FAQs - Your questions on PV answered

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 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 financial benefits 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 (kilowatt hours, = units as measured by an electricity meter) per year.

A display, like that shown right, can show you how much electricity the array is producing each day. This particular model, which is powered by its own PV cell on the back face, receives its information from the inverter by Bluetooth and can communicate with your own computer. We also fit a Generation Meter, which shows the accumulated number of kilowatt-hours generated and can be read at any time. Thus you will know precisely how much energy you are generating and can compare this with our predictions.

It is difficult to assess how much energy you will use yourself and how much you will export to the grid. 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. You will know the amount of energy generated (from our Generation Meter) and the amount you are still taking from the Grid (from the Grid's meter, as before), but not how much you export. Unless you have an Export meter fitted (by the electricity company, not us), for domestic systems, the Government assumes you are exporting half of what you generate, and your electricity company will pay you the export tariff accordingly.

How much carbon emissions will this save?

We will tell you when we make the energy assessment and you can easily calculate it from the Generation Meter. 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. According to DEFRA, 1 kWh of mains electricity is responsible for 0.545 kg of carbon dioxiode emissions; thus a solar PV array generating 2000 kWh per year would prevent 1.09 tonnes of carbon dioxide emissions each year – a sizeable amount!

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 are looking into options for the PV to run local systems when the power is down. Such systems are made and used in Germany, and we hope will soon be authorized for use here.

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. It will also receive the Feed in Tariffs.

How big do the panels have to be?

About 7 sq. m for each kWp of output. 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. Bigger roofs give a better return (up to 4 kWp) but good systems can be installed on small roofs.

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!

If there is light shading and this will fall differently on each panel, then we can fit a micro-inverter under each panel, rather than connect up the panels into a string and fit one inverter. This means that if one panel is shaded, its own performance will drop but it will not hold back the other panels. This is a little more expensive but worth doing in cases where roofs suffer differential shading.

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.

© Soltrac 2011