PhotoVoltaic Energy FAQ

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Questions answered on this page:

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 hot water cylinder. You can use electricity from your PV panels to heat water too, via an immersion heater.

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, you can either store the extra in a battery, for later or export it 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.

The efficiency of PV panels has increased a great deal over the 10-15 years. However, if you are limited by available roof area then you should look at kWp (kilo-watt-peak) per unit area. Otherwise you might be more interested in kWp/ for installation.

Solar PV panels are less efficient than solar thermal panels but they are more versatile. You can use it for any electrical appliances, not just heating your hot water, and you can export what you do not need.

We often talk about self consumption - this is the proportion of your PV power that you use yourself. Without a battery you are unlikely to improve on about 30%. With a battery you can do better but unless you have very high electrical demand in summer you will not use it all. With a battery you can also (to some extent) choose when to export the excess - in combination with a variable (Time Of Use = TOU) tariff you can improve your financial return and/or your carbon savings.

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.

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 from a standard level of sunlight which is 1 kW/m2 only achieved on a very bright sunny day here.

The power generated also depends on the temperature of the panels - they do less well when they are hot. Unfortunately they get hot, not just from the air temperature but also because they are not 100% efficient (just like a battery or a power transformer gets hot when it is in use). So it is very important that they have a free flow of air around them (or some other cooling mechanism).

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 is much more important 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.
  • keep your panels clean - dirt has the same effect as shading Do they need cleaning

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% but you do get more hours in the day.

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%. The vertical panel will produce more power in winter when the sun is low, but the sun is weaker than because the radiation skimming the surface of the earth has a lot more atmosphere to go through.

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. The sun is highest in June and yet there is a dip then probably because the panels over-heat.

The angle of incidence is also significant as the sun is higher and stronger in the summer. The optimum angle (in terms of annual yield) is close to (90 minus latitude) so that the panels are at right angles to the sun when it is high.

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 from October 2023, a typical tariff would be 34p/kWh but it depends on your supplier
  • By exporting it to the grid and receiving a payment for this. Since the Feed in Tariffs closed there is a new scheme called the Smart Export Guarantee. Energy suppliers are required to offer a guaranteed price for renewable energy but there is no minimum price and there may be conditions attached. You will need to shop around to check where to get a good deal. They are highly variable. The best right now (September 2023) is 15p/kWh from Octopus.
    • Some retailers offer tariffs which vary through the day. For example this one from Octopus has increased export tariff during 4-7pm. If you use your battery to 'shift' your export you can reduce your bills.

Suppose you have a 4 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 34p/kWh

1 kWp generates 880 kWh so 4kWp gives you 3520 kWh on average

Export tariff3520/2*15p264
Avoided cost of buying from the grid3520/2*27p475
Annual Total 739

Saving carbon emissions with Solar PV and a battery.

Your panels generate electricity which is very low carbon. By using this instead of taking power from the grid you are reducing your carbon emissions, regardless of the time. However, if you have a battery as well, you have some flexibility as to when you use the power you have generated that you did not use at the time. If you can use this when carbon emissions are high, or if you export it when carbon emissions are high, you can reduce your emissions still further. However, you should also remember that your battery and inverter system is not 100% efficient. For each 1kWh you use in charging, you may only get 0.9 kWh out again. If it was 10% loss overall, that means that you need a 10% difference in emissions intensity to get any reduction in emissions from using the battery.

It is possible to find predictions of day-ahead carbon intensity of the grid for example from https://carbonintensity.org.uk/. However unless you have a suitable control system it is easier to program your battery to charge/discharge at fixed times each day. This chart shows typical variation in emissions through the day at different times of the year. It is normalised so that the average emissions intensity is 1 at all times of the year.

From this you can see that in the summer time the lowest emissions are about 2pm (when you are likely to be getting lots of free electricity) and the worst are after about 6pm. The difference in emissions is typically about 30% - so you are able to save quite a bit by organising your charge and discharge times so that you discharge your battery (either for export or personal use) later in the evening.

During the winter, you will not have much spare electricity during the day and not nearly so much PV generation overall. However you can take advantage of the low carbon emissions overnight. To optimise your emissions you can charge fro the grid overnight, e.g. between 2am & 6am, discharge during the morning, then top-up from 1pm to 4pm and discharge after 4pm.

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, Solar iBoost+ or SOLiC 200.

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?

Having solar panels increases the cost of rebuilding your home in case of a disaster. This could increase your premiums a little. It is important to notify your insurer. Compare the market has more advice on this.

Can I install it myself?

Some manufacturers offer installation kits but you would need to a competent electrician and roofer to do this - and it may affect your insurance.

Do they need cleaning?

Dirt has the same effect as shadow and this can make a big difference. 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 when there is a prolonged drought then they can still 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.

Does it need maintenance?

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.

Problems with squirrels

There have been quite a few incidences of squirrels nesting under the panels and chewing on the electrics. This is bad - a short can prevent the panels from working at all. If you get a lot of squirrels in your garden, be sure to put up nets to stop them, or any other animals, getting under the panels.

Will it make any noise?

If you have a central inverter (the bit that converts DC current to AC) it will have a cooling fan which makes a humming noise. Some are quieter than others. Micro-inverters do not have fans so they do not have this noise issue.

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: https://www.bre.co.uk/filelibrary/pdf/rpts/Guide_to_the_installation_of_PV_systems_2nd_Edition.pdf.

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. If you have a smart meter this will record both import and export separately. Older meters do not record the export (though they should not go backwards as they do in some other countries). Using this information it is possible to work out how much energy you are using (import + generation - export) and the amount of the PV you generate that you consume yourself. (generation - export)/generation.

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