Micro inverters are a great alternative to the common 'string' solar inverter and are used in over half of all solar installations in North America. Micro inverters, also known as micros, have several advantages over string solar inverters but a marginally higher upfront cost. In this article, we examine whether it is worth paying extra and what advantages micro inverters have over common string inverters. For those new to solar learn the basics of how solar works here.
Enphase Energy founded in 2006 has long been regarded as the world leading micro inverter manufacturer. Based in the US, Enphase launched the first micro inverter, the M175 in 2008 but it wasn’t until the next-gen M190 was launched in 2009 that sales really took off. Ever since Enphase has established itself as an industry leader in micro inverter technology and have a huge market share in North America. Sales in other regions such as Europe and in Australia have been low in comparison but are growing as both consumers and installers realise the many benefits of micro inverters.
What is a Micro Inverter?
A micro inverter is a very small inverter designed to be attached to each individual solar panel. This is much different to common string solar inverters which are usually located on a wall some distance from the string of solar panels and connected via DC cable. In string inverter systems, DC power from the string of the panels is then converted to AC at the inverter. Since micro Inverters are attached to every solar panel, each panel works independently from the rest of the solar array and DC power is converted to AC immediately on the roof. Some solar panels are also available with factory installed micro inverters, these are referred to as AC solar modules which we discuss in detail later.
Note: most benefits of micro inverters only apply to high-quality micro inverters such as those from Enphase Energy. Unknown or poor quality micros should be avoided due to the history of high failure rates and poor service.
Advantages of Micro Inverters
The main advantage of micro inverters in that each panel is monitored and optimized individually to generate maximum power. Solar panels can generate different amounts of power due to a number of reasons such as shading, dirt and mismatch as explained in detail below. In a standard string installation, the panels are linked together in series and the power output of the entire string can be greatly reduced by a single poor performing, shaded or dirty panel. However, using micro's can overcome this and help avoid the many reasons solar systems can under-perform including:
Shading – Trees, buildings, poles, wires, antennas, or rooftop structures
Soiling – Dust, snow, bird droppings or birds
Panel mismatch – manufacturing tolerances
Panel faults – micro cracking, hot spots or water ingress, or LID
Temperature difference – thermal mismatch at different times of the day
Panel Ageing - Panel degradation at different rates over time
No PID – Eliminates Potential voltage and current leakage issues
Odd roof layouts - Panels mounted at different orientations and angles
Small roof - Roof area’s which can only fit a small number of panels
Considering the issues listed above, micro inverters improve performance as they allow all panels to operate at their maximum power point and not be affected by the other panels. Another advantage of micro's is the ability to locate panels on multiple sections of complex roof layouts with different orientations and tilt angles. Unlike string inverters, micro inverters are not restricted to panel placement which is generally limited to two different orientations.
Micro inverter systems also have no single point of failure, which is particularly important in a large system with many panels. This means that unlike with string inverter systems, if one panel or micro inverter has a serious fault the rest of the system is unaffected and it doesn't result in complete loss of generation.
Further advantages when using micro Inverters:
No single point of failure
Not limited by string voltages
Easily expanded with different or newer panels
Enphase Micro Inverter Range with IQ7X
Enphase Energy have a family of micro inverters and have recently released the IQ7 and IQ7+ which is the seventh generation micro inverter. The latest high-powered smart grid-ready Enphase IQ 7 Micro and Enphase IQ 7+ Micro achieve very high system efficiency at 96.5% and are compatible with the latest 60 cell and 72 cell high powered solar panels up to 460W. See full datasheet here - Enphase IQ7, IQ7+ & IQ7X specification sheet.
System and panel level monitoring
Micro inverters offer better system monitoring compared to string inverters as each panel can be monitored individually. For common string inverter systems additional panel mounted 'DC optimizers' such as those from Tigo or SolarEdge can also enable individual panel level monitoring and optimisation.
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 panels are under-performing which can prevent system downtime and help with any warranty claims. Enphase Energy have the Envoy monitoring hub for all systems to enable setup, commissioning and remote monitoring. They also monitor all registered systems worldwide from the US headquarters which provides valuable information to monitor fault conditions and continue to improve their products.
