String Inverter or microinverter, which is the right way to go?

String inverters and microinverters each have distinct advantages and disadvantages whose relative importance may vary significantly for different kinds of PV installations

Microinverter schematic diagram (Microchip Technology, Inc.)

A solar inverter or photovoltaic
(PV) inverter is a critical
component in a photovoltaic system. Its function is converting the intrinsic direct
current (DC) output of the solar modules into a utility frequency alternating
current (AC) that can be fed into the commercial electrical grid or used
directly by standard electrical devices. In
conventional PV installations, a
number of solar modules are wired in series, creating a PV 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
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 whose relative importance may vary significantly
for different kinds of PV installations.

String Inverters

Inverters are a major part of the balance of system (BOS)
cost of PV installations, i.e., the cost of everything but the solar
panels. Central string inverters offer substantial
economies of scale in large systems, particularly those in excess of 100 kW in
size. Megawatt inverters can reduce the
dollars-per-watt cost of power conversion by 90%. For utility-scale PV systems, total costs
including operating and maintenance costs are paramount, and large central
inverters are dominant in that market.

The main shortcoming of the central inverter is that when solar
panels are strung together, the string of panels will be performance-limited by
the worst of the individual panels within it. This is not only determined by intrinsic
panel-to-panel variations but is also dynamic. For example, if a single panel is shaded, its output drops dramatically,
which affects the output of the string as a whole even if the other panels are
not shaded. In addition, inverters use
a technique known as maximum power point tracking (MPPT) to optimize PV 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 diverging 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 disproportionately affect
the output of an 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 rather than being a catastrophic event.

The main disadvantage of the microinverter concept has traditionally
been cost. Because each panel has to have its own inverter with much of the functionality
of a central inverter, the costs are marginally greater. On the other hand, the economics of
microinverters are rapidly improving as microelectronics steadily increase in
level of integration and decrease in price. 

Microinverter-based PV systems
also simplify the overall BOS. For
example, 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 small and
maximizing performance from every panel is an important consideration. In
smaller systems, the differential in price-per-watt for the inverters has less
effect on overall system cost. The improvement in energy collection given a
fixed size array can offset this difference in cost. For this reason, microinverters
have been most successful in the residential market, where limited roof space
and multiple sources of shade constrain system performance. As the price of microinverters comes down,
the types of systems for which this technology is attractive will continue to