Inverters are the ‘heart’ of every PV system. Their main task is to convert the direct current (DC) from the solar panels into alternate current (AC) that can be used by loads onsite and be exported to the electric grid. Inverters also regulate frequency and voltage, and, in almost all cases, gather production data from the system, and make it available on the internet.
There are two types of inverters that are used in residential and commercial systems: string (or central) inverters and microinverters. String inverters are fed by multiple strings of solar modules , and come in numerous sizes, measured in kilowatts. They are usually installed near the meter or the main electrical panel. On the other hand, microinverters are small boxes that attach directly to frame of the solar panels, and their output is measured in watts. Most times, their count will equal the number of solar modules, although some types of microinverters can be connected to two to four modules.
Here are the benefits and drawbacks for each solution:
String inverters Pros:
-Less hardware: easier to install and to replace in case of malfunction
-Slightly cheaper: string inverters are generally more affordable and installing them is cheaper than installing microinverters
]-Multiple inputs: by using one input for each mounting plane or string, the inverter can maximize each array’s output even when they are in different orientations and/or shading conditions.
-Not compliant with Rapid Shutdown requirements: string inverters need additional hardware (i.e. a Module Level Power Electronics unit) if installed in a region where NEC 2017 or NEC 2020 rapid shutdown is required. MLPE will add about $40 for each solar module to the total cost of the system
Shorter lifespan and warranty terms: string inverters are expected to last 10 to 14 years. In comparison, solar modules are expected to last around 30 years
-Lower power output in partial shade conditions: if only part of an array is shaded, the whole string’s output will be impacted (i.e. will be proportional to the output of the most shaded modules)
-Optimal module and string output: each microinverter is constantly optimizing the output of the module, so even if a string is partially shaded, its output will be optimized (i.e. will be proportionally higher than the most shaded modules)
-Compliant with most stringent Rapid shutdown requirements
-Long warranty and expected life cycle: most microinverter manufacturers now offer 25 year warranties
More points of failure: even though they are covered by warranty, microinverters, like any appliance, may fail at any point during their lifetime
-Higher price and installation cost -Located on the roof: in case it needs to be replaced or moved, microinverters can be only be accessed by accessing the roof and removing the solar modules.
So which is better?
The debate on what’s the better choice between string and microinverters is a recurrent one. Cost, roof shading, local requirements, warranty considerations are all determining factors. Whereas we don’t think that there is an absolute best solution that can be applied to all systems and for all locations, we certainly advise anyone interested in installing solar to consult with a local solar expert!
Written by: Marco Casalegno