Solar Panel Inverters
The basics about inverters are covered in Solar Buying Guide - Step 2. This article adds some additional information for those interested.
The function of inverters is to "invert" every other pulse of DC (direct current) electricity produced by the solar panels. This changes it into an alternating series of positive and negative pulses called AC (alternating current) electricity, which is the type used by our homes.
There are two types that are used in solar-PV systems: string and micro.
The function of inverters is to "invert" every other pulse of DC (direct current) electricity produced by the solar panels. This changes it into an alternating series of positive and negative pulses called AC (alternating current) electricity, which is the type used by our homes.
There are two types that are used in solar-PV systems: string and micro.
String Inverters
A string inverter converts the electricity from a "string" of solar panels from DC to AC. They are good-sized boxes usually mounted in your garage, one (occasionally two) for an entire solar-PV system.
There is a substantial difference in quality among string inverters. The best ones are warrantied for ten years, but usually last twenty. Some of the best ones have a Global Peak MPP (maximum power-point) tracking feature that minimizes losses in shaded conditions. Some also offer an optional Secure Power Supply that includes an outlet providing 1500 Watts of power directly from your PV system, in case the grid is down, making the system do double duty as a backup power generator.
There is a substantial difference in quality among string inverters. The best ones are warrantied for ten years, but usually last twenty. Some of the best ones have a Global Peak MPP (maximum power-point) tracking feature that minimizes losses in shaded conditions. Some also offer an optional Secure Power Supply that includes an outlet providing 1500 Watts of power directly from your PV system, in case the grid is down, making the system do double duty as a backup power generator.
Micro Inverters
A micro inverter is a small box attached to the back of each solar panel. They have some advantages over string inverters, especially in shady conditions. The point at which to consider switching from string to micro inverters is at about 6% shading.* More than that and micro inverters will probably produce more energy. Less than that and the advantages are mostly on the side of string inverters.
The other advantages of micro inverters are these:
The cost of micro inverters may be a bit less than a string inverter for very small systems. String inverters may be a bit cheaper on larger systems. But the differences in costs between the two is minor and is getting smaller all the time.
The other advantages of micro inverters are these:
- Panels don't all have to face the same direction, as string inverted panels do.
- It's much easier to add on to such a system than to one with string inverters.
- If one micro inverter fails, the rest of the panels function normally.
- If a string inverter fails, the whole string of panels goes down.
- Panels with integrated micro inverters are a bit easier, quicker, and cheaper to install.
The cost of micro inverters may be a bit less than a string inverter for very small systems. String inverters may be a bit cheaper on larger systems. But the differences in costs between the two is minor and is getting smaller all the time.
Inverter Efficiency
Inverters always waste a bit of electricity in the conversion process. Top-quality inverters will have high efficiency: the percentage of DC current they convert to AC. The best measure of efficiency is the inverter's CEC efficiency rating. 98% is about the very top. Anything above 96.5%** is considered very good.
In addition to higher efficiency, the best inverters maintain more of their output as temperatures rise.
In addition to higher efficiency, the best inverters maintain more of their output as temperatures rise.
Choosing the Right Inverter
Inverters have to match the electrical characteristics of the panel or panels they will be attached to, so the choice of inverter size is best left to the vendor or installer.
It is worth getting the spec sheet for the inverter, though, and making sure its rated nominal AC output power is not appreciably lower than the PTC power rating of the panels it's attached to, whether a whole string or individual panels.
Inverters can handle about 10% above their nominal power rating, and even the best panels will rarely meet even their PTC-rated power output. So as long as the two ratings are close, you're fine. On the other hand, you don't want oversized inverters, because inverters are most efficient when running close to their rated power output.
It is worth getting the spec sheet for the inverter, though, and making sure its rated nominal AC output power is not appreciably lower than the PTC power rating of the panels it's attached to, whether a whole string or individual panels.
Inverters can handle about 10% above their nominal power rating, and even the best panels will rarely meet even their PTC-rated power output. So as long as the two ratings are close, you're fine. On the other hand, you don't want oversized inverters, because inverters are most efficient when running close to their rated power output.
* "Shade Study Engineering Report," PV Evolution Labs, Report #138 5-4, October 17, 2014.
** 2016 Renewable Energy Handbook, Solar Power World: 110-121.
