String Inverters VS Micro Inverters
Depending on the application, both inverter technologies have their pros and cons. String inverters, also called central inverters, are used when the array will not experience shading; mainly for trackers. An example of a string inverter is the Power One Aurora 5 kW inverter. Microinverters allow you much more freedom with your installation. They are mounted under each panel and plug directly into the panel's junction box. Enphase microinverters isolate each panel which reduces the effects of shading, panel mismatch and non favourable orientation. They also feature maximum power point tracking (MPPT) at the module level which increases array production by up to 20%. Using Enphase microinverters also allows you to monitor each solar module through their Enlighten platform. However to make a fair comparison between the two inverter technologies, Tigo Maximizers need to be added to the string inverter system so it operates in a similar way to the microinverters.
Each 5 kW string inverter is approximately 3' tall by 1' wide and weighs over 60 lbs, there are two per 10 kW installation. They are either mounted on the outside of your home or in your garage. They also have a slight humming sound when operating due to the large amount of power they handle. Also due to the complex installation there is usually more visible conduit, cabling, and junction boxes.
Microinverters are mounted under each panel, staying completely concealed. The cabling is simpler and much neater between the microinverters and meter base making the installation more aesthetically pleasing.
10 kW of string inverters can be 30-40% less expensive than 10 kW of Enphase micro inverters. However, a 5 kW string inverter costs about $2400 CAD, therefore a total cost of $4800 for a 10 kW microFIT system. Now consider replacement of the string inverters after 10 years. This brings the cost of operation with string inverters up to $9600. For 40 Enphase micro M215 inverters you can expect to pay approximately $6600 plus approximately $1000 for trunk cabling which means the cost of longterm ownership of the solar system is less with micro inverters even though the initial cost is higher. However, string inverters and microinverter systems operate differently. Microinverters isolate each panel and also perform MPPT tracking at the module level instead of the string level, increasing energy harvest. To compare apples to apples Tigo Module Maximizers must be added to the string inverter system. Tigo Maximizers are a DC-DC optimizer which isolate each panel and perform module level MPPT, much like Enphase microinverters do. This increases your array efficiency by approximately 7% compared to just using string inverters. Tigo also allows per module monitoring on a similar platform to Enphase Enlighten. The addition of Tigo to a 40 panel system will cost approximately $2300.
Overall, Enphase M215 microinverters have about the same upfront cost as a string inverter system with Tigo but they have a lower cost of ownership, higher reliability, and a longer warranty of 25 years.
String inverters usually will need to be replaced within 10-15 years. Micro inverters on the other hand have a projected mean time between failures (MTBF) of 331 years. Enphase micro inverters are much more reliable than string inverters because they do not handle as much power as a central string inverter. They only handle the power of a single panel, not a row or multiple rows of panels. This means that there is much less thermal stress on each micro inverter unit which adds to its life. Also, since Enphase micro inverter are mounted on a per module basis the overall surface area of the combination of the micro inverters is much larger than the surface area of a string inverter. This means that the micro inverters dissipate heat more efficiently than string inverters, which also means that the units work more efficiently to convert power (cooler electronics work better).
String inverters are able to reach high conversion efficiencies and for this reason they are sometimes used in dual axis trackers, and large scale commercial and utility scale PV plants. However micro inverters outperform string inverters when the solar array experiences shading or snow cover. This is because when a solar array experiences shading with a string inverter the whole string or row of panels becomes limited to the output of the shaded solar panel or panels. This does not happen with Enphase micro inverters because each panel operates independantly of eachother.
String Inverter Annual Performance
Micro Inverter Annual Performance
Micro Inverter Snow Cover Performance
Note that the Enphase micro inverters produced 4.58 kWh which is 2.7 time more than the Power One system on that same day.
String Inverter Snow Cover Performance
Note that the 2 Power One 5 kW string inverters produced only 1.72 kWh.
More about the Enphase M215 Micro Inverter
More about the Power One String Inverter
More about Tigo Maximizers