Most micro inverters have been rigorously tested in extreme weather and climate conditions. These units are designed to last the life of a solar panel and generally have a warranty to back that up. Many micro inverters such as those from Enphase come with a 25-year warranty, which is far longer than the standard 10 years for most string inverters. Again, bear in mind that this is only relevant with high-quality micro inverters such as those from Enphase. To date, there have been very low failure rates reported from our solar specialists, and Enphase has proven to be one of the most reliable inverter manufacturers in the world.
Enphase tested and monitored the performance of their micro's during the 2014 Australia summer in Adelaide 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 high temperature” during this period. Other manufacturers also have 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 competing brands 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. Unlike common string solar systems when single module is damaged it cannot turn into a dangerous hot spot which can happen with strings as the higher voltage and current can cause a thermal runaway.
In a micro inverter system panels still produce DC power but it is converted to AC immediately, right on the roof. This is important because it is voltage from DC electricity that is more likely to cause arcing when a cable is damaged, which in turn can lead to heating and a potential fire. It is very important to bear in mind that fire from any solar system is very unlikely, especially if correct installation methods are taken.
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 limited AC modules are available using the now Sunpower micros.
In mid 2018 Enphase purchased Sunpower’s micro-inverter business and has taken to developing AC modules again. The new 7th generation IQ series from Enphase have being specifically designed for integration into AC modules.
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 inverters 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.
This brings us to the second point; cost. The upfront cost will be slightly more expensive as mentioned before. On smaller sytems (3-4kW) micros are less of a premium than string systems, because instead of having one string inverter you have 8-12 micros. Although on a 6kW system, the cost difference becomes greater as one single string inverter becomes significantly cheaper than 16-20 micro inverters.
The final and most obvious reason why micro inverters may not be the best choice is due to the mounting location on the roof where they are generally exposed to extreme heat, humidity and potentially salt mist when located near the ocean. This is where string inverters have the big advantage of being mounted in more protected locations or even indoors. Most micro inverters are tested and built for these weather extremes, but over the years some micro inverters have had multiple faults especially when in harsh conditions such as high humidity and in close proximity to the ocean with extreme salt corrosion.
Micro Inverters and batteries
Unfortunately, many of the well-known DC battery storage systems on the market cannot be used together with micro inverters. Since most battery systems are DC coupled, a compatible hybrid string inverter or multi-mode inverter is required. Because micro inverters output AC power they cannot be used with these common DC battery systems, however, they can be used with popular AC battery systems such as the Tesla Powerwall 2 and Sonnen ECO.
Enphase Energy manufacturer a compact 1.2kWh AC battery module which is compatible with the Enphase micro inverter system and can be managed through the Enphase Enlighten portal. This unique modular system can be expanded if required and since it communicates with the Enphase Envoy it 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. Fortunately there are several popular AC batteries now available which have backup capability including the Tesla Powerwall 2 and the new Enphase Empower batteries described below.
New Enphase Encharge batteries
New for 2021 - Enphase has announced a completely new energy storage system known as Enphase Ensemble built around the new Enphase Encharge batteries. The new AC-coupled batteries will be available in 2 sizes, a small 3.4kWh model and a large 10.1kWh model which is technically three of the smaller batteries combined into one unit. Enphase has again engineered a very unique battery system using new micro inverter technology configured to work in parallel and deliver continuous power of 3.8kW and peak power of 5.7kW. This is over 10 times greater than the old generation Enphase battery and is well-sized for the average household.
More importantly, the new energy storage system has full backup capability thanks to embedded grid forming micro inverters and the additional Empower smart switch unit which contains the grid isolation device.
There are many advantages to using Enphase micro inverters and AC modules, especially when individual panel level monitoring is required. For smaller systems (up to 6kW), micro Inverters have a slightly higher upfront cost compared to string inverter systems and this price difference increases considerably for larger systems. If you are on a tight budget and have large open roof area's with no shading issues then a good quality string inverter is going to be more economical. For those looking at energy storage then micro inverters do have some limitations and will only work with AC coupled battery options.
If you are looking for a quality, safe, high-performance solar system with great monitoring, then quality micro inverters from Enphase are easily one of the best options. For roofs with multiple aspects or shading issues, micros are a perfect choice, especially if each roof section is limited to a small number of panels. For commercial systems with many roof pitches, obstacles or where performance monitoring is required, micros will offer many advantages and can offer a greater payback over the life of the system.
It is obvious Enphase is considered the best micro inverter supplier on the market. This is due to the solid track record, low failure rate, high quality, advanced system monitoring and generally very good service.