity. A typical cell comprises a wafer of silicon which is manufactured with a “p-n junction” within it. This junction is the boundary between “n type” silicon on one side of the wafer (silicon with an excess of free electrons) and “p type” on the other (silicon with a deficit of free electrons). The energy in light frees electrons within the silicon cell - these are captured by the electric field of the p-n junction. Metal contacts on the front and the rear of the cell are used to draw off the electric current produced by this flow of electrons.
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The terms monocrystalline, polycrystalline and amorphous describe the nature of the silicon used to make the PV cell. Monocrystalline cells use a wafer of single crystal silicon, typically cut from a larger ingot of single crystal silicon. Monocrystalline cells are uniform in appearance. Polycrystalline cells use multi-crystalline silicon, this is cheaper to produce but slightly less efficient. A patchwork of individual crystals can be clearly seen in a polycrystalline cell. Amorphous cells have no crystalline structure and are the least efficient of the three technologies.
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Photovoltaic or PV modules typically consist of a number of interconnected PV cells encapsulated between a sheet of glass and a backing sheet. This laminate is often mounted within a rigid aluminum frame, but can also be manufactured in a wide range of other configurations to create modules that suit many different applications.
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The output from a grid connected PV system is fed directly into the existing circuits in the building, usually by a connection at the main distribution board. Besides smaller electricity bills, there is no difference in the way the electricity is used.
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If the solar PV system is generating more electricity than the load demand in the building, all excess is exported to the grid. Export occurs automatically and can be measured (and income collected) using an “export meter”.
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A grid connected system instantly switches off when a power cut occurs. This is required to meet electricity regulations and is implemented by control circuits integrated within the inverter. Disconnection during a power cut is necessary to ensure power quality and protect workers who may be repairing the fault. The system will automatically re-start once the power cut ends (a 3 minute restart delay is required by regulations).
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A grid connected system does not need batteries. Batteries add significantly to the cost of an installation and also significantly reduce the overall efficiency (typically by 20-30%). They are also toxic and have a relatively short lifetime. Batteries are only used where they can’t be avoided such as for an off grid application.
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A grid connected system feeds into the whole building electricity network, feeding any loads that are running at the time. Feeding a single load in such circumstances is not feasible. However, when designing the PV system, it can be sized to meet the notional demand of a single load.
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Solar PV systems are entirely modular and can be of almost any size. Systems are usually sized to meet one of three criteria: a) to cover a defined area (eg a certain section of roof); b) to meet a defined energy target (eg to generate 10% of a buildings annual electricity requirements); or c) to fit within budgetary requirements.
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PV systems are typically rated in kWp - kilowatt (peak) – the combined installed capacity of all the PV modules installed. All modules are supplied with a Wp – Watt (peak) – rating; 1kWp equals 1,000Wp.
The Wp rating of a module expresses the rated output of that module under “standard test conditions” (stc) – internationally agreed temperature and sunlight conditions. To enable comparison, all modules are tested (and rated) using the same stc conditions.
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PV systems installed in the UK operate for much of the time below the rated capacity, though the rated output can be met or exceeded on occasions. This is because the internationally agreed standard test conditions (see above) equate to a better “solar day” than is the average in the UK.
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Solar PV systems work very effectively in the UK, though the annual output is obviously lower than in sunnier countries. That they provide a viable means of electricity generation in the UK is demonstrated by ongoing government support and from the thousands of systems that have already been installed.
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An optimally orientated and un-shaded 1kWp PV array will typically produce between 750 to 900kWh (units) of electricity per year in the UK (depending on location and system design). Sundog energy can provide site specific estimates, including computer simulations, on request. In making initial assessments, an output of 750kWh/kWp is commonly used as an initial “rule of thumb” – hence a 2kWp system would produce approx: 2 x 750 = 1,500kWh / year.
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kWh (kilowatt-hour) is a measure of electricity use (or generation) over time. A 1kW (kilowatt) load running for 1 hour will use 1kWh of electricity. Similarly, a 1kW generator running for 1 hour will generate 1kWh. Electricity is bought and sold in kWh (often termed “units”).
kWp is used solely when expressing the nameplate rating of a PV module or array (see above). The actual instantaneous output of an array is measured in kW and the energy production over time is measured in kWh.
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Every 1kWh generated by a PV system means 1kWh less has to be generated by conventional power stations. These power stations currently emit (on average) 0.43kg of CO2 per kWh – hence each solar kWh generated prevents the emission of 0.43kg of CO2. (source: Energy Savings Trust)
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Roofs usually provide an ideal location for a PV array. Most buildings have a suitable roof and mounting systems are available for most roof claddings. In selecting the best location, a building should be assessed for the roof with the most southerly aspect and also for the roof that is free of shade from surrounding obstacles or neighbouring roofs for the majority of the day.
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No, arrays work effectively and are commonly installed at other orientations - from East through to West. However, annual output is reduced if the array faces significantly away from south. With information on roof pitch and orientation, Sundog energy can provide site specific output estimates.
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The ideal array pitch is between 30 and 45 degrees from horizontal. However, pitches between 5 and 60 degrees will result in less than 10% losses for a south facing array. With information on array pitch and orientation, Sundog energy can provide site specific output estimates.
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Shade can make a large impact on the performance of a PV system. Even a small degree of shading on a small section of an array has a significant impact on the overall array output. To work effectively, the whole PV array needs to be located in a predominantly shade free position. Shade can be produced by obstacles such as trees or adjacent buildings, or be inherent from the building or roof design.
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An extremely wide range of products and mounting solutions are available – suitable for roofs, facades, glazing and ground mounted applications. Information can be found elsewhere on the Sundog energy website.
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This can depend on the type of building, site and geographical location. Generally planners look very favourably on PV systems and are under pressure to accept them.
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There should be little or no concern for the life of the installation. Most solar modules come with a 25 year guarantee and inverters generally have a 5 year guarantee. However all system components should last considerably longer. Sundog energy provides a 5 year workmanship warranty on all its installations.
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With no moving parts, most installations require no regular maintenance. However some sites opt for periodic inspection and “health checks”.
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In the majority of locations, a solar array is self cleaned by rainfall. Arrays with very shallow pitches or in dusty or dirty sites may require a cleaning regime.
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