About Photovoltaics

Overview

Rather than having just a handful of ageing and polluting power stations we now have the possibility of decentralisation; millions of small generators producing power where it is needed, solving the problem of large energy losses during transmission.

Although photovoltaics work better in direct sunlight, they are not reliant on it. They only need daylight to produce electricity.

PV's are silent and unobtrusive, they are designed to withstand the elements and weather very little with age.

PV's have no moving parts and are very reliable. Most PV modules are guaranteed for 20/25 years, with a life expectancy far exceeding this. Maintenance requirements are negligible.

Solar PV produces electricity, not hot water. Please see our solar thermal page for information on thermal systems.

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Terminology

Module
A PV (photovoltaic) panel, which is made up of a number of 'cells', each of which produces a small electrical charge under light.

kWp (kilo-watt peak)
The number of thousand watts a photovoltaic system will produce in peak conditions.

kWh (kilo watt hours)
A unit of electricity. This is defined as power of 1000 watts (being consumed or produced) over the period of one hour. e.g. 2000w over 1/2 hour or 100w over 10 hours.

For example:
A PV module could have a rating of 200 Wp (produces 200 watts in peak conditions). 12 modules are connected together on a roof to form a 2.4 kWp array (200 * 12 = 2400 Wp or 2.4kWp). Using SAP (see 'electricity produced' below), this array will generate 2.4 * 858 = 1992kWh / year (30/40 deg pitch, south-facing & un-shaded).

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Module Types and Efficiency

Solstice Energy can supply and install any PV module accredited for use in the UK, such as Sanyo, Sunpower, Sharp, Kyocera, BP or any others. If you have a particular requirement or preference for any reason please let us know.

People often confuse efficiency with quality, but all it means is that high efficiency modules can generate the same amount of power as lower efficiency modules in a smaller area. For example, 210Wp Sanyo hybrid and Sunpower monocrystalline modules are almost the same dimensions as 170Wp Sharp polycrystalline modules. Our design team tends to recommend the (proportionally more expensive) high efficiency modules for smaller roof areas or where a client really wants to maximise output, and less expensive polycrystalline modules for large roof areas.

Despite some manufacturers claims, we have found that hybrid, mono and polycrystalline all give the same kWh yield / kWp over the year. Quality and guarantee periods are similarly high with all the common brands available for use in the UK.

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Electricity Produced

Daylight levels in the UK mean that PV will produce an average of 858 kWh of electricity per kWp system size, per year (South-facing & un-shaded). This figure is reached by Standard Assessment Procedure, and forms the basis of our predictions.

An average UK household uses around 3000 kWh per year (so a 1kWp system would supply a quarter of the yearly use). Simple energy efficiency measures in the building will increase the proportion of energy coming from the PV's.

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Roof Suitability

Positioning

The PV array will usually be mounted on a Southerly facing, pitched roof.

The "solar sundial" below shows that a fair bit of deviation in the positioning of the array will still yield a large percentage of the energy expected from an optimum orientation (a pitch of 30/40 degrees facing due South).

solar sundial

Shading

Whenever shading decreases the power output of any one module in an array, then the output of the whole array is similarly decreased. It is therefore very important to avoid shading (from trees, other buildings etc) when siting a PV array. However, if, for example, the shading only occurs late on winter afternoons then it will have less impact on the annual output of the array than shading that occurs every morning until 12:00.

For complicated shading situations, our designers can plot the PV array and the surrounding objects and run a PVSYST simulation to show predicted output throughout the year.

Roof Area Required

Allow approximately 8m2 of pitched roof area (16m2 of flat roof) for every 1kWp of PV. Obstructions like dormer windows and roof vents need to be considered when surveying the site.

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Planning Permission

Planning permission is not normally required unless it is a listed building or in a conservation area - please see the Town and Country Planning Amendment of April 2008. If in doubt, check with your local council, and if you do need to make a planning application we will advise you and supply all the drawings, photographs etc that you need for a successful outcome.

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Where Does It Go?

There are two ways of storing the electrical energy produced by a photovoltaic array. One is by using batteries in a stand alone system (the remote house, boat, caravan etc); the other is by connecting to the national grid.

In a grid connect system, the grid works in effect as a battery. During the day the PV array might produce more energy than is required by the building it serves. This excess will be exported to the building next door or down the road via the mains. In the evening, or any other time of high use / low sun, electricity is drawn in the usual way from the grid, and supplemented by the PV system. This all happens automatically due to natural ebb and flow of electricity as it finds the shortest route to a load. Grid-connect PV is often referred to as 'fit and forget' because of the lack of user interaction or maintenance required.

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Power Cuts

If there is a power-cut the inverter will shut the system down, and there will be no generation. This is a safety mechanism to avoid 'islanding' - where workers on a supposedly dead grid might get a shock from PV systems in the area exporting power. Power cuts are thankfully few and far between in the UK at the moment, and the considerable problems associated with batteries (expense, maintenance, lifetime, storage, disposal etc) mean that grid / standalone systems are very rare. Advances have been made in the switching technology though - see http://www.sma.de/en/products/backup-systems.html.

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G83 / G59 Regulations

G83 regulations deal with the export of power below a maximum of 16A per phase, and cover most domestic systems. Under G83 we can install a system and notify the DNO (District Network Operator) afterwards.

If we multiply 16A by 230volts you’ll see the maximum system size is 3.68kW. However, we can install up to 4kW under G83, which is the point at which the best economies of scale meet the highest Feed in Tariff, by under-sizing the inverter i.e. installing an inverter that limits the maximum a.c. output to <3.68kW. Under-sizing the inverter is normal practice anyway, and is explained further here.

If a building has a three-phase electrical supply then we have three times as much capacity under G83 regs, so we can install up to 11.04kW (perhaps 12kW with inverter under-sizing) without the need for an application to the DNO, as long as the system is divided so that it is <16A on each phase.

Tip: the easiest way of telling if you have single or three / phase electrics in your building is to check the number of wires connected at the bottom of the electricity meter. Single phase = 4 (1 live in, 1 live out, 1 neutral in and 1 neutral out). Three phase = 8 (3 lives in, 3 lives out, 1 neutral in and 1 neutral out).  

G59: For systems above 16A / phase we need to apply for permission from the DNO.  Most inverters have built in safety features that satisfy G59 regs, and so you don’t need a separate G59 safety relay, contrary to popular belief. However sometimes there may be a requirement for the DNO to upgrade the local transformer (and charge for it) prior to connection so that it can safely handle the amount of power being fed into grid. This depends on the size of the system and also the location, and becomes more of an issue in rural areas ‘at the end of the line’. We’ve seen a lot of flexibility from DNO’s with our applications, and have had 50kWp systems (73A / phase) accepted without the need for any additional work or charges. It’s always best to get that application in early during the design stage so everyone is certain about this.

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Exporting Your Electricity

We install a generation meter as part of our installation, but not an export meter. If your levels of generation are high and annual daytime consumption is low then an export meter will worthwhile, and you will need to arrange this through your electricity company.

However, as a good alternative to fitting an export meter, most supply companies offer 'deeming' of 50%, where they assume that you are exporting 50% of the power you generate. This obviously suits people well when they are using the majority of the generated power in the house, and exporting less than 50%. If you find you are exporting more than 50%, you should find out what fee (if any) your supplier will charge for installing an export meter, and if the 3p / kWh paid for exported power makes this worth it.

Our advice for new domestic installations is to go with the 50% deeming, and see how you feel after a year of getting used to the PV system. It helps to change your patterns of use where possible, so that you use generated power as it comes off the roof - eg set timers so the dishwasher comes on at 10am, and the washing machine at 12. This way you will save a higher proportion of expensive imported power (at say 16p), and decrease the proportion of power being exported (for 3p).

You may find this doesn't work for you, but it will be easier to arrange installation of an export meter after a year than to fit it straight away and then have it removed.

Commercial buildings often use much more daytime power than the PV’s will generate so an export meter is definitely not required. In such a case, all generated power is used on-site and the owners will benefit from the FiT and the saving on import bills.

OLD METERS Old meters – Modern digital import meters will slow down and stop when PV’s are supplying more power to the building than is being consumed. Old meters (with the spinning wheel) will actually reverse under these conditions and so any exported power becomes worth 3p + the 16p saved from the bill. However, using an old meter under these circumstances could be considered fraudulent.

The introduction of ‘smart metering’ will likely start with micro-generators, and these meters will be capable of recording import & export, and so will replace old meters as well as deeming.

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Feed in Tariff (Clean Energy Cashback)

Feed in Tariffs replace the old grant and Renewable Obligation Certificate system, and are intended to be incentive enough for people to install renewable energy without grant funding. Payments are linked to inflation, and continue for a period of 25 years. The FiT is subject to degression which will reduce the rate, but only for new installations - once you install the system the rate is set.

The rates are:

Size & nature of system Payment / kWh generated
<4kWp 21p
4 - 10kWp 16.8p
10 - 50kWp 15.2p
50 - 250kWp 12.9p
250kWp - 5MW 8.9p

So for an example 4kWp system generating 3400kWh / year, you could expect:

1. 3400 x 21p = £714
2. 1700 x 16p = £272
3. 1700 x 3p = £51
Total 1st year income = £1,037

Note:

  • No2 assumes 50% of the power generated is used in the home (during sunny days), offsetting usual import cost of around 16p / kWh (inc VAT)
  • No3 refers to the other 50% exported to the grid at minimum export price of 3p / kWh.

The 4kWp system above would typically cost £9000 (including 2 storey scaffolding & 5% VAT).

Payback – At a cost of £9000 and with an annual benefit of £1037, this example system will pay for itself in under 9 years. This ignores the effect of inflation and electricity price rises which will continually increase the value of the system.

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Energy Performance Certificates

An EPC of band D or above is now needed in order to claim the standard FiT rate for the size of your system.

Solstice Energy is qualified to provide domestic EPC's, and we know that band D is not hard to reach. Those 100 year-old, solid walled, sash windowed properties are still achieving a D as long as the basics are in place - loft insulation, good heating controls etc, and there are companies offering "no D, no fee" promises because of this confidence. The addition of a solar PV or thermal system adds a lot of points and can be included on the certificate.

The EPC for your property would be provided free with any installation. If we have doubts about the ability of the building to reach a D then we will produce a 'mock' EPC after our initial site-visit so there would be no nasty surprises.

Non-domestic buildings also require an EPC to get the standard FiT rate, and we could arrange this through an external energy assessor.

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