The solar inverter is the heart of the solar power system. Inverters have two primary functions, firstly to optimise energy harvest from the panels and then to convert the DC power from the panels into AC power for your home.
There are three types of inverter systems, string inverters, micro inverters and string inverters with power optimisers. Let's look at them individually.
String inverters are the most commonly used, due to ease and low cost. With a string inverter, generally one inverter is mounted on the wall and all the panels are connected to it, in a string. Most residential sized inverters 3 kW and over will have two separate trackers, to increase yield and provide greater design flexibility. String inverters are simple, cost effective, easy to maintain and value for money, which is why they’ve been so popular. Fronius and SMA are the two leading string inverter manufacturers in terms of quality. Of the lower cost brands, Sungrow is one of the leaders and Zeversolar has benefited from SMA ownership. Huawei is the world's largest inverter manufacturer by volume and we're just starting to see them in Australia, so they could be another reasonable cost effective option.
String inverters are fine for most applications, however for installations where there’s shading concerns or requirements for panels on multiple orientations or pitches, string inverters may not be possible nor are they the best choice. While the monitoring capabilities on the better string inverters is very good, you won't be able to see panel level performance.
- 3 - 8.2 kW models
- 2 MPPTs
- WiFi & online monitoring
- Snap in design
- Designed and made in Austria
SMA Sunny Boy
- 3 - 5 kW models
- 2 MPPTs
- WiFi & online monitoring
- Designed in Germany, made in China
Micro inverters do the same thing as a string inverter, however they're much smaller and you mount one under each panel. This creates the primary benefit of panel level optimisation (a concept we discuss in our choosing an inverter blog), which increases your yield by approximately 5%, even in perfect conditions. Micro inverters allow total design flexibility, meaning each panel could have it's own unique orientation or pitch. They also isolate the impact of a shaded panel to just that panel, rather than on a string inverter where all panels on the same string would be effected. If you have shading issues or a tricky roof, you might have heard a consultant say "you'll need micro inverters", but even in ideal conditions they'll produce more than a string inverter.
While micro inverters are great, there are some drawbacks. Having multiple inverters unnecessarily can increase cost, and a common failure point on the roof is harder to access. That being said, not having one point of failure is a plus. Micro inverter systems cost more than string inverter systems and are harder to oversize. Enphase is the undisputed leader in this field, with proven reliability and increased functionality in their micro inverter systems.
iNVERTERS WITH pOWER oPTIMISERS (oPTIMISED sYSTEMS)
Optimised systems combine the standard string inverter on the wall with devices under the panels called power optimisers. We mentioned at the start that inverters have two functions, to optimise energy harvest from the panels and to convert DC into AC. String inverters do both of these on the wall and micro inverters do both of these under the panel. Optimised systems convert the DC to AC on the wall, but have power optimisers which are mounted under the panels to optimise energy harvest. This allows the benefits and cuts away some of the drawbacks of both string and micro inverters, like having panel level optimisation of micro inverters but only having one point of DC to AC conversion like string inverters.
There's a couple of ways to achieve this type of system. You can use a common string inverter, like the Fronius or SMA, and incorporate agnostic optimisers, like Tigo or Maxim, into some or all of the panels. Another way is to use a simplified inverter with no tracker which converts DC into AC, but mount optimisers under each panel. SolarEdge is a system that does this, and it is designed from the ground up entirely on the optimised approach. This has enabled them to leverage all possibilities of optimised systems. Some of these include their safe DC concept, longer strings, panel level monitoring and control, and a simplified battery add-on solution.