String vs. Central Inverters in Commercial Applications - How to decide?

After solar modules, inverters represent the single largest hardware cost in any solar project.  While most commercial and utility-scale solar projects in the United States have traditionally employed central inverters, there is increasing interest in the use of string inverters for certain applications.

Solar cells produce electricity as direct current (DC).  Inverters convert DC power to alternating current (AC) power that is used by businesses and homes.  Individual solar panels are connected in series to form strings.  When using a central inverter, the DC power produced from each string runs along wires to combiner boxes where they are connected in parallel with other strings.  From there, the DC power is then run into the central inverter and converted to AC power.

When using string inverters, there are multiple smaller inverters for several strings, so the DC power from a few strings runs directly into a string inverter rather than a combiner box and is converted to AC.  Here’s what the two models look like graphically:

The decision on whether to use a central inverter or string inverters needs to be made on a case-by-case basis taking into account primarily two elements:  (1) total system cost (including space constraints) and (2) total energy production.

Total System Cost:  Notice that the first consideration, total system cost, is not the same as comparing the cost of the central inverter to the cost of the aggregate number of string inverters needed to handle the same production of electricity.  Here are the primary pros and cons related to system cost:

Total Energy Production: The efficiency of string inverters versus central inverters is essentially a wash, string inverters often may be slightly more efficient, but the difference is often negligible.  Other key inverter differentiations related to system production are set forth below:

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