Solar Electricity - FAQs
Below we list common questions about solar electricity and solar PV systems. If you have any other questions that you we might be able to answer please contact us.
General QuestionsGeneral solar PV questions, terms, definitions and concepts |
What is kWp?kWp stands for Kilowatt Peak or put another way the maximum performance obtainable from a solar panel / module when operating under perfect conditions. kWp is a useful measurement to use when selecting and integrating components such as solar PV panels, cables and solar inverters in system sizing calculations.Ensuring that cables, solar inverters and isolators are of a sufficient size to handle a solar PV arrays kWp output will ensure that these components will not overheat, get damaged or be a cause of power losses. The actual output of a PV array is dependent on the light available to it. In the UK under normal operating conditions a PV array will rarely if ever reach it's kWp potential; for example a 2kWp system will rarely (if ever) achieve a constant 2kWp, instead PV array outputs fluctuate second by second based on the constantly changing light conditions. |
What is the difference between a kilowatt (kW) and a kilowatt hour (kWh)?A kW is a thousand watts and a unit of power. A kWh is the amount of energy used (or in our case generated) when a kW of power works for one hour. We use Watts (W), kilowatts (kW) and kilowatt peak (kWp) to measure power when sizing a system to ensure that all of the different components will work efficiently and safely together when connected. We use kilowatt hour (kWh) and kilowatt hours (kWhrs) to measure how much usable power will be generated and or available for export/sale. A constant 1kW working for one hour Conventional electricity bills are measured and payable based on kWhrs, Feed in Tariff payments are also based on kWhrs. Accurate kWh estimates can be prepared in advance of an installation based on historical solar radiation data combined with the characteristics of the installation and the location. In the UK we typically see that for every 1kWp of installed PV you get a return of between 800 and 1000 kwhrs of electricity over the course of a year. The better the system design in relation to it's location the greater the kWhrs return Vs kWp installed will be. |
What are the components of a solar photovoltaic (PV) system?
We have written an article where we describe the different components of a solar PV system and the functions they carry out here: Solar PV System Components |
How does a solar PV system work?Daylight hits the photovoltaic cells and is converted to electricity. The solar inverter converts the electricity from direct current (dc) to alternating current (ac) which can then power local loads, charge batteries or be exported to the grid. Further information about the types of solar PV system and an overview of how they work can be found here: Types of Solar Photovoltaic (PV) System |
What's the difference between solar photovoltaics (PV) and other solar energy technologies?There are four main types of solar energy technologies:
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What does energy conversion efficiency mean?Energy conversion efficiency is an expression of the amount of energy produced in proportion to the amount of energy consumed or in our case available to a device. The sun produces a lot of energy in a wide light spectrum, energy conversion efficiency for Solar PV devices in general are typically from 10% to 20% efficiency. The energy conversion efficiency of a solar panel has a direct effect on the kWh yield and is a key differentiator between different solar panels when specifying equipment. |
What is net metering?Net metering allows buildings with PV systems to use any excess electricity they produce to offset their electricity bill. As the PV system produces electricity, the kilowatts are first used for any local requirements within the building. When the PV system produces more electricity than the building needs, the extra kilowatts are fed into the utility grid. |
Does a solar PV system need batteries?If the building is connected to the National Grid then your system will not need batteries as any excess energy can be sold to and fed into the grid. During the winter this process can be reversed and the grid can feed your buildings' electrical system, you will essentially use the grid as your energy store. If mains electricity is not available or if you'd like to store solar electricity locally, you can use batteries. Batteries can also be integrated into hybrid grid tie / battery systems as a backup, protecting against mains power cuts. |
What is a Grid-Tied Solar Inverter?A grid-ties solar inverter converts the DC power being supplied from the solar panels into AC power to be used by any local electrical loads within the property's electrical system, with any excess being exported to the grid/mains. The solar inverter also controls the DC power being generated by the solar panels through it's use of MPPT trackers. It actively monitors the grid/mains power supply conditions and is configured in accordance with engineering recommendation G.83/G.59, replaced by G.98/G.99, to only connect the solar PV system to the mains/grid supply when it's stable. As part of a solar inverter's startup sequence (grid-tied inverters usually shutdown at sunset and startup at sunrise), it tests the integrity of all of the wiring connected to it, including the cabling between solar panels and it's own internal safety components before making the AC connection allowing the solar PV system to supply the property's electrical system and export to the grid/mains. |
Design and InstallationPlanning, designing and installing a solar PV System |
How is a Solar PV system connected to the national Grid?Connecting a PV system to the distribution network is arranged by the installer working with the Distribution Network Operator (DNO). The DNOs are the power companies that run the power distribution networks (EDF, Scottish & Southern, Western Power, UK Power Networks etc). There are three engineering recommendations (G98, G99 & 100) that are followed by both the installer and the DNO to ensure that the solar PV system will integrate safely with the national grid. Solar PV and energy storage battery systems under 16A per phase (i.e under 3.68kWp) come under engineering recommendation G.98. Solar PV and energy storage battery systems installed under G.98 can be installed without prior notice to the DNO, with the DNO being informed afterwards (within 30 days). Solar PV and battery energy storage systems over 16A per phase will need to consult the DNO before the systems are connected to ensure that the distribution network can handle the extra power being provided. larger solar and bettery storage systems are installed under G.99. Installations employing current regulation are installed under G.100. We've written more about connecing a solar PV or battery system under 16Amp per phase here: Grid connections for Solar PV systems and batteries |
What are the effects of shade on solar PV Panels?Shading has a disproportionate affect on a systems performance. This is because the cell with the lowest illumination determines the operating current of the series string in which it is connected. This is one of the areas that must be covered in a survey carried out before the installation. Modern solar panels can bypass the effected diodes to minimise shade effects; but these effects must still be considered as system yields (kWhrs) as well as other factors will be affected. |
How does the angle a solar PV Panel is installed affect it's performance?The maximum total annual solar radiation available in the UK is usually at an orientation of due south and at a tilt from the horizontal equal to the latitude of the site minus approximately 10-15 degrees. For example 30 degrees is an optimal tilt at the tip of Southern England, increasing to around 40 degrees in Northern Scotland. If the optimum angle is not achievable, over 90% of the maximum annual energy can still be achieved at 10 degree and 50 degree tilts. South-facing vertical facades generate around 70% of the maximum. We've written more about the free tools that can be used to accurately calculate yields here: Free Solar PV Calculators, tools and software |
What other points should be considered with a solar PV installation?
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What are the relevant codes of practice and articles of legislation?
You can find copies of most of these documents here: Further information, Calculators and Inverter Fault Codes |
How long do solar photovoltaic (PV) systems last?The basic solar PV panel (interconnected, enclosed panel of PV cells) has no moving parts and can last more than 50 years. It is estimated that performance will decrease by less than 1% per year, which would mean that in 50 years they'd still be 60% efficient. Most solar PV panels are provided with 25 year power guarantees direct from the manufacturer as standard. As well as solar PV panels a DC to AC solar inverter is needed to supply power to a property. The current lifetime estimates for solar PV DC/AC solar inverters is from 8-12 years. Anyone installing solar should allow for solar inverter replacement costs every 10 years. |
Will I need Planning Permission for a solar PV system?Installations above a certain size and in certain locations such as in national parks and world heritage sites will need planning permission. Planning permission is not usually required for residential properties unless the installation is over 4m high (unless on a roof, then no bigger than the roof and no higher than 200mm above the roof line). You can find further information about planning permission for solar PV systems here: Planning Permission for Solar Photovoltaic Systems |
Do Building Regulations Apply when installing a Solar PV System?You may have read elsewhere that compliance with the Building Regulations is not applicable to solar PV systems, this is absolutely not true and potentially dangerous. Building Regulations are not the same as Planning Permission. As with any other building work a solar PV installation must be installed in compliance with the Building Regulations. You can find further information about the applicable Building Regulations here: Solar PV: Safety and The Building Regulations |
How to size a solar photovoltaic (PV) system?Excluding electricity that may be used for heating and cooking, the Energy Saving Trust estimates that the average 3 bedroom house consumes 3,300 units of electricity (kWh) a year. If you are on mains electricity at the moment and have access to the electricity metre in the property, your usage can be worked out by taking incoming mains/grid supply meter readings at the same time of the day each day and working out the minimum, average and peak usage. In a new build, off-grid situation or a new home a useful exercise is to make a list of all the appliances that will use electricity in the property, note their wattage which will be printed on the devices and work out how often and for how long these devices will be used. Once you have this information you can work out the total usage or 'peak load' for a typical day. The common measurement for power usage is Watt Hours or Kilowatt Hours (kWh) i.e. how many watts used in a hour. We've written more here about how to calculate how many panels will fit on a pitched roof and how much power they will produce here: Solar Panels: PV System Sizing and Power Yield Calculator If you have a flat roof, you can work on how many panels can be mounted to it whilst avoiding self-shading here: Flat Roof: Solar Array/Shading Calculator You can find a list of free solar PV calculators and tools here: Free Solar PV Tools and Calculators |
Repairs and MaintenanceOwning a PV System |
How do I know if my solar panels are working?There are a couple of non technical ways to find out if the solar panels are working:
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What maintenance and cleaning do solar PV systems need?Solar photovoltaic (PV) systems are silent in operation, have no moving parts and require minimal maintenance anthough depending on the environment they will benefit from cleaning. Horizontal glazing and frame bars can trap debris which could lead to shading of part of the array. Birds, lichen, dust and leaves can reduce the light and therefore the effectiveness of panels, the design of the system should aim to minimise uneven soiling when possible. Panels are pretty robust so if they get dirty rub off the dirt with a damp cloth, blast them with a hose (no detergent) or hire a window cleaner with a pure water system and a fibreglass pole. We've written more about cleaning solar PV panels here: Cleaning Solar PV Panels |
Can I add more solar panels to my system?Yes, if space is available, more panels can be added to a property but consider that new panels cannot be added to a system and share a generation meter with one that has already been signed off for a feed in tariff (FIT). Additional panels and a new system may take the overall amount of power that can be exported from the property over 16A per phase and over the limit that has previously been agreed to with the DNO (Distribution Network Operator). If this is the case, the additional export capability needs to be agreed with the DNO before the new system is commissioned. |
Articles: Further Information, Calculators & Solar Inverter Fault Codes
Lots of articles, calculators and technical information including links for further reading. Covering a range of topics related to the installation and maintenance of solar photovoltaic and electrical systems in the UK.
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Engineering Recommendation G98Grid Connections for Micro-Generators including Solar PV Systems and Electricity Storage Systems in the UK. Under 16Amps Per Phase, grid synchronised. BSI - PAS 63100:2024 - Protection Against Fire of Battery Energy Storage Systems for use in DwellingsThis Publically Available Specification (PAS) from the British Standards Institution (BSI) was sponsored by The Department for Energy Security and Net Zero. Although not yet a British Standard, this guidance was developed in response to an urgent demand for clarity as to good practice across the industry including product, service and process standards. How to provide backup power to a house using a portable generatorIn this article we show you how to provide backup power to your home using a portable diesel, petrol or LPG backup generator. We look at changeover switches, the importance of earthing, backup generator loadings, how to isolate non essential loads. We ask where to locate the backup generator when it's in operation, how best to safely isolate the grid/mains power supply and switch to a backup generator supply. Ground Faults, Isolation (ISO) Faults, RISO Low Faults and Insulation Resistance Faults with Solar PV SystemsAfter a number of years exposed to wind, rain, snow, ice and sometimes animals; solar panel systems can start to develop faults. The most common faults we find related to exposure are ground faults, isolation (ISO) faults, RISO low faults and insulation resistance faults. In this article we take a look at what these faults are, the possible causes and what steps are taken to identify and resolve them. Surge Protection for UK Solar PV SystemsRecent changes to the BS7671 UK Wiring Regulations 18th Edition in the form of amendment 2 have introduced requirements and considerations for surge protection on both the AC and DC side of a solar PV System. In this article we take a look at surge protection, particularly in the context of the updated UK wiring regulations and how surge protection could best be incorporated into new and existing solar PV systems installed in the UK |
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