INVERTERS

Solar PV inverters are 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 solar 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.

Inverter Sizes

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 solar inverters available, with most manufacturers offering both single and three-phase solutions.

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String Inverters

In the current market, string inverters are widely used due to their affordability and reliability. They are named after their ability to connect a group of solar panels, called a “string,” to the unit. The panels are connected in a DC circuit and the string is then linked to the inverter’s DC input for conversion.

Despite the popularity, there are disadvantages to using string inverters. The main disadvantage being string-specific limitation is 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 produced 300W, the remaining 9 modules would also be capped to 300W on a standard string installation.

MPPT’s

Built into inverters are the Maximum Power Point Trackers (MMPTs). Their primary role is allowing the inverter to automatically extract as much power available from the connected string as possible. By using an algorithm, the system operates at the most efficient voltage. It will allow some voltages and currents to be detected and corrected between modules. As a result, there is a 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 connected to different MPPTs. MMPTs are made by joining two strings together. Shading one panel in one string by 50% will reduce all the other modules in both strings by 50%. Splitting those two strings across two MPPTs would only affect the MMPT with the shaded module.

If any one solar panel connected to a tracker fails or degrades, all solar panels connected to that same tracker will lose energy. Therefore, it is good practice for commercial systems to use an inverter with higher MPPTS. Three, four or even six trackers are built into the three-phase inverters from Huawei and Solis to ensure minimum losses over time.

Optimisation

Beyond MPPT’s there are optimisers. DC optimisers sit between each panel and the inverter (usually connected directly to the junction box of the module) and optimise the panel output. This process takes the maximum power point tracker out of the inverter and places it behind the module. Therefore, this can increase the price, installation time, and labour requirements.

However, the benefits it can bring are incomparable. The optimiser isolates panel-specific issues to the connected modules, meaning shading is no longer a problem for modules physically in the shade. Solaredge inverters of the same rating as standard string inverters can have much longer string lengths. This is due to optimisers modifying the string current to push out a fixed voltage. Additionally, an optimiser behind each panel allows module-level monitoring when using the correct monitoring kit.

All inverters from SolarEdge must be installed with their optimisers to operate. Tigo is an alternative, third-party optimiser which works with most standard string inverters on the market.

Jinko has a range of Maxim modules with inbuilt cell-level optimisation. Therefore, it will bring many advantages of optimisation at a lower cost, used alongside a traditional string inverter.

Microinverters

There is a distinction between a Microinverter and an optimiser. This is because an optimiser takes DC from the module and optimises it to be more suitable for the inverter. 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. Furthermore, they’re considered a safe solution since DC power is isolated to the roof, and AC power is used inside.

Inverter Replacement Scheme

If you are replacing out-of-warranty solar inverters and can provide us with your serial number, we will apply an additional 5% discount to a replacement Solis, Huawei, 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 solar inverters.

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