Micro inverters or micros are newer to the market than string inverters and are fast becoming very popular. In the US more than half of all new solar installations used micros. They have some benefits over string inverters but also a higher upfront cost. In this article we will look at whether it is worth paying extra for micros.
First, what is a micro inverter? For an understanding of the basics of how solar works click here. A micro inverter is as very small inverter designed to be attached directly to one or two solar panels. This is different to a more common string inverter which is usually attached to a wall some distance from the string of solar panels and connected via DC cable. Being attached to an individual panel means that each panel or pair of panels works independently from the rest of the solar array and DC power is converted to AC immediately. There are also solar panels available with a micro inverter already attached at the factory, these are referred to as AC modules.
Note: most benefits of this technology only apply to high quality micro inverters. Poor quality micros on the market should be avoided.
A biggest advantage of small inverters being attached to individual panels is that even seemingly identical solar panels can output different amounts of power. In a string inverter situation, panels are linked together in strings and the output of the entire string can be reduced by any poor performing panels. Panels can under-perform for a number of reasons; shade from trees or power lines, soiling from dust and leaves, ageing and degradation at different rates and temperature differences are all factors. Micro inverters allow all panels to output their maximum power and not be brought down by neighbouring panels. Another advantage of this is being able to place panels on any part of a roof and they do not have to be primarily in one orientation as with a string inverter system.
Micro systems offer better system monitoring than string inverters because micros can be monitored at an individual panel level. A homeowner will be able to see the performance of the system as a whole, but the installer will have a separate login where they can monitor all their systems at a panel level and are warned when any are under-performing. Enphase for one also monitors all of their systems worldwide from their headquarters in the US. This allows an excellent opportunity to monitor fault conditions and continue to improve their product.
Most micro inverters, including the two common manufacturers Enphase and SolarBridge, have been rigorously tested in extreme weather conditions. These units are designed to outlast the life of a solar panel, and have a warranty to back that up. Many micro inverters come with a 25 year warranty, compared to a standard 5 years for string inverters. Again, bear in mind that this is only relevant with high quality micro inverters. A poor quality micro inverter with a 25 year warranty is not worth the hassle of having to deal with failures on a regular basis.
Micro inverter systems have no single point of failure, which is particular important in a large system with many panels. It means that unlike with string inverter systems, if one panel or micro inverter has a fault the rest of the system is unaffected.
Enphase monitored the performance of their products in the 2014 Adelaide, Australia summer, which had the most consecutive days over 40C (104F) in its recorded history and peaked at 45.9C (115C). Because of the advanced monitoring of their inverters, they were able to announce that “No Enphase Micro inverter across Australia shut down due to temperature” during this period. SolarBridge also has very stringent testing of their inverters and have performed well in hot and freezing temperatures. It cannot be inferred that all micros are up to this standard though and there are several brands including APsystems (APS) which have recorded much higher failure rates by installers.
Safety - AC vs DC
In a rooftop string inverter system DC electricity is sent from the roof top, usually through roof or wall cavities to an inverter which then converts the DC electricity to AC electricity. In a micro inverter system panels still produce DC power but it is converted to AC immediately, right there on the roof. This is important because it is voltage from DC electricity that is more likely to cause arcing when a wire is damaged, which in turn can cause fires. It is very important to bear in mind that fire from any solar system is very unlikely, especially if correct installation methods are taken to protect wiring.
Micro inverter manufacturers argue that whilst the upfront cost of their technology is higher than string inverters, they are better value over time. It stands to reason that if micros allow for greater performance and output of your solar system and are more reliable, then over time they will have a better pay back. Whether they do and if so by how much, depends on many factors including; difference in cost to string inverter system, shading and soiling of system, and electricity tariffs.
AC panel advantages
AC modules (panels with factory attached micros) have some advantages in addition to detached micros. I will focus on SolarBridge as they are considered the leader in AC modules and not all AC modules have the same testing and quality procedures.
SolarBridge have partnered with a number of panel manufacturers, from the highest quality to more affordable mid-range panels. SolarBridge inverters along with the panels they are mounted to, are tested as a unit in the manufacturing process, ensuring that the products work together. The panel manufacturer then sells the unit with a single warranty, meaning that if anything goes wrong with a unit in future there will only be one contact point (no passing the buck as can happen).
Another significant advantage for installers, is the speed at which they can install an AC module. As the inverters have already been mounted to the panels, installers simply need to lay the panels on the roof and plug them together. This may also translate to installation cost savings for consumers.
Disadvantages and what to avoid
A factor that must be considered when looking at micro inverters is that there are far more units in one solar installation than with a string inverter. In a 5kW system using 250W panels and micros on each panel, there will be 20 inverters. Which means that if the micros being used have the same failure rate as a mid range string inverter, there is 20 times more chance of a failure than with that string inverter. So, if you decide to use micro inverters you must use the best quality, because you cannot afford for them to fail as often as a mid range string inverter.
Which brings me to my second point; cost. The upfront cost will be more expensive as mentioned before. On smaller sytems (2-3kW) micros are less of a premium than string systems, because instead of having one string inverter you have 8-12 micros. But on a 5kW system, the cost difference becomes greater as one string inverter becomes significantly cheaper than 20 micros.
Finally, another reason to get the highest quality micros is that because micros are mounted to panels on roofs they are subject to much greater weather extremes than string inverters, which are usually placed in protected areas. Most micro inverters are tested and built for these weather extremes, but some are simply not cut out to withstand these pressures.
Micro Inverters and batteries
The huge variety of battery storage systems now on the market can make things complicated when you want to use micro inverters together with batteries. Unfortunately many of the battery systems are DC coupled batteries which means you need a compatible hybrid string inverter and multi-mode inverter. Because micro inverters output AC power these cannot be used with the DC battery systems.
Fortunately there are several AC batteries now available including the Tesla Powerwall 2. Enphase Energy also manufacturers an compact AC battery module (see image) which is compatible with the Enphase micro inverter system. This unique modular system can be expanded if required and since it communicates with the Enphase micro inverter control system enables a complete range of load shifting and demand management features. The only disadvantage of the Enphase AC battery is (at this stage) it cannot be used as a back-up power supply.
There are many advantages to using micro inverters and particularly AC modules. However, if using micros you must use the best quality, and this can mean a considerable upfront cost difference to a string inverter system. If the cheapest system with the shortest payback period is required then a string inverter is probably called for. If you are looking for the highest quality, safest and best performing system, then high quality micro inverters should be considered. If a roof has multiple aspects or shading issues micros offer a big advantage. Also, on commercial systems where monitoring and high output is required micros can offer greater payback over the life time of the system.
Micro inverter manufacturer Enphase is considered an industry leader.
As is probably evident from this article our favourite micro brands are SolarBridge and Enphase. This is because not only do they market themselves better but they have a very solid track record. Remember, for the reasons I have mentioned above, micro inverters need to be of the highest quality. Whatever you choose ensure the units have been rigorously tested in extreme conditions and the company has a very strong track record.
If you are also in the market for a large battery system for load shifting or back-up power supply then micros will limit the battery options you can use. Also many of the AC battery options cannot work during a blackout so you will have to weigh up whether a back-up power supply is critical or just a convenience.