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Solar Electricity - FAQs

Below we list common questions about solar electricity and solar PV systems. If you have any other questions that you think we may be able to help answer then please contact us.

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What is kWp?

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

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

How does a solar PV system work?

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

What does energy conversion efficiency mean?

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

What is net metering?

Does a solar PV system need batteries?

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 do I know if my solar panels are working?

How long do solar photovoltaic (PV) systems last?

Can I add more solar panels to my system?

What maintenance and cleaning do solar panels need?

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?

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 inverters in system sizing calculations.

Ensuring that cables, 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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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. 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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What are the components of a solar photovoltaic (PV) system?

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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How does a solar PV system work?

Daylight hits the photovoltaic cells and is converted to electricity.  The 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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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 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 specifiying or choosing which is the best equipment to use.

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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, UK Power Networks etc). There are two engineering recomendations (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 recomendation 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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What is net metering?

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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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' 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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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 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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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.

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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?

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How do I know if my solar panels are working?

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

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 inverter is needed to supply power to a property. The current lifetime estimates for solar PV DC/AC inverters is from 8-12 years. Anyone installing solar should allow for inverter replacement costs every 10 years.

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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. At low tilts (less than 16°) horizontal glazing bars can sometimes trap debris which could lead to shading of part of the array. The design of the system should aim to minimise uneven soiling. Panels are pretty robust so if they get dirty rub off the dirt with a damp cloth or blast them with a hose (no detergent).

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Can I add more solar panels to my system?

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

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

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How to size a solar photovoltaic (PV) system?

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 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 excercise 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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Professional Solar PV System Design & Planning

The success of any renewable energy project is always in the quality and robustness of the system design, our experienced in-house design team has been designing innovative and ground breaking solar PV and other renewable energy systems since 2007.

Solar PV System Design
We take a collaborative and open approach and often find the very best systems are those where the system owner/developer has taken an active involvement with the design.

We can help prepare cost effective designs, roof layouts and documentation for all types and size of solar PV project; including drawings, bills of materials and detailed custom installation/owners manuals. We work with installers and developers as well as private clients and businesses, so if you need any technical information prepared then contact us, we have specialist knowledge and experience at your disposal, you will find us easy to work with and happy to help.

Solar PV System Design & Planning

Solar PV Installation & Commissioning

Need experienced hands and heads on-site? Our engineers and electricians are some of the best and most experienced in the business. We can add real value to any project through our flexibility, efficiency and the quality of our work.

Solar Panel Installers

Our solar PV installers, roofers and electricians have carried out installations of all shapes and sizes; building solar PV arrays onto most types of roof including commercial roofs and flat roofs. We have built ground mounted frames and installed solar panel mounting structures such as carports and canopies; we have safely installed solar panels on top of barns, in open fields, on factories, houses, workshops, stables, garages and sheds.

Solar PV Installation & Commissioning

 

Articles

Further reading and calculators related to designing, installing, repairing and maintaining solar photovoltaic and electrical systems:

Types of Solar Photovoltaic (PV) System

Introduction to the different types of Photovoltaic (PV) System available including Grid Tied, Off-Grid, Hybrid and Batteryless solar PV systems.

Solar PV System Components

Overview of the basic components needed to install a complete solar PV system. Introduction to solar PV panels. power inverters, AC & DC isolators and mounting systems.

Free Solar PV Calculators

A list of free solar PV calculators, solar design tools and software, Use to calculate solar yields and the Return on Investment (ROI) for solar PV systems.

Evaluating Solar PV Panels - Which panels to buy?

What to look for when evaluating and selecting solar PV panels for UK installations. Electrical characteristics of solar PV arrays and the safety factors used for selecting BoS equipment, also mounting options.

Solar Panels - PV Array Calculator

Solar Panels: Solar PV System sizing and power yield calculator. Use to work out roof layouts, PV array sizes, No. of panels and power yields. Based on SAP 2009.

Solar PV: Smart Export Guarantee

Interesting times ... The Smart Export Guarantee came into affect in January 2020. The Smart Export Guarantee is an obligation set by the government for licensed electricity suppliers to offer a tariff and to pay small-scale low carbon Generators and Micro-generators for any electricity that they export to the National Grid.

Flat Roof Solar PV Array Spacing / Shade Calculator

The minimum required space between parallel rows to avoid shading is decided by the height of the array immediately in front, the slope of the roof and the lattitude of the installation site. This table illustrates the different row spacings required for optimum positioning in different locations.

Solar Electricity - Frequently Asked Questions (FAQs)

How to size a system?, What is kWp?, What is the difference between a kilowatt (kW) and a kilowatt hour ( kWh)?, How does a solar PV system work?

Engineering Recommendation G98

Grid Connections for Micro-Generators including Solar PV Systems and Elecricity Storage Systems in the UK. Under 16Amps Per Phase, grid synchronised.

Preparing for a Solar Panel Installation

As with any construction project the success and efficiency of a solar PV panel installation comes down to good planning. Some advice for potential system owners preparing for a new solar panel installation.

Planning Permission for Solar Photovoltaic (PV) Systems

A solar PV installation can be classed as 'permitted development' subject to conditions and when not located within a conservation area, AONB or world heritage site.

Solar PV, Safety & The Building Regulations

Residential PV systems notifiable under Part P. Special consideration needed for Part A. A combination of serious risks for solar PV installers.

Safely Retrofitting Solar PV Roof Mounts

The things to consider before installing a retrofited solar PV system on a roof and an introduction to the type of equipment used to secure an on-roof solar PV system.

Smart Switching / Solar PV Power Switching

Available as an addition to existing solar PV systems or installed as a package alongside a new system, Smart switching puts full control of a solar PV system's power output firmly in the hands of the system owner.

Mains Power Supplies & Earthing Systems

Mains Supply and Earthing Systems: Location & accessability, supply systems, earthing, bonding, labelling, installing RCDs.

Electrical Warning Signs & Labelling

A quick overview of some common electrical warning signs and labels that you might find attached to electrical equipment.

ABB / Power One Aurora Inverter Faults and Repairs

Power One, at one point were the second ranked inverter manufacturer in the world and there are many Power One Aurora Inverters installed in the UK. The most popular models being the Uno PVI-3.0-TL-OUTD and the Uno PVI-3.6-TL-OUTD.

Fronius IG / IG Plus Inverter Faults and Repairs

Fronius IG and IG Plus series inverters have LCD displays on the front of the chassis that providing it's working, will highlight any errors with the inverter or the solar PV system that it runs.

Mastervolt Inverter Faults and Repairs

Mastervolt Sunmaster and the smaller Soladin ranges of inverters were widely installed in the UK between 2011 and 2014. Popular Sunmaster models being the Sunmaster XS2000, Sunmaster XS3200 and the Sunmaster XS4300.

SMA Sunnyboy Inverter Faults and Repairs

SMA Sunnyboy Inverters have been widely installed in the UK, some of the most popular being the SB1200, SB2000 and SB3000. High frequency models include the SB2000HF, SB2500HF, and SB3000HF. Transformerless models include the SB3000TL and the SB3600TL.

SolarEdge Inverter and Optimser Faults and Repairs

The SolarEdge system is unique and in our opinion unrivalled in terms of it's ability to monitor system performance down to panel level. This is achieved through the installation of a small unit called an optimiser.

Sonnen Macht Inverter Faults and Repairs

The problem we often find with these inverters are damaged relays, the tell tale sign of a relay failure is an Error 19: Relay or Error 19: Relay Fault warning displayed on the inverter's display.

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