Solar inverters.

Virtually every home has an electricity connection, regardless of the size or type of inverter. Residential homes in the UK usually have a single-phase connection, while commercial buildings have a three-phase connection. This does not apply to every site, but it 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.

As string inverters are affordable and reliable inverters, they are widely available on the market. The name “string” comes from their ability to connect a group of solar panels to the device. The panels are connected in a DC circuit, and the string is then connected to convert the DC input of the inverter.

While string inverters are very popular, there are some disadvantages to using them. One of the main disadvantages of a string inverter is that the module with the lowest output limits the output of the other modules. For example, in a string of 10 x 320 W panels, if one of these panels is damaged and only produces 300 W, the remaining nine panels would also be limited to 300 W in a standard string installation.

Maximum power point trackers (MPPT) are built into inverters. Their main task is to enable the inverter to automatically draw as much power as possible from the connected string. With the help of an algorithm, the system works with the most efficient voltage. This allows certain voltages and currents between the modules to be detected and corrected. The result is higher performance from the connected strings.

Each MMPT monitors the connected strings independently so that a problem with one string in the array does not affect the others. Since MMPTs are made by joining two strings together, shading a panel in one string by 50% will reduce all other modules in both strings by 50%. Splitting these two strings into two MPPTs would only affect the MMPT with the shaded module.

If even a single solar module connected to a tracker fails or degrades, all solar modules connected to the same tracker will lose power. It therefore makes sense for commercial systems to use an inverter with a higher MPPTS. Huawei and Solis three-phase inverters incorporate three, four or even six trackers to minimise losses over time.

In addition to MPPTs, there are also optimisers. DC optimisers sit between each module and the inverter (usually connected directly to the junction box of the module) and optimise the power of the module. In this process, the maximum power point tracker is removed from the inverter and placed behind the module. This can increase the price, installation time, and labour costs.

However, the resulting advantages are incomparable. The optimiser isolates module-specific problems to the connected modules, which means that shading is no longer a problem for modules that are in the shade. SolarEdge inverters with the same output as standard string inverters can have much longer strings. This is because the optimisers modify the string current to output a fixed voltage. In addition, an optimiser behind each module allows monitoring at the module level if the correct monitoring kit is used.

All SolarEdge inverters must be installed with their optimisers in order to function. Tigo is an alternative third-party optimiser that works with most commercially available string inverters.

The difference between an optimiser and a microinverter is that an optimiser takes the direct current from the module and optimises it so that it is suitable for the inverter. A micro inverter, on the other hand, converts the direct current power directly behind the module into alternating current. Microinverters are considered a safe solution as the direct current is isolated on the roof, and the alternating current is used inside.