What are the pros and cons of a microinverter vs. a string inverter?
When you install solar panels on your roof, you need an inverter which basically makes the electricity from the panels useful for your home or business. It does this by converting the electricity from a varied voltage DC, to 240v AC, just like the electricity that comes from your energy provider. There are pros and cons of all major inverter brands to consider. This article shows the differences in the two types of technology and doesn’t focus much on individual brands. For more reviews on specific brands, click here.
In a 100m solo race the best performer wins every time. However, put them in a relay race with an injured teammate and what would happen? A microinverter system allows all your panels to perform individually whereas a string inverter you rely on all the panels in the string as teammates.
The traditional and most commonly used type of inverter is called a string inverter. The reason it is called a string inverter is because the solar panels need to be connected in strings to work properly. Simply put, this means that the solar panels are wired in series and the voltage for the whole string of panels adds up as you add more panