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Solar Electricity - FAQs

Below we have listed some of the common questions we have been asked 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?

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 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 10-12 years. Although some manufacturers offer warranty extensions up to 25 years, solar pv system owners to be on the safe side should factor in a cost to replace the inverter every 10 years. Inverters cost from approx £800 for a sub 4kWp system.

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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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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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What grants and incentives are available?

From April 1st 2010 the UK Solar PV Feed in Tariff (FIT) sometimes refered to as the 'Clean Energy Cashback Scheme' was introduced. This is an incentive designed to encourage the installation of solar PV systems by providing a guaranteed repayment on the investment.

Payments are based on the size of system that is installed (the kWp) and the actual amount of power the system generates (kWhrs measured by an OFGEM approved generation meter). The Feed in Tariff (FIT) incentive is available for all types or customer including, homeowners, businesses, industry, schools and charities. We provide more information about the Solar PV Feed in Tariff (FIT) here: Solar PV: UK Feed in Tariff (FIT)

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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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What is The Code for Sustainable Homes (CSH)?

The Code for Sustainable Homes (CSH) aims to achieve a step-change in environmental performance of new UK homes setting out a timetable to reduce carbon emission in new build housing to Zero by 2016 via new Building Regulations.

CSH is targeted at architects, home designers and builders of new homes and covers  water use, waste generation, and the use of low-polluting materials and processes as well as energy.

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What is the 'Merton Rule'?

All Local Authorities are required by government to publish a ‘Local Plan’ in which they set out their proposals to reduce local energy use and carbon emissions. 
In 2002 Merton Council developed a ground breaking local plan which required new developments to supply 10% of their energy use from on-site renewable sources. Many local authorities, including all those in Greater London have increased the requirement to 20%. Increasing numbers of local authorities are have adopting it and many more are developing similar ‘On-site renewable energy requirement’ plans.

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