Solar PV Knowledge Bank

What The Installation Process Looks Like

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There are many installers to choose from, some good and some not so good. Obviously we would like you to choose Spirit to install your system. Even if you don’t choose us, please have a look at the following things that you should consider before choosing an installer for your system.

We are assuming you know that your installer should be MCS certified. Unless you use an MCS certified installer, your system will not be eligible for export tariffs.

Once you have checked the installer is MCS certified, here are the other issues to consider:

1) Some roofs need some strengthening before panels can be put on the roof. Does your quote include a structural report?

Our prices include provision of this structural report from a qualified structural engineer (and member of the Institute of Structural Engineers).

The report looks at whether the roof is strong enough to take the additional load introduced by the panels. There are in fact two loads – one being the down weight of the panels and the other being an upward force on the roof resulting from the potential for the wind to get under the panels and lift the panels and / or roof off the house. Around 20% of roofs need strengthening against this uplift.

We do not provide a “generic” report to cover lots of roofs, we provide a specific report, to cover your roof. We have found there are too many combinations of rafter size, spacing, truss manufacturer, roof covering and wind zone to produce “generic” calculations.

Some roofs need to be strengthened before panels can be safely fixed to them. If this is the case with your roof we will discuss this with you before going ahead with the installation. You will have the right to cancel your contract with us (with full refund of your deposit) if you do not wish the work to be carried out. If strengthening is required it is likely to add about £100 – £300 to the contract price.

Be wary of any installer that doesn’t provide a structural report. They either do not understand the risks they are imposing on your house or they are doing a very cursory check on the roof. Many installers dismiss the structural issues on the grounds that either “the load’s not material” or “the load increase is less than a 15% increase on the current load so it will be fine”.

Our structural engineers have done calculations to show that in general an installation will increase the load on the roof by more than 25%, which certainly is material.

Ask the installer if they have ever had to strengthen a roof before fixing panels on the roof. If the answer is no, what they are essentially saying is that all British roofs are ‘over engineered’ – in other words they were built to take the weight and uplift forces on panels, even though it was never envisaged that panels would be added to the roof. This is unlikely – modern roofs are designed to minimise cost – they are not usually built with ‘spare’ load capacity. In addition they were not designed for a ‘net uplift’ situation – with panels on the roof the overall force on the roof can be upwards rather than downwards.

If you fail to strengthen your roof, the result is unlikely to be catastrophic failure of the roof. Rather it will cause a gradual sagging in the beams which you will only start to notice when the ceiling starts to crack. Once this has happened, only major remedial works will sort the problem out. In our view it is probably not worth saving a few hundred pounds at the outset only to look at a bill for thousands in a few years’ time.

2) Is shading an issue and what shading analysis does the installer carry out?

Shading is the enemy of solar PV systems and can have a devastating effect on energy yield. It is very important that the system is designed to optimise return on investment and minimise losses. In considering the impact of shade on a solar PV system, three questions are relevant:

  1. Is there shading and if so at what time of year will it occur, which parts of the array will it shade and for how long each day?

  2. What impact will shading have on system performance?

  3. How can the performance of a shaded system be optimised?

Unless the shading is partial winter shading or early morning / late evening shading, the best way to ensure that shading doesn’t severely compromise the efficiency of the system is to build a computer model of the roof and the objects which shade it and carry out an hour by hour simulation of the system performance.

A good installer will provide a computer simulated shading analysis. This can be used to evaluate the performance of different sized systems on a given roof. Many customers ask if they can just “squeeze in” an extra two panels in near a chimney or a dormer. The only way to answer the question with any degree of accuracy is to simulate the performance of the system with and without the extra two panels.

The impact of shading on photovoltaic systems is a complex subject about which much has been written. We have written an eight page technical note on shading; please ask if you would like a copy. In the meantime here are the most salient points:

  1. Shading can cause a significant reduction in system output:

    Shading of 6% can reduce output by 33%.

    If there are a small number of panels on a string, they will only just fire up the inverter even without shading. If one part of a panel is shaded, there may be insufficient voltage to fire the inverter. With small strings, very little shading (4%) is required to shut down the whole system.

    The inverter optimises the efficiency of a string using MPP tracking. This ensures that the maximum power is fed into the mains. If some panels on a string are shaded and others are not, the inverter will operate sub optimally, unless state of the art functionality is used (eg SMA’s OptiTrac Global Peak).

