String Inverter or Microinverter?
String inverters and microinverters each have their own advantages and disadvantages.
An inverter is a critical component in a solar power system. The inverters function is to convert the direct current (DC) of the solar panels into alternating current (AC) that can be fed into the electrical grid or used directly by standard electrical devices. In solar power installations, a number of solar modules are wired in series, creating a solar array or string. Multiple strings are then connected in parallel to a central inverter that feeds the grid.
In contrast, a solar microinverter is a small device installed on the back of each solar panel that converts DC from a single solar panel to AC. The electric power from multiple microinverters is then combined and fed into the grid. The two approaches each have distinct advantages and disadvantages which varies for different kinds of installations.
Inverters are a major part of the cost of a solar power installation. Central string inverters offer substantial economies of scale in large systems, particularly those in excess of 100 kW in size. For utility-scale PV systems, total costs including operating and maintenance costs are major and therefore, large central inverters are dominant in that market.
The main shortcoming of the central inverter is that when solar panels are strung together in a string, the panels performance is limited by the worst of the individual panels within it. If a single panel is shaded, its output drops dramatically, thus affecting the output of the entire string of panels even if the other panels are not shaded. In addition, inverters use a technique known as maximum power point tracking (MPPT) to optimize solar power output by adjusting applied loads. Once again, variations in an individual panel in a string will cause the inverter to change the MPPT settings, thereby deviating from optimal performance.
Microinverters have several advantages over conventional central inverters. The main advantage is that since each panel has its own inverter, reducing or even losing the output from a single panel does not affect the output of the entire array. Each microinverter is able to obtain optimum power by performing MPPT for its own panel. From a maintenance/reliability perspective, the failure of a single panel or inverter in this type of system will have a minimal impact on overall system performance.
Microinverter-based solar power systems also simplify the overall system. They eliminate DC wiring, high-voltage DC switch boxes and protection circuits, and the facilities that house central inverters.
Microinverters have become common where array sizes are smaller and maximizing performance from every panel is an important consideration. In residential or light commercial systems, the differential price-per-watt for the inverters has less effect on overall system investment. The improvement in energy collection given a fixed size array can offset this difference. For this reason, microinverters have been most successful in the residential market, where limited roof space and multiple sources of shade constrain system performance.