Compare String, Optimizer and Micro-Inverters
String Inverters vs. Optimizers vs. Micro-Inverters: Which is the Best?
The solar inverter converts direct current (DC) electricity from the solar panels into alternating current (AC) electricity used in the home. For grid-connected systems, the first consideration is to determine if you want a central string inverter, a string inverter with module optimizers, or a micro-inverter system. Each has unique costs, benefits, lifespan and performance differences that can be easily compared.
Rooftop mounted systems following 2017 NEC must be equipped with Rapid Shutdown. Optimizers or micro-inverters meet the Code requirements. Ground mounted systems do not require the same rapid shutdown capability. Read more about string inverters, optimizers and micro-inverters below.
A solar array setup with a string inverter means there is one central converter box, sometimes more for larger systems. They are compatible with every kind of solar panel, which are wired together using the output cables from one solar panel connected to another to create a DC (direct current) string, There are typically 10 or 14 panels, up to 600Vdc per string. Multiple strings can be wired to the inverter using two #2 to #12 AWG wires. Given this type of wiring, if a solar panel on a DC string fails or is partially shaded, it can negatively impact the performance of all the solar panels on that string. However, most string inverters offered today minimize such losses with software and by allowing multiple strings (2 or 3) to better mitigate the reduction in any one panel.
Most string inverters include an integrated DC safety disconnect switch to meet NEC electrical code standards. They operate at higher DC input voltage, typically 600V per string in the U.S, and produce standard 240Vac output for use in the home.
If shade is NOT an issue, then a string inverter will produce plenty of power at the lowest cost. In direct Sun, an optimizer or micro-inverter system can produce 2% to 3% more power than a string system. It may be cheaper to simply add one more panel to achieve the same results.
If shade is an issue, then all types of inverters (string, optimizer or micro) will produce less power. However, optimizers or micro-inverters can lose 10% to 25% less power than a string inverter in shady conditions.
String inverter systems are the most economical approach. They typically cost 10% less than an optimizer or micro-inverter system. Most grid-tied string inverters have a 15 to 20 year expected performance life and can work in most any conditions, mounted indoors or outside. String inverter warranties are usually 10 years, and most manufacturers offer optional extended warranties up to 20 years. Most string inverters include an LED display on the box, with web and smartphone monitoring available with a WiFi connection.
For more information, visit the GRID TIED INVERTER BUYER'S GUIDE
String Inverter with Module Optimizers
An optimizer system has a small device, called an optimizer or maximizer, attached to each solar panel using the panel output cables. Then the optimizers are connected to each other using the included cables. Between 8 to 25 optimizers can be wired together to form a DC (direct current) string. The difference depends on the panel voltage, but generally strings can be up to 500 volts or 5,250 watts per string. Each string can then be wired to the inverter. Given this type of wiring, if a solar panel on a string fails or is partially shaded, it should NOT impact the performance of the other solar panels on that string. Compared to string inverters, optimizers can produce 2% more power in direct Sun, and up to 25% more power in shady conditions.
Compared to micro-inverters, optimizers produce about the same power. This option offers the economy of a string inverter with some performance benefits like micro-inverters. Each solar module will have an optimizer (or maximizer) to manage DC power production and provide individual module reporting. Common optimizer brands are SolarEdge and Tigo Energy. Optimizers allow mixing of solar panels, and odd string sizes for greater installation flexibility. They have a monitoring system to track performance and troubleshoot each individual module if needed. They have a 25 year warranty that is separate from the string inverter warranty. A PV system with optimizers will generally cost about 10% more than a string system and roughly the same as a microinverter system.
For more information, read TIPS HOW TO DESIGN SOLAREDGE PV SYSTEMS
A micro-inverter system has a smaller, mini converter attached to each solar panel using the panel output cables. Then the micro-inverters are connected to each other using special AC cables. Depending on the model, between 11 to 16 micro-inverters can be wired together to form an AC (alternating current) branch circuit. Each branch circuit can then be wired (using #2 to #12 AWG wires) to a 20 Amp circuit breaker in the household electrical panel or in a combiner box. Given this type of wiring, if a solar panel on an AC branch circuit fails or is partially shaded, it should not impact the performance of the other solar panels on that branch circuit.
Micro-inverters operate at lower-voltage 240Vac output for use in the home, with each inverter producing around 1 to 2 Amps. A separate AC safety disconnect switch is required near the utility meter to allow for manual disconnect of the system to the utility power grid for fire safety and utility maintenance.
It is important to note the conversion Efficiency when comparing the specific model of string or micro-inverter being considered. Some string inverters have conversion efficiency of 99% while a micro-inverter may convert power at 97% efficiency. In addition, the AC Power Output rating of the micro-inverter should be adequate to handle the real-world maximum or PTC rating of the solar panel. If the micro-inverter is under-sized to the PTC rating, it may intermittently clip the maximum power available.
Micro-inverter systems generally cost 10% more than string inverter systems without optimizers. They require extra cables, hardware and monitoring equipment. Compared to string inverters, micro-inverters can produce 2% more power in direct Sun, and up to 25% more power in shady conditions. Compared to optimizers, micro-inverters produce about the same power. They also work well when using different roof areas and different orientations. They often have 25 year warranties and a longer performance lifetime. Micro-inverters are great for easy system expansion. They are used with a monitoring system to track performance and troubleshoot each individual module if needed.
For more information, compare MICRO-INVERTERS