  2. Shade is best avoided. However if this is not possible there are things that can be done to minimise its impact:

    Use state of the art technology with MPP tracking at the panel level. This ensures that a shaded panel has no impact on the performance of the rest of the system. Both Enecsys and SolarEdge provide systems with MPP tracking at the panel level.

    Put the shaded panels on a separate string with their own MPP tracker. The shaded panels must have their own MPP tracker, otherwise putting them on a separate string could actually reduce the energy yield.

3) How will the panels be held on the roof?

You should be aware that there are two types of fixing kit on the market:

There are German engineered bracket and rail systems by companies such as Schuco or K2. The brackets are screwed into the rafters so that the weight of the panels is held by the rafters. If the wind blows the upward force on the panels and roof is resisted by the rafters.

The second system is called Click Fit. The panels are basically hung without screws on the roof battens. The roof battens are the thin bits of wood which go across the rafters. They are only secured by one thin nail per rafter – so essentially all that is holding the panels in place is a few nails.

The Click Fit system is used in Germany and the Netherlands, where the roofs are built differently with much stronger battens. However it has only been tested in this country on brand new roofs constructed indoors.

The reason other installers use it is that it is very quick to install – since there is no requirement to put any screws in place. It is a good way of cutting corners and being in and out in a day or two. But it is not yet fully tested.

We don’t use Click Fit, we use the K2 system.

The risk of the Click Fit system is that the panels and potentially the roof covering / battens as well could quite simply be blown off the roof in the high October / November winds that we have had in recent years.

Putting solar panels on your roof is like putting a sail on the roof. There needs to be a gap behind the panels for ventilation purposes. The wind can get behind this and rip the panels / and or roof off the house. This won’t happen if the correct fixings have been used, but it might well happen if the fixings are inadequate. It might not happen this year but it might happen in five or ten years’ time.

4) Is the installer really the installer, or do they use subcontractors?

Many solar PV companies are in fact marketing organisations. They do not actually carry out the installation themselves, they just ‘sell the business’ and then pass it on.

In this case you need to be aware of what happens if there is a problem with the installation. Who will sort it out, who is giving the warranty for the work? You should also ask about quality control – how does the company you are contracting with make sure their subcontractor carries the work out correctly?

Apart from bringing in the occasional extra pair of hands on an installation team, Spirit Energy does not use subcontractors to install systems. The only subcontractors we use are scaffolders. We vet these very carefully, checking their insurance and health and safety policy. We require scaff tags on all of our scaffolding.

5) Can they provide testimonials? How many installations have they done?

Installing solar panels is like any other building work – it can be done properly using quality components or it can be done in a shoddy manner using cheap components.

The best way to find out which type of installer a company is, is to speak to previous customers. They will soon give you an idea of what you can expect from the company you choose to install for you.

Spirit Energy will always find two or three customers for you to ring and / or visit to discuss the work that we have done for them. You can also see our testimonials on our website. We have testimonial after testimonial from satisfied customers. You can speak to our customers to prove we didn’t make them up!

Ask how long the installers have been installing. Ask about training given to installers. Ask about health and safety policy and quality control.

Our lead installers have been installing for several years, and as a company we have installed over 2,000 systems, both domestic and non-domestic.

6) The system itself and the warranties on offer (the “small print”)

Finally there is the system itself.

There are many panel manufacturers. There is not a lot that can go wrong with a panel and since all panels have to be MCS certified to qualify for export tariffs, you shouldn’t get too hung up on which panel to choose.

You should give some attention to the inverter. This is the heart of the system and the thing that could go wrong. We use the SMA (Sunny Boy) inverters from the German company SMA. They are the market leader and have been around for many years. We use them because we know from experience that they are robust and reliable. We don’t want to have to come back to your house to sort out inverter problems!

Also check whether the price includes a wireless display or online monitoring. This is very useful and enables you to see the instantaneous power output of your system. You can also see daily generation without having to go and look at the inverter or generation meter which may well be in the loft. You can also see monthly generation charts, total generation to date, and download data to the computer.

Finally see (2) – make sure you know how the panels will be fixed to the roof.


On solar PV we give a full 5 year workmanship warranty on domestic installations that is insurance backed by HIP.

The panels come with a 25 year performance guarantee and a 5 year defects guarantee – this is fairly standard. The inverter comes with a 5 year defects guarantee, extendable to 10, 15, 20 or 25 years at additional cost. We extend the inverter warranty to 10 years as standard. Where there is a need to claim on a manufacturer’s warranty within the first 5 years we will make the claim from the manufacturer on your behalf and we will replace the faulty equipment free of charge.