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A comparison of Central Inverters and Microinverters

One of the most essential pieces in a photovoltaic solar array is the solar inverter. Its purpose is to convert the direct current output of the solar array into an alternating current that can be tied to the electrical grid or used by external devices in off-grid situations. In the past, photovoltaic systems generally used central (string) inverters which contain several solar arrays that are wired in series, which generates a photovoltaic string. These strings are connected in parallel to an inverter which is tied into the grid. On the contrary, a solar microinverter converts direct current to alternating current on a panel-by-panel basis. Both central inverters and microinverters have advantages and disadvantages: each has an importance that varies for different types photovoltaic installations.

Central Inverters

Central inverters, or string inverters, have been around for quite a long time. They have been proven to efficiently convert DC (direct current) to AC (alternating current), and thus remain to be the most popular option in photovoltaic systems.

Central inverters are the less expensive option of the two, and contain fewer moving parts. For large-scale systems (especially systems greater than 100kW in size), central inverters can reduce the per-watt cost of power conversion up to 90%. For smaller systems, the operating and maintenance costs remain much lower than the cost of microinverters.

The largest issue associated with central inverters occurs when solar panels are connected together. If a single solar module experiences a period of shade, the output of the entire system can be reduced by up to 50%. The shaded panels act as a “weakest link”. The overall system performance is affected greatly because of that weak panel’s output. Another important drawback of central inverters is that they can be extremely dangerous, especially for workers performing routine maintenance. Each inverter carries 700-1600 volts, and continues to collect energy even when completely shut off.

Microinverters

Microinverters are a fairly new technology, having only been produced since the 1990’s. They were designed to resolve some of the issues associated with typical central inverters. The biggest advantage is that each panel has its own inverter. Because of this, when the output is reduced in a single panel, it does not have a negative effect on the output of the entire system. This improved technology creates an ideal option for solar energy consumers that may be prone to above average periods of shade.

Another great advantage of microinverters is safety. Direct current is converted to alternating current at the panel level which allows for a low individual total volt output, whereas central inverters contain a single dangerous voltage level.

Microinverters are a great improvement over central inverters in most areas except cost. Each panel in a microinverter requires its own inverter which escalates costs quite greatly.
In smaller solar panel array systems (less than 10kW), microinverters are becoming the more popular option, especially in systems designed to achieve the highest performance level from each solar panel. In these small systems, the cost of the inverters has a much less effect on the overall cost of the system. Microinverters have become the favourite among residential solar arrays, where roof space and contributors to shade (ie. trees, neighboring houses, etc.) are important factors for system performance. The decrease of cost for microinverters will lead to an increase of use in much larger solar power systems over time.