Solar Panels convert light energy into DC (Direct Current) form of electricity. Inverters convert this DC electricity to the AC (Alternating Current) form – this is the type of electricity we use in our homes through the AC outlets and that is provided by the electric utility. The conversion can take place at each solar panel (micro-inverters) or the conversion of all the panels can take place centrally (string inverters).
One type is not inherently “better” than the other – they are just different ways of accomplishing the same thing; but, since they are different, one may be a better choice than the other depending on the situation. One other device, the power optimizer, which is not an inverter, is commonly used in a string optimizer based system to provide a functionality that is inherent with micro-inverters.
Since an inverter converts energy form one form to another, there will be a loss of energy in the process and the efficiency rating provides this value. Today, the gap in the efficiencies of the micro vs string inverters are almost negligible.
For technical reasons, for some solar panels, only the string inverter can be used (today), e.g. panels with 72 or 96 cells since most micro-inverters today support only 60 cell panels.
From a cost perspective, micro-inverters and the string inverter plus power optimizer options are about the same.
Most micro-inverters connect to one solar panel (there are some that connect to more panels). The micro-inverter takes the DC output from the solar panel as its input and then outputs the AC form. The AC outputs of each micro-inverter are connected together, close to the panels, and a single set of AC wires are routed to the cut-off switch at ground level.
The major advantages of micro-inverters are: 1) The variance in the outputs of any of the individual solar panels does not negatively impact the system as a whole, i.e. if any one or more of solar panels are shaded, or facing different directions, or not functioning at all, the system as a whole will still continue to function and the output will be degraded only by the amount of the lower functioning panels. 2) The failure of any of the micro-inverters will not affect the functioning of the system as a whole and the failed inverter can be replaced when convenient. 3) The system can easily be expanded and different types of panels and micro-inverters can be utilized in the system. 4) The health and output of each panel can be monitored and reported on an individual basis. 5) Micro-inverters can be, and are already being integrated into the solar panel module for easier and speedier installation.
Microinverters work with many, but not all solar panels, based on their technical specifications. For example, many micro-inverters do not support 72 or 96 cell panels (string inverters need to be used in these cases). Also, individual micro-inverters have a limit on their AC output so regardless of the solar panels maximum output, the micro-inverter will only output up to its maximum limit, which may be less than the panel’s.
The DC outputs of all solar panels are connected to a common set of wires on the roof and then routed to the DC cut-off switch at ground level. From the DC cut-off switch, the DC is inputted into the central string inverter that then outputs AC. They are call “string” inverters because each string inverter is connected to a grouping of panels called a “string.”
The major advantages of string inverters are: 1) String inverters work with almost all solar panels (except those with integrated micro-inverters). 2) High wattage panels can effectively be taken advantage of by using appropriately higher output string inverter or by using multiple string inverters. 3) Servicing the string inverter will not require working on the roof. 4) String inverters have been in use for solar since the beginning and are generally cheaper (not including the power optimizers) than micro-inverters.
A string inverter’s maximum output is affected by the least performing solar panel making it non-ideal for use where shading so some panels in a string may be an issue or if panels within a string will be installed on roofs with different orientation. Since string inverters are “central,” when a string inverter fails the system as a whole fails. Also, without the use of power optimizers, string inverters cannot monitor and report on health and production on an individual panel basis. String inverters have certain technical conditions that make the design process a little more “complicated” than that of micro-inverters.
Power optimizers take the DC from the solar panel and provides an “optimized” DC output that is then fed into a string inverters. Power optimizers were developed to address some of the short-comings of string inverters such as marginal performance of some panels affecting the whole system and per panel monitoring. Although power optimizers are cheaper than micro-inverters, one power optimizer is required for each panel, in addition to the string inverter, making the total system about the same.
As alluded to in the micro-inverter section, some manufacturers are integrating micro-inverters with the solar panel. This makes the design and installation process easier, quicker, and cheaper, albeit reducing some design flexibility.
LA Solar Group
We carry the best-of-breed micro-inverter, string inverter, power optimizer, and smart module solutions to design a system based on your needs. Some of our partners include: Chilicon Power, Enphase, SMA, Solaredge, and Sunpower.