PhotoVoltaic Energy FAQ

What is solar PV? – how is it different from solar thermal energy?

Solar photovoltaic (PV) panels generate electricity from sunlight. This is different from solar thermal panels which absorb heat from sunlight and use it to warm up your water.

PV panels generate a DC voltage and this is converted to 240V AC, compatible with your domestic supply, using a grid connect inverter. When you generate more than you are actually using, the extra will be exported to the grid.

This is not the only way to use PV – for example small panels can also be used to charge up a battery that powers an off-grid appliance such as an outdoor light.

PV panels require large areas of doped semi-conductors and although the materials, such as silicon, are very common the purification and processing is expensive. There are a range of types which more or less efficient at converting sunlight into useful power. Needless to say, the more efficient ones are the most expensive. However, unless you have a very small roof the efficiency is not so important as the cost.

You should certainly consider solar thermal panels instead of or as well as solar PV. However, there is only so much hot water you can use and most households will only need 2-4 m2 of solar thermal panels whereas you can have as much solar PV as you have space for and export the excess power to the grid. The savings on your electricity bill plus the 'feed ­in tariffs' (FITS) subsidy will give you a reasonable return on your investment as long as you use a good proportion of the power yourself. You can also add batteries into the system, or use spare electricity to heat your hot water tank if you have one.

How much electricity will I generate?

The energy intensity of sunlight varies through the year and from place to place. Solar panels will still function on cloudy days but the overall intensity is less than in clear weather and when the sun is low in the sky the energy is also reduced. Also the efficiency of solar cells varies.

System prices are usually expressed by power rating rather than area and the rating is usually expressed as so many kWp (kilo-watts-peak). This indicates how much power (kW) they can produce under optimum conditions (depending also on load and temperature) from a standard level of sunlight which is 1 kW/m2 – only achieved on a very bright sunny day here.

In Cambridge, you can reckon on a total of about 850-900 kWh/year yield from a 1kWp installed system, with no shading at optimum orientation and tilt, see below. A typical size installation is 3 kWp which would give you about 11 kWh/day in July, but only about 2.4 kWh/day in January. A typical household uses 9-10 kWh/day. Two useful websites with solar calculators are: PVGIS and PVWATTS (used below)

You may be limited as to how much energy you can generate either by how many panels you can afford or how much space you have to put them on. You can reckon on 6-7m2 space to get 1 kWp depending on the efficiency of the cell technology. In any case, domestic installations are rarely more than 4 kWp because then you need special permission to connect to the grid.

Also, cells do degrade over time, some types more than others. You should have a guaranteed level of performance such as 80% of rated power after 20 years.

What about shading?

Because of the way that the individual cells of the array are connected together, the effect of shading can be greater than you might expect. If the array is 10% in shadow the drop in output may be much greater than 10%. To minimise this impact, it is important to take care with the arrangement of your array:

  • try to avoid putting panels where they will be shaded
  • if shading is unavoidable consider using optimisers or micro-inverters so that shading on one panel does not affect the others.
  • be aware that trees and other vegetation may need regular pruning so as to avoid shading where there was none before.

What about the angle and orientation of my roof?

These graphs were derived from an online solar calculator called PVWATTS using London as the reference location.

Most installations are fixed tilt and orientation for simplicity. You would get a better yield by adjusting the panels to face the sun at all times but this means moving parts and potential maintenance issues and you don't want to keep having to climb onto your roof.

Assuming the position is fixed, the ideal is to face directly south. However, if your roof faces SE or SW you will still get 90% of the ideal. If it faces east or west then it would be down to about 75%.

Ideally your panels should be tilted between 30° and 40° to the horizontal. If you have a flat roof then you can fix your panels onto tilted frames. Most sloping roofs are in the right ballpark anyhow. A horizontal panel will capture around 85% of the ideal but a vertical panel is much worse: only 64%.

Seasonal variation

Again, the graph was derived from an online solar calculator called PVWATTS using London as the reference location.

