Upgrading your system

Technical Information

Wired in series vs wired in parallel

When your solar panels are wired ‘in series’, essentially each panel is connected to the next in a “string” and the circuit travels along one path. Because of this, all the current in the circuit must flow through all the panels, therefore one underperforming or shaded panel will restrict the total flow of current. Likewise, as it’s because it is a closed loop, breaking the circuit at any point stops the entire series from working, similar to the way old Christmas tree lights work.

If you install solar panels in parallel, each panel’s wires are connected to a centralised wire leading from the roof. This effectively means that each panel is operating in isolation, feeding energy to the one cable, therefore none of the panels are affected by the performance of other panels.

Watch how it works

SolarEdge Technical Video

Your system, in the expert’s hands

Online troubleshooting
Being the leading SolarEdge installer in the UK and a Certified SolarEdge Service Partner means that we are experienced and knowledgeable at installing, trouble shooting and repairing SolarEdge systems, sometimes being called to repair systems not originally installed by us.

We have a dedicated monitoring department that have a sole responsibility for assessing the performance of customers systems when alerts are triggered from SolarEdge HQ. As an installer, we get increased technical access that allows us remotely repair and send system updates via wifi.

A non-exhaustive list of what we can look at includes:

The overall generation of the system

this can be compared with other similar systems in the area to work out an average and to see if there is a noticeable difference that requires further investigation..

Any sort of shading (obstructions)

we use the ‘playback feature’ for this – it’s unbelievable how something small can cause a huge effect on the panel.

We can check the voltage of panels and individual optimisers

we have remote access to the inverter (as long as the communications are active) so we can see if any error codes have been triggered and remotely try and fix them.

Highest performing panel vs lowest performing

To see what sort of % difference there is between the two – if its more than 20% we will then go down the route of panel replacement, helping customers to claim on manufacturers’ warranties.

The voltage of panels

A lot of panels we have replaced in the past has been down to faulty bypass diodes, we can see from the charts if the panel has a faulty bypass diode from the voltage being 1/3 or 2/3 lower than the rest of the panels.

A/C voltage for the property

over voltage can be an issue for sending exported energy back to the grid, assessing the level of A/C voltage allows us to recommend whether the property would benefit from a Voltage Optimiser.

Variable degradation affecting
your system performance

Degradation performance issues
Solar panels degrade over time, which means that their performance reduces as the panel ages. Even though panel manfacturers’ quote a product guarantee of around 25 years for their modules, they also predict the estimated decline in performance through a performance warranty.

Panel companies are only comfortable offering this guarantee because of a 2012 NREL study (“Photovoltaic Degradation Rates—An Analytical Review”) that found solar panels degrade about 0.5% to 3% each year, barring any equipment issues.

The reason why performance declines can be due to many naturally occurring factors, including and not limited to;

• Natural cell degradation through UV and weather exposure,
• Thermal cycling can cause solder bond failures and cracks in solar cells,
• Damp heat has been associated with delamination of encapsulants and corrosion of cells,
• Humidity freezing can cause junction box adhesion to fail,
• UV exposure contributes to discoloration and back-sheet degradation.

However, panel degradation is a complex matter, and whilst these factors can and will have an adverse affect of a panel’s performance, there could be weaknesses in the design and manufacturing process, along with how modules are installed, that can also elicit a decline in performance.

Manufacturers are continually testing and refining every part of the manufacturing process, all the way down to the encapsulants and adhesion materials, to try to slow degradation rates.

In tandem to this, PV system technology can actually bring about what is known as Potential Induced Degredation (PID). One of the culprits of this is the increasingly popular ‘transformerless’ inverters that allow different modules, and different parts of that module (ie individual cells and the frame) to perform at different voltage levels within a system, which can allow electrical current to leak and modules to lose their peak performance.

Often, simply negatively grounding a system removes this issue, but transformerless inverters are ungrounded.

When electrical current leaks, sodium ions in the glass move toward the solar cell or the frame, depending on how the system is grounded.

Frameless modules can help reduce the PID possibility (since there’s no metal frame to disrupt voltages). And many module manufacturers take extra steps to ensure modules are PID-free now. It’s important for installers to know what products they’re combining into a full system to know if something besides the panel may contribute to degradation.

Cheaper panels and less material

A few years ago, as module companies started to compete to lower their prices, they made their frames thinner to reduce the aluminum being used which unfortunately meant they can bend. Bent frames can strain the whole panel, allowing seepages of water under the frame, that when freezes, expands, bending the frame further. This can be especially bad as panels get thinner and less mechanically robust

More, thinner busbars

Solar panels sometimes fail because of busbar solder bond failures. With the trend of more busbars on solar cells, you would think there is a higher chance of solder bond failures. That’s not entirely true.

Cells can easily break. If you have a big ribbon with a big solder bond, it puts more local stress on the cell and causes them to be more likely to break. By reducing the size of those solder bonds, you can reduce the amount of stress at the point where that ribbon gets connected to the cells.

With more busbars and more solder bonds, there is a higher probability of solder bond failure. But the importance of one solder bond failure goes down when there are more busbars to pick up the slack. Also, more busbars across a solar cell can decrease the chance of full cell breakage.

Flexible panels and installation

As module companies decrease their costs, they may turn to ultra-thin solar cells that use less silicon. Thinner solar panels are more flexible and not as rigid as older module models, which makes installation a delicate process.

Hand-to-hand transport can affect a module, especially if installers are carrying modules on top of their hardhats. That flexing and bouncing up and down can take a real toll and lead to microcracks in the cells. Same with dropping a module and the biggest no-no—standing or walking on top of solar modules.

What can we do?

Not all new technologies are bad, nor are all modules destined for failure. And although the types of problems may be changing, panel warranties are increasing and system lifespans are getting longer.

Smart buying and installation of solar panels and other project components can help mitigate degradation. Using trusted products and installing them with care will ensure a solar system will perform at its best—with no more than 3% power loss each year.

The big offender

Manufacture tollerances
A power or manufacturer’s tolerance is the quoted range of the amount of power a solar panel can produce.

The wattage (amps x volts) of a panel can vary but it is issued a nameplate wattage. What a manufacturer will show in its specification of the product is a power tolerance and that is what the panel could produce in its given range. This is all completed under Standard Test Conditions (STC) by the manufacturer.


A 100 watt (W) panel could have a power tolerance of +/- 10%. This means the panel could produce as high as 110 W or as low as 90 W.

Solar systems are typically wired in either a series or a parallel circuit, or even a combination of the two. In this particular scenario, it is important to highlight the relationship between the series circuit and the quoted power tolerance of the panels.

The series circuit is a closed continuous loop which the current travels through. If it is broken at any part then the whole circuit would be incomplete and the current would stop flowing. In extreme cases, that means if one solar panel was to stop working then every solar panel in the circuit would also stop working. In regards to the power tolerance, if one solar panel was operating at the lower end of its power tolerance spectrum, the energy production of the rest of the panels in the circuit would also be restricted to that lower performing panel.

Heat: the enemy of electrical products

Thermal mismatch
Thermal mismatch happens when PV cells are affected by heat negatively. It may be surprising to know that heat can actually cause damage to PV cells because of the actual functionality of solar PV modules. A manufacturer will only design a module to withstand so much heat therefore if it were to operate at higher temperatures then the module would underperform or even stop working. However, if the module temperature were to drop then the efficiency can increase.

A PV module will consist of a number of PV cells – 60 is often an industry standard but different models will have more or less. With many older models it is easy to see each PV cell as they are usually in a rectangular shape on the laminated side of the panel. The specification sheet of the model will detail the PV module temperature coefficient. This means the temperature for the PV module to operate in will be listed but also the decrease/increase in efficiency for every degrees celsius (°C) it goes above/below that temperature.

Typically, the two types of PV modules which are installed are either manufactured with monocrystalline silicon or polycrystalline silicon. However, thin film PV modules are becoming more popular. The monocrystalline module is of a purer quality silicon but doesn’t work as efficiently as a polycrystalline module in higher temperatures. The monocrystalline PV module is the most common type installed in the U.K.

In standard test conditions (STC) the testing of a PV module is carried out at 25°C. A solar PV module will also have a specified ‘Module Efficiency’ which is portrayed as a percentage (%). The ‘Module Efficiency’ is the amount of solar irradiance which is converted into direct current (DC). So, the temperature coefficient is the increase/decrease of efficiency in % per °C.

Each PV cell is unique and will perform differently to the other PV cells in their module. Depending on a number of factors, PV cells may be at different temperatures. The issue is that PV cells are installed in a series circuit. So, if one PV cell were to be affected negatively then it may affect the other cells in its group (bypass diodes segregate cells in groups of 20[1]) or the entire module and then possibly the whole array. Cases such as these are commonly referred to as “hot spots” in the industry.

Examples which can cause thermal mismatch include:

  • Dissipated power to one area
  • Lack of air to cool the PV modules in the middle of an array
  • Cell mismatch (cells of varying current production connected in series)
  • Cell damage

In the U.K., the temperature doesn’t increase over 25 °c very often but as the PV cells absorb solar irradiance, the module can heat up to temperatures higher than the surrounding air temperature.

For STC, an irradiance of 1000 watts (W) per square metre is produced. This stimulates peak sunshine on the tested PV module which would be directly facing the sun without clouds. In real life conditions, weather like this will occur during Summer. This is the most important season for any investor as it will be when their PV system produces the most for the year. So, if their PV system were to suffer from a decreased efficiency during this season then it lowers their return on investment (ROI). It also increases the amount of energy then required from the grid and in turn increases energy costs.

The specification sheet will also show test results of the PV module in a nominal operating cell temperature (NOCT). This is when the manufacturer creates conditions which are similar to real climate. This data shows that the PV module will typically operate at a higher temperature in lower insolation conditions and perform at lower ratings than STC.

If PV cells suffer from high temperatures over their lifespan then it will cause degradation and have a long term negative effect resulting in a lower efficiency and a lower power output. Affected PV cells may also have a shorter lifespan than the other cells in the module. There are cases when irreversible damage is caused to PV cells.

Thermal mismatch not only affects your production and ROI. It also increases the hazard of a potential electrical fire. As with any electric equipment, a PV module has a threshold for temperatures which are too high for it to operate in. The temperature should be spread across each of the PV cells so the module can complete its task safely but this isn’t always the case.

There are a number of things an installer can do to ensure the prevention of thermal mismatch in a solar PV system and mitigate the effect of any thermal mismatch already present

  • Installing the PV modules a safe distance (1 – 2 inches) from the roof to allow for effective air cooling
  • Wiring the panels in parallel (using SolarEdge or Enphase technology) to ensure that lower performing panel affected by thermal mismatch does not affect the performance of the others in an array
  • Live monitoring for each PV module to help identify potential issues with panels
  • Correct and safe wiring
  • Use of high quality panels with robust warranties

An investor should also aim to maintain a regular cleaning cycle for the PV system. If there is any concern that there may be damage then an infrared thermal inspection can be completed to see if a PV module is suffering from thermal mismatch.

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0800 856 2200

0800 856 2200