Your solar energy system’s inverter converts the DC current generated by your solar panels into AC current and synchronizes your solar electricity at the same frequency as the electricity supplied by your utility. That’s a long way of saying that the inverter makes the electricity generated by your solar panels compatible with the electricity supplied by the grid.
Types of Solar Power Inverters
Depending on its design, a solar power system will use one or more of these types of inverters:
Central inverters are used in solar energy systems where the entire DC output of an array is converted to AC at one central point. One or more inverters may be used for this job. Central inverters are typically used for large-scale solar installations such as utility or commercial solar power projects.
String inverters are used in solar energy systems where solar panel arrays are configured into individual groupings, also called strings. Power conversion occurs at each string. String converters are typically used for residential and small commercial solar power applications.
Microinverters convert the DC electricity generated by solar panels to AC at each individual solar panel.
Transformerless (non-isolated) inverters are a variation of string/central inverters, but without isolation transformers. They are brand new to the U.S. market.
Power optimizers are used with string or central inverters. They offer solar panel-level optimization (like microinverters), but convert DC to AC power at the string or central level.
Considerations When Selecting Solar Inverters
Switch to Solar will recommend the most appropriate inverter configuration for your solar energy project. When selecting inverters, there are a number of considerations to keep in mind.
Many solar energy systems use multiple string or central inverters so that equipment malfunctions or scheduled maintenance will impact only portions of the solar power system. In the case of microinverters, if one fails, solar electricity production is reduced by only one solar panel, while the remaining solar panels continue to produce power.
[accordion-item title=”Power Tracking (MPPT)”]
Maximum power point tracking (MPPT) is the standard method used for tracking the point where the solar panel is producing the optimal combination of current and voltage.
- Microinverters can optimize (or achieve MPPT) on a solar panel by solar panel basis.
- String inverters optimize at the string level.
- Central inverters optimize at the solar panel array level.
- Power optimizers offer MPPT and monitoring at the solar panel level.
85% of solar installations experience some shading. With both centralized and string inverters, shading on one solar panel will affect the entire string of panels. If shade reduces the power of one solar panel by 20%, the production of the entire string will be reduced by 20%. Because of this, shading is one of the major threats to the output of residential and small commercial solar energy systems.
To combat this problem, solar panel strings can be made smaller. This concentrates the problem on a single area of the system. Another approach is to use microinverters or power optimizers to isolate shaded solar panels and reduce the effects of shading.
[accordion-item title=”Efficiency Ratings”]
Efficiency ratings for each type of inverter vary:
- Microinverters: 91% to 95.5%
- Central inverters: 97% to 99%
- String inverters: 95.5% to 97.5%
The actual efficiency of your solar energy system will be dependent on site conditions.
[accordion-item title=”Power Harvest”]
The goal of your entire solar energy system is to maximize the amount of solar power harvested from the solar panels. Different types of inverters harvest different amounts of solar energy, but keep in mind that the inverters are only one factor in the system’s ability to harvest solar power.
DC optimizers offer greater solar power harvest, but this is coupled with additional efficiency loss. Microinverters claim as much as a 25% increase in solar power harvest, but this varies from site to site. Microinverter and power optimizer manufacturers state that the additional solar power harvested from these devices far outweighs the single-digit efficiency losses.
Solar inverters often have the ability to monitor solar panel production, which is important for detecting system malfunctions. Because microinverters and DC optimizers report this information at the solar panel level, they offer more timely diagnosis of any problems. Read more about system monitoring.
String and central inverters are time-tested, reliable technologies, while microinverters are still relatively new and unproven.
For example, Enphase had unusually high failure rates with their M190 microinverters because they used electrolytic capacitors. They’ve changed the technology with the M215s, but now installation is more complicated compared to string/central inverters or even string/central inverters plus optimizers.
The average costs for the various solar inverter types are:
- Microinverters: $0.50 to $1.00 per watt
- Central inverters: Below $0.30 per watt
- String inverters: $0.25 to $0.50 per watt
- Power optimizers: $0.10 to $1.00 per watt
String and central inverters by themselves are the least expensive, but Switch to Solar has found that, for the same size solar energy system, optimizers plus string inverters are less expensive than microinverters. The question is whether the percent gained in output is greater than percent increase in price.
Because conditions vary from site to site, so does the output gain.