A solar PV inverter is required on any PV system where AC power needs to be utilised. This is because it is the function of the Inverter to convert DC power generated by the solar, into useable AC power that can feed the electrical loads within the property. There are many different inverters on the market today and below we will look at some of the main distinguishing features to help guide you in selecting the right inverter for your system.
In terms of inverter sizes and sizing, essentially all properties have an electrical supply. Typically, in the UK, for domestic properties this will be a single-phase connection and for commercial properties this will be a three-phase connection. This is not the case with every single site but is a good rule of thumb. The electrical supply will influence the choice of inverter available, with most manufacturers offering both single and three phase solutions. You are not restricted to only using single phase inverters on single phase sites, multiple single phase inverters can be connected to a single or three phase supply, however you are not able to connect a 3ph inverter to a property with a single phase supply, only a three phase.
String inverters are the most common solution found in the market today. They tend to be reliable yet cheap and are known as string inverters because they allow for the connection of ‘string(s)’ of solar panels back to the unit. When a group of panels are connected in a DC circuit, we call this a string. That string is then brought back and connected to an Inverters DC input where it feeds the device DC for conversion. Despite the popularity, there are disadvantages to using string inverters. The main disadvantage being string specific limitation, such as the fact that the lowest performing panel on that string will limit the output of the rest of the modules. So, for example on a string of 10 x 320W panels, if 1 of those panels was damaged and only producing 300W, the remaining 9 modules would also be capped to 300W on a standard string installation.
An MPPT (or Maximum Power Point Trackers) are built into inverters and their primary role is to allow the inverter to automatically extract as much power as is available from the connected string as possible. This is done via an algorithm which makes the system operate at the most efficient voltage. This allows them to detect and correct some of the voltage and current variations between each module, resulting in higher output across the connected string(s).
As each MPPT tracks its connected strings independently, any string specific issue will not affect other strings in the array that are connected to different MPPTs. For example, if you have 2 strings connected into 1 x mppt and you shade one panel in one string by 50% – all of the remaining modules in both strings will be reduced by 50% each. If we split those 2 strings down across 2 x MPPT’s instead, only the mppt with the shaded module would be affected.
For larger commercial systems it is good practice to use an inverter with a higher amount of MPPTS, as a failure or degradation in any one panel connected to a tracker will cause a loss of energy from all solar panels connected to the same tracker. The three-phase inverters from Huawei and Solis have three, four or even six trackers to ensure the minimum losses over the life of the system.
Beyond MPPT’s there are optimisers. DC optimisers sit between each panel and the inverter (normally connected directly to the junction box of the module) and they optimise the panel output. This process essentially takes the maximum power point tracker out of the inverter and places it behind the module. This can increase price and installation time along with the labour requirements, but the benefits it can bring are unparalleled. The optimiser isolates panel specific issues to the connected module, meaning most shading issues become a thing of the past, only the modules physically in the shade will be affected by it. On top of this, optimisers are modifying string current to push out a fixed string voltage which means you can get away with much longer string lengths on Solaredge inverters of the same rating as standard string inverters. Additionally an optimiser behind each panel even allows module level monitoring when using the correct monitoring kit.
All inverters from SolarEdge must be installed with their own optimisers in order to operate. Tigo is an alternative, third party optimiser which works with most standard string inverters on the market.
Jinko have a range of Maxim modules with inbuilt cell-level optimisation, which bring many of the advantages of optimisation at a lower cost, used alongside a traditional string inverter.
A Microinverter is different to an optimiser in the sense that an optimiser is taking DC from the module and optimising this to make it more palatable for the inverter. Whereas a Microinverter converts the DC power to AC directly behind the module itself. Microinverters share a lot of benefits with optimisers in terms of module specific monitoring and shade optimisation and are considered to be a very safe solution when you consider that DC power is isolated to the roof and everything back inside the property is AC.
Inverter Replacement Scheme
If you are replacing an out-of-warranty inverter and can provide us with your serial number, we will apply an additional 5% discount to a replacement Solis or Huawei inverter or hybrid-inverter. This offer applies regardless of the make or model of the original inverter, or where you initially purchased it from. Our online Design Tool will help you size your replacement.
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