Micro inverters are a popular alternative to the more common 'string' solar inverter and are now used in more than half of all solar installations in the US. Micro Inverters or micro's have several benefits over string inverters but also a higher upfront cost. In this article we examine whether it is worth paying extra and what advantages micro inverters have over string inverters.
What is a Micro Inverter?
A micro inverter is as very small inverter designed to be attached directly to one or two solar panels. This is different to a common string solar inverter which is usually located 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 factory attached, these are referred to as AC solar modules. For an understanding of the basics of how solar works click here.
Note: most benefits of this technology only apply to high quality micro inverters. Poor quality micros on the market should be avoided.
Advantages of Micro Inverters
A big advantage of micro 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 or shaded 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 as each panel can be monitored individually. Additional 'optimisers' can also perform this function for regular solar string inverters. With micros or optimisers an owner can see the performance of the system as a whole, plus at the individual panel level. Also the installer can have a separate login where they can monitor all their systems at a panel level and are warned when any are under-performing. Enphase Energy 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 leading manufacturer Enphase 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 10 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.
APsystems are another big micro inverter manufacturer although in the past industry specialists and solar installers reported much higher failure and reliability issues than Enphase. We do not have much feedback on the performance on the new generation micros from APsystems.
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 solar modules
AC solar modules (panels with factory attached micros) have some advantages to add-on micros. SolarBridge was considered the leader in AC modules but was acquired by the highly regarded solar panel manufacturer SunPower in 2014 and since only very limited AC modules are available using Solarbridge micros.
More recently Enphase has taken to developing AC modules again and teamed up with two world leading solar panel manufacturers LG Energy and Jinko Solar. The new 7th generation IQ series from Enphase have being specifically designed for integration into AC modules and will be available in 2018.
A significant advantage for installers is the speed at which they can install AC modules. As the inverters have already been mounted to the panels, installers simply need to lay the panels on the roof and plug them together. As the micro inverter is integrated into the panel there are also less points of failure and the micro has usually been optimised for the particular panel it is assembled to. This is generally more reliable with less handling can 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 should always use the best quality such as those from Enphase simply 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 slightly 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
Unfortunately many battery storage systems now on the market cannot be used together with micro inverters. Since most battery systems are DC coupled 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 you should only use the best quality units. Micro Inverters generally have a higher upfront cost compared to a string inverter system and this can be considerably more for systems over 3kW. If the cheapest system with the shortest payback period is required and you have no shading issues then a decent quality 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 huge 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 evident from this article our favorite micro brands is Enphase. This is because not only do they market themselves better but they have a very solid track record and good service. 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.