Obviously, you will get a lot more sunlight in the summer than in the winter - nearly 5 times as much. Also, you don't get any at night.

Since you will be supplementing your energy supply from the grid, this does not make much difference to you but in case you are interested, here is a chart showing typical variation through the year. No I don't know why it dips in June. It must be something to do with typical cloud conditions.

Annual variation

The weather is not reliable but solar panels don't need direct sunshine and the energy you generate is quite similar from year to year. The left chart below shows how much sunshine varied in 2008-2010 and the right hand graph shows how the solar cells performed. Overall, there was only +/- 2% variation in energy generated, while the sunshine was +/- 8%.

2 graphs showing sunshine hours/month and PV yield ditto for 2008-2010

Sunshine data from the Met office Cambridge weather stations. PV Yield data from Clarke Brunt in Milton

What will I earn from generating this electricity?

There are two ways that you earn money from your solar cells:

  • By using it yourself and not buying from the grid – typically saving 14p-15p/kWh but it depends on your supplier
  • From government subsidy through the Feed in Tariff. There is a cap on the number of installations that can qualify for FiTs each month. However, at the moment the number of applications is less than the cap so there should not be any problem getting it.

The Feed in Tariff (FiT) comes in two parts:

  • Generator tariff applies whether or not you use the electricity yourself. as of April 2017 this is 4.14p/kWh for installations of up to 10 kWp. Whatever level you get is guaranteed for 20 years.
  • Export tariff applies to electricity you export to the grid: currently 5.03p/kWh. In practice this is not metered on domestic installations, and it is just assumed that 50% of the energy that is generated is exported.

In total you earn 6.56p/kWh from FiT and this is index linked and tax free. If you use it yourself you effectively get more than 20p/kWh.

For the latest FIT rates go the OFGEM site here

NB. To qualify for these payments you will need to have an energy performance certificate for your house at level D or above. Otherwise you will get a lower rate.

Suppose you have a 3 kWp system in an ideal location and you use half the power from it in your house (bear in mind that if the house is normally empty during the day and your usage is mainly early mornings and evenings, then you will be seriously out of step). Suppose you normally pay 14p/kWh

1 kWp generates 880 kWh so 3kWp gives you 2640 kWh on average

Generator and export tariff2640*6.56p£173
Avoided cost of buying from the grid2640/2*14p£185
Annual Total £358

For a 3 kWp system costing about £5000 including 5% VAT. So your annual return is about 7.2% (tax free), varying somewhat with the weather of course, and provided there are no other costs.

The returns on these systems depend very much on your usage pattern - whether or not you can use electricity at the time it is generated or not. Here is a spreadsheet which you can use to calculate your returns, in a simple way.

For more information about the FIT from the horse's mouth, so to speak, go to OFGEM.

Can I heat up my hot water tank with spare electricity?

Rather than exporting all your 'spare' electricity onto the grid it is possible to connect up the panels to your immersion heater so that when you aren't using all your power you can at least get hot water out of it. This is not a simple connection because you need a system which detects how much power you are using and diverts only what you can spare into the immersion heater. Also, it needs to turn off if the water tank gets too hot.

There are several products on the market which you can choose from, for example, look for Immersun, SolarCache+ or SOLiC 200.

See also Nicola's blog post here.

What is the energy payback time?

The energy payback time is the time taken to generate the energy which was used to manufacture and install the solar cells. Most studies on this estimate a payback time of less than 3 years, even in the North of England. The lifetime of the system will be typically at least 25 years, so even the worst estimates give an overall positive payback. There are some nice charts here

Will my solar PV system protect me from power cuts?

With a standard grid connected system, you can't use it in a power cut because if you did the voltage would feed back into the grid and you could electrocute any poor maintenance man from the electric company trying to fix the problems.

You could install a battery based system with a separate inverter and a change over switch, and keep the batteries charged with a solar array. If you really want to run your entire house from it in the event of a power cut, this will be expensive however because you will need a lot of battery storage but you should be able to run important equipment like your refrigerator and freezer from this.

What do solar panels look like and how do they fix to my roof?

There are several options. The panels are typically 1x1.5m but they do vary considerably.

There are lots of pictures of installations by a local supplier here.

For most installations, a few tiles or slates are removed and brackets are fitted to the roof timbers underneath. The tiles/slates are then replaced, and an aluminium frame attached to the brackets, and the panels mounted on the frame. Alternatively, it may be possible to fit the panels flush with the roof tiles – although this is usually more expensive when fitting to an existing roof.

However, PV panels need good ventilation (because they are much less efficient if they get hot) and solar PV tiles, fitted instead of normal tiles, may not get enough, so they may not be a good choice.

It is possible to fix solar panels to most types of roof, but if in doubt you had best ask your supplier.

Do I need planning permission?

You don't need planning permission to put solar panels on the roof of your house unless it is a listed building. Even if you are in a conservation area, you are allowed to put panels on the roof, as long as they do not protrude above the ridge or 20mm from the slope. If you aren't in a conservation area, you have more freedom to put PV panels in various places. The rules are explained in more detail here

You will need building regulations approval: the inspectors will be concerned that your roof is strong enough. (You might be surprised. Recent building works on my house revealed that the old ridge beam did not extend as far as the wall on one side – it must have originally been sitting on a chimney breast that had since been removed). Registered 'Competent persons' can self-certify – so this will normally be handled by your installer.

Will this affect my insurance premiums?

From our survey so far, no, but you should check to be on the safe side.

Can I install it myself?

Yes - but it probably won't be economic to do so, because in order to qualify for the FIT, your system must have been installed by a company recognised by the Microgeneration Certification Scheme (MCS). You can find a list of such suppliers here.

We know happy customers who have used Midummer Energy and Cambridge Solar. These are both recommended and locally based.

The inverter you use has to be certified as 'G83 approved'.

Does it need maintenance?

Some installers recommend that you clean your panels twice a year or so and you may get a few percent more power if you do. If they are tilted (at least 15 degrees above horizontal) then the rain will wash dirt away but if there is a prolonged drought then they might get dusty. Most reports of the benefits of cleaning panels are based on field tests in dry dusty places like Arizona - not England! If you do clean your panels with tap water, be sure to wipe drops of water away with a squeegee just as you would for windows - Cambridge tap water is hard and will leave scale deposits if you don't. Rain water is very soft and does not have this problem.

The likeliest bit to go wrong is the inverter and they are usually guaranteed for only 5 years although you will be able to buy an extension. The inverter is not on the roof, so it is relatively easy to get at.

One problem is that you may not notice if the system has gone wrong at all. The best thing to do is to regularly check the generator meter and ensure that you are actually generating. Many system come with a wireless monitor so that you can see at a glance if the system is generating.

Will it make any noise?

The inverter can make a humming noise, from its cooling fans. Some inverters are quieter than others.

I have heard there is a fire risk - is this true?

There is no more risk of a fire starting than with other electrical equipment in the house.

Inverters are designed to protect against "islanding", which is when there is a grid power failure but on-site generation still supplies the building. This can be dangerous if someone doesn't realise the power is live and comes to work on the system. In the event of a fire destroying the mains connection, this should operate so that the 'mains' side of the inverter is not live either.

There is a potential danger however if the fire destroys the insulation on the DC cables that lead from the solar array to the inverter. In this case, in strong sunlight there would be a very high DC voltage present, which could potentially be dangerous to firefighters – especially if they are spraying water around the place. So if your house does has the misfortune to catch fire with a PV array on the roof, don't attempt to fight it yourself – leave it to the professionals, who should know what to do!

How does it connect to my electricity wiring?

There is a clear discussion of this here:

Basically, the panels are connected via the inverter, which is often in your loft, to your main distribution board where there will be a new meter showing how much you generate. You can also have an export meter measuring how much you export to the grid but most people don't bother with it, as the export tariff is so small, and this meter has to be installed by your network operator (who will charge) rather than by the installer. If you don't have an export meter it is assumed that 50% of the generated electricity is exported.