What are Solar PV Inverters?
Solar PV panels produce electricity from sunlight, and with over 500,000 systems now installed on people’s roofs in the UK, they have never been more popular.
The average solar PV system in the UK is 3.5kW, which with an average panel being 250w mean it will consist of 14 panels (find more solar PV tips here).
The electricity produced by solar PV panels is in the form direct current (DC). In the UK, the electricity that we use on the whole is in the form of alternating current (AC), and so before we can use the electricity produced from the solar PV panels, we need to convert it into AC – an inverter carries out this conversion.
Traditionally, individual solar PV panels are installed in series that lead to one string inverter. These string inverters effectively treat all the different panels as one giant panel.
There is one big issue with these string inverters. If there is a problem with one of the panels (for example a tree causing shading), all the other panels will also suffer an equivalent output loss. The panels will operate at the output of the worst performing solar panel.
You may think this might put solar PV installers from using them, however they are still by far the most popular setup mainly because they are cheaper to install and also they are easier to replace, since you just replace the one unit.
Micro inverters are attached to each individual panel, which allows the panels to operate independently from one another.
This gives them a huge advantage where different solar PV panels in your solar PV array may produce different outputs for whatever reason, since this will not impact all the rest of the panels.
For example, imagine a chimney breast situated on a roof of a mid-terrace house, this may cast a shadow for a couple of hours each afternoon. In a system with a string inverter, this would massively reduce daily output from the solar system. With micro inverters, the shadow would obviously impact the output of one panel, but this would not impact the other panels.
For this reason, it is estimated that for micro inverters can increase output from solar systems by up to 20%. Since you get paid for each kWh of electricity produced, any way you can maximise output should be encouraged!
Other differences between Micro inverters and string inverters
Most string inverters will only come with a 5 year warranty and even the most optimistic solar installers will admit that string inverters will need to be replaced at least once over the life-time of the solar panels.
In contrast micro inverters will last far longer; some manufacturers offer a 25 year warranty for their products. The reason for this is that string inverters need to deal with kilowatts of power and high input DC voltages, so they need high power transistors and electrolytic capacitors – these components unfortunately just don’t last as long. Micro inverters only need to deal with 250w of power (the size of a panel) and the DC voltage is far lower so only require thin film capacitors – eliminating the need for large transformers and electrolytic capacitors.
Complexity of install
With micro inverters, you need to install one unit for each individual solar panel – this is obviously more time consuming to do.
The benefit is that micro inverters tend to be easier to fix, typically because there is a single point of failure in the inverter, but the larger inverters can be far harder to fix because they are more complicated bits of kit.
For a 3.5kW solar PV system, 14 micro inverters would be required (1 for each panel). With each unit cost approximately £200, you are looking at a cost of approximately £2,800.
A string inverter for a 3.5kW system should only cost £1,500, which is far cheaper; this will mean that the initial install cost of a solar system with a string inverter will be cheaper.
Remember though that these string inverters won’t last nearly as long – maybe 10 years tops, while a micro inverter might last as long as 20 – 25 years.
When the sun is shining, solar panel systems produce a lot of electricity – this means high voltages, especially when you are dealing with string inverters. These can cause arc faults in some cases where cabling running from panels are unplugged or vermin chews through cables. Arc faults are dangerous because they don’t normally trip the circuit breaker – which means that heat can build up potentially causing fire.
Micro inverters, running at lower voltages don’t suffer from these problems, although it is worth mentioning that for both types of inverter – a problem like this would be extremely unusual. It is just highlighting the point of operating a system with high voltages!
So are micro inverters worth it?
Is it worth the extra upfront expense of installing micro inverters? It really depends on whether your system is likely to suffer any shading. If this is the case, then we would definitely recommend micro inverters. The fact is your system will produce more electricity with these installed, which not only produces higher financial returns via the feed-in tariff but also gives you more free electricity to use in the home.
Since the micro inverters last longer and most manufacturers offer far longer warranties, we would where possible advise people to consider getting these installed!
UPDATE: The Feed-In Tariff is now closed for new applications. To find out about the new scheme designed to replace it, click here.
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