General solar PV questions, terms, definitions and concepts

What is kWp?

What is the difference between a kilowatt (kW) and a kilowatt hour (kWh)?

What's the difference between solar photovoltaics (PV), solar thermal, solar hot water and other solar energy technologies?

How does a solar PV system work?

What is net metering?

Does a solar PV system need batteries?

What does energy conversion efficiency mean?

Planning, designing and installing a solar PV System

What are the components of a solar photovoltaic (PV) system?

How is a Solar PV system connected to the national Grid?

What are the effects of shade on solar PV Panels?

How does the angle a solar PV Panel is installed affect it's performance?

What other points should be considered with a solar PV installation?

What are the relevant codes of practice and articles of legislation?

How long do solar photovoltaic (PV) systems last?

Will I need Planning Permission for a solar PV system?

Do Building Regulations Apply when installing a Solar PV System?

How to size a solar photovoltaic (PV) system?

Owning a PV System

How do I know if my solar panels are working?

What maintenance and cleaning do solar panels need?

Can I add more solar panels to my system?

General solar PV questions, terms, definitions and concepts

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 not run at 2kWp all of the time, instead PV array outputs fluctuate second by second based on the changing light conditions.

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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. 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 per year. The better the system design in relation to it's location the greater the kWhrs return will be.

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• Solar PV panels: are the front end of a PV system that converts daylight into electrical direct current (DC)
• Solar Power Inverters: convert direct current (DC) into alternating current (AC)
• Solar PV Mounting Systems: can be ground, roof mounted as well as integrated into buildings, designed to secure the solar PV panels
• Solar Cables & Connectors: are used to connect the components
• Isolators: are used to disconnect parts or all of the system for maintenance and/or in an emergency
• PV Junction Boxes: are sometimes used to combine multiple solar PV strings on the DC side before the solar inverter
• Monitors & Displays: Used to provide performance data
• Solar Charge Controllers: Used to protect batteries when used (NB: Not needed for grid connected systems)
• Solar Batteries: are used to store and release electricity.(NB: Not needed for grid connected systems

If you would like more information about the various components found in both on-grid and off-grid solar PV systems; we have written an article where we describe the different components and the functions they carry out here: Solar PV System Components

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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 either integrate with existing power systems or charge batteries.

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

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There are four main types of solar energy technologies:

• Photovoltaic (PV) systems: which convert sunlight directly to electricity by means of PV cells made of semiconductor materials such as silicon.
• Concentrating solar power (CSP) systems: which concentrate the sun's energy using reflective devices such as troughs or mirrored panels to produce heat that is then used to generate steam and then electricity.
• Solar water heating systems: which contain a solar collector that faces the sun and either heats water directly or heats a "working fluid" that, in turn, is used to heat water.
• Transpired solar collectors, or "solar walls,": which use solar energy to preheat ventilation air for a building.

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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 7% to 20% efficient. Commercially available solar PV modules and panels typically operate between 12%-17% 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 / modules when specifying or choosing which is the best equipment to use.

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Net metering allows buildings with PV systems to use any excess electricity they produce to offset their electric 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.

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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' electricity system, you will essentially use the grid as your energy store.

If mains electricity is not available or you want to store 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.

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Planning, designing and installing a solar PV System

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, UK Power Networks etc). There are two engineering recommendations (G83/1 & G59/1) 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 systems under 16A per phase (i.e under 3.68kWp) come under engineering recommendation G.83/1. Solar PV systems installed under G.83/1 can be installed without prior notice to the DNO, with the DNO being informed afterwards (within 30 days).

Solar PV Systems over 16A per phase will need to consult the DNO before the PV system is connected to ensure that the distribution network can handle the extra power being provided. Depending on the system size and the DNO's preferences, larger systems may be installed either under G.83/1 or G.59/1.

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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 arrays 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.

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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.

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• Method of fixing/ integration into the building
• Ensure that the fixings do not cover or shade any part of the PV cells
• The fixing must allow for thermal expansion without breaking the glass
• Weather sealing
• Ventilation, the back of PV panels can reach 80 degrees if poorly ventilated
• The mounting option must allow for safe maintenance and possible replacement of individual solar PV panels and modules
• The life of the support structure must be at least that of the PV array. The preferred materials are aluminium, stainless steel or glass-fibre
• Protection from corrosion
• Wind loading
• Any extra weight
• How and where to run electrical wiring
• Where to place AC and DC junction boxes, solar generation meters and solar inverters

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• BS EN 61215
• BS EN 61646
• The Electric Supply Regulations 1988
• The Building Regulations 1991 (and amendments)
• The Construction (Design and Management) Regulations 1994

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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.

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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). There is more about planning permission for solar PV systems here: Planning Permission for Solar Photovoltaic Systems

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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. We have some more detailed information about the applicable Building Regulations here: Solar PV: Safety and The Building Regulations

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Excluding electricity that may in some properties 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 easily worked out through taking incoming mains/grid supply meter readings at the same time of the day in a typical week 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 these devices are used.

Once you have this information we need to work out your 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 you use in a hour.

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Owning a PV System

There are a couple of non technical ways to find out if the solar panels are working:

• At the solar inverter:The solar inverter sits at the heart of the system and is the piece of equipment that converts the DC power generated by the solar panels into AC power for use in your home and export to the grid. The vast majority of solar inverters have LCD displays on the front which will show the status of the system. If the solar panels are working and the solar inverter has a display it will likely show you here. Common measurements that are displayed are 1) The DC voltage from the panels 2) The Current measured in Amps from the panels 3) The overall power in Watts being generated by the panels. 4) Often the mains voltage will also be measured and shown on the display. Solar inverters are usually around the size of a small TV or a microwave oven, if you can't find the solar inverter in the house, these are often installed in loft spaces and adjoining garages.
• At the solar generation meter:A solar generation meter is a small piece of equipment that counts the solar power that is being generated. Common places for these to be installed are 1) Near to the consumer unit/fuse box 2) Near the incoming mains/grid supply meter (which can sometimes be outside in a box on the side of the house) 3) In the loft space, if this is where the rest of the solar equipment has been installed. If the solar panels are working, the solar generation meter meter will count the power that they generate, measured in kWhrs and you will see the number increase.

Solar inverters and solar generation meters aren't always installed in convenient locations. The good news is that if you would like to keep a closer eye on what the solar PV system is doing, this can easily be done by either installing a comms device into the solar inverter which will then provide data to a laptop/PC/smartphone via a network cable/GPRS or wireless and or there are also devices that will take readings directly from the solar generation meter and display the results such as overall and current power production on a handheld or wall mounted display.

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

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Yes, more panels can be added to your system at any time. However you will need to bear in mind that new panels cannot be added to a system that has already been signed off for a feed in tariff (FIT), in real terms this means that although more panels can be put on your roof, the new panels cannot supply power through the same solar generation meter that is used to count generation for FIT payments i.e. you cannot install more panels to an existing system in order to increase FIT income.

But if this describes your situation, there is no reason to feel sad, the good news is, particularly if you will use the extra power that you generate, is that solar PV installation prices have dropped considerably since the days of the Feed in Tariff.

Solar now stands on it's own as a good investment for those looking to generate their own power and unhook themselves from ever increasing mains electricity prices. The other thing to remember is that additional panels may take the overall amount of power coming from your property over the 16A per phase that is allowed to be connected to the grid without prior permission from the DNO. If this is the case you will need to agree that additional capacity can be connected to the grid with the DNO before the new panels are commissioned.

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Deliveries to anywhere in the UK are quick, tracked and accurate, technical support (by phone and on site if needed) and design/product advice is available before, during and after installation. The recording and collating of serial numbers, factory and other test results, manuals any other information required for equipment warranties, add ons and extensions is included.

Trade, DIY and Retail Clients Welcome.

A whole house surge protector is installed to provide protection from transient overvoltages originating from the mains/grid. A whole house surge protector is installed directly inline and as close as possible to the incoming mains/grid supply meter, this allows for surge protection for all circuits and equipment including solar inverters, routers, stereos and other sensitive electrical equipment within the network. The addition of a 100Amp lockable isolator also allows for safe and convenient isolation of all electrical equipment within the network including consumer units, solar inverters, battery storage units and EV chargers from the mains/grid in one place.

Surge protectors are in compliance with the recently updated 18th edition amendment 2 of BS7671.

DC surge protection devices (SPDs) are installed between the solar panels and the solar inverter to protect both the solar inverter and the downstream electrical equipment from transient overvoltages of an atmospheric origin impacting the electrical system via the DC side of the system / the solar panels.

DC Surge protectors are in compliance with the recently updated 18th edition amendment 2 of BS7671.

Engineering Recommendation G98

Grid 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 Dwellings

This 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 generator

In 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 Systems

After 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.