What is PV? 

PV means Photovoltaic. A solar PV panel is one that converts the sun’s energy into electricity – power that you can then use for your fridge, cooker, computer or any other electrical device, or sell to the electricity grid. The panels produce electricity that is Direct Current or DC. An inverter converts that into AC (alternating current) that you can then use or export. We wire the panels into an inverter (with a big switch for safety reasons) and then the inverter to a spare 'way' in your electrical consumer unit, again with 2 more switches and with a generation meter. The system is then ready to go.

Is it economical?

It used not to be, but on 1 April 2010, the government started its generous Feed-in Tariff (FIT) for electricity produced by small-scale producers using renewable systems, including PV, hydro-power and wind. This makes PV a very good financial option; please see our separate page on FITs and the economics of PV.

How much electricity will I produce?

A PV system is rated at peak kilowatt capacity (kWp), that is the kilowatts produced under strong sun, precisely of 1000 watts per sq. metre. We will tell you this capacity of the system. And it is this value that determines which band of payments you will receive.

Computer simulation programmes will enable us to predict how many kilowatt-hours of electricity you will produce each year. This depends on the peak watt assessment but also the angle of the roof, the orientation and shading. In general, in the south of England, an array of 1 kWp will generate about 850 kWh per year.

It is difficult to assess how much energy you will use yourself and how much you will export to the grid. Remember that your production will peak at midday in summer, when perhaps only your fridge and deep-freeze will be drawing power, whereas the maximum consumption of most families is around 5 pm on winter afternoons – when there is no solar energy.

How much carbon emissions will this save?

We will tell you when we make the energy assessment. At present 80% of the UK’s electricity is generated from fossil fuels, very inefficiently, and also much is lost in transmission on the grid. So the carbon savings from your investment could be considerable and really help to reduce your carbon footprint. Industry sources claim each kWh of mains electricity is responsible for 460 g of carbon emissions, but some experts dispute these figures and claim the emissions are higher; taking 500 g/kWh as a reasonable assumption, you would save 1 tonne (=1000 kg) of carbon dioxide emissions for each 2000 kWh of energy you generated from the PV array.

Can I also get a grant?

No, the grants for domestic producers ended for new applicants two days after the government announced the Feed in Tariffs.

The situation for public sector, charitable and community bodies is still to be clarified.

Will a PV system allow me to have electricity when there is a power cut?

No, the system has to be programmed to shut down the solar power when there is a power cut, otherwise the electricity you produce could electrocute the engineers from the supply company working to get the power running again. We know that this is a concern for some rural customers who have frequent power cuts and will look into any options possible for the PV to run local systems when the power is down. Farms, hospitals and other facilities often have oil-powered generators to supply electricity when the power is down, so it must surely be possible to develop similar systems for PV arrays.

Can I connect the PV panels to a battery and so be self-sufficient?

Yes, you can and in the early days most PV systems were done this way. But for the domestic user the time of maximum production does not match the time of maximum consumption. And batteries are temperamental, expensive, space-consuming and most important, have a large carbon footprint in their manufacture. Far better is to treat the grid as a large battery, to which we all contribute and from which we all draw off electricity as and when we need it. The grid is in place and works; there is no environmental gain decoupling from it.

That said, a stand-alone PV system is a good option for sites not connected to the grid and where a grid connection would be exorbitantly expensive.

How big do the panels have to be?

Solar PV needs larger areas than solar thermal because the process is not so efficient, but the panels are usually smaller, typically 1600 x 800 mm, so they can be assembled in many different-sized arrays. Solar PV panels typically convert 10-15% of the sun’s energy into electricity, whereas solar thermal panels, used to heat water, usually convert 60-80% of the sun’s energy falling upon them. We will assess the potential of your roof for PV at the site survey. Our page on FITs and the economics of PV gives some sample sizes and costings of PV arrays.

Does the roof have to face south?

The biggest component of the cost of PV systems is the panels, and so they are usually best fitted only in optimal situations. We will run a simulation for you showing the energy produced, from which you can calculate the return. But in general we would be sceptical whether PV is financially worthwhile unless your roof faces between southwest and southeast. This is in contrast to solar thermal, where east- or west-facing systems can be designed to work well at reasonable cost.

The best angle for solar PV is between 30 and 45º to the horizontal.

Is shading important?

Yes, more so than with solar thermal.

The panels are composed of small cells connected in series, and the current builds up across them from cell to cell. If the shadow of say a TV aerial or electric cable crosses the panel, it may disrupt the flow of electricity from cell to cell, and so reduce the electricity produced by a far greater amount than the proportion of the panel that is shaded. So avoid roofs that are shaded!

Does it work on cloudy days?

Partly. Just like solar thermal, it works best under full sun. But even on dark, heavily clouded days in winter, some electricity may be produced.

What controls whether the electricity is used locally or exported?

The inverter, which converts the DC circuit from the panels to AC, senses the amplitude and periodicity of the AC current provided by the grid. Using the power from the panels, it matches the periodicity but produces current with a voltage 2–3V more than that of the grid. This means that when you are producing more current than you need, because of its slightly higher potential, the excess current will flow into the grid, fuelling other homes. No clever switchgear is needed; it is all taken care of by the inverter.

How does this fit into the bigger scheme of things?

According to DECC, we currently get around 5.5% of our electricity from renewable sources. To meet the 15% target for all energy by 2020, we will have to increase the 5.5% figure to around 30%. Modelling studies for DECC show that small-scale renewable installations could meet 2% of electricity demand in 2020. Elsewhere DECC say that they expect that by 2020 Feed-in Tariffs will support over 750,000 small-scale low-carbon electricity installations and will have saved 7 million tonnes of carbon dioxide.

Despite critical comments from some sections of the press, scientists and decision-makers remain convinced that climate change is real, is happening now, is caused mainly by burning of fossil fuels and could itself cause major disruption to life styles in years to come. If you are not convinced, please see our web pages that answer the concerns put by the “sceptics”.

And some practical issues on the installation

• We offer free site surveys, which are just that, not sales presentations. After the site survey, we will prepare a quotation and assessment of the energy generated.
• For houses, planning permission is only required where:

1. The panel protrudes more than a fixed amount above the roof plane, eg it is on A-frames on a flat roof; or
2. The house is a listed building, in which case listed building consent is also needed; or
3. The house is in a Conservation Area (usually a village or urban designation) AND the panels would be visible from a public highway.

For commercial buildings planning permission is still needed but the rules are due to change soon, to bring them into line with domestic properties.

• Scaffolding is usually needed; we will arrange it and it is included in the cost.
• Small installations typically take 2 days, larger ones longer.
• The circuit will be connected up by a fully qualified and experienced electrician, who has further trained in PV and whose work has been inspected by the Microgeneration Scheme. This is not a job for amateurs!
• A generation meter will show you the amount of electricity generated.
• We provide a User Manual on completion of the job and will register the installation with Gemserv so you can claim the Feed-in Tariffs.
• You will not notice any difference to your daily life. From the electricity you produce, first priority goes to its use in your home; if there is more power than is needed, then the balance is exported to the grid for other customers.
• All our work is guaranteed for 5 years. The panels come with longer guarantees, which we will explain to you. We will also offer service contracts to those that need them.
• And if something goes wrong, please call us and we will put it right. We pride ourselves on our customer service.

© Soltrac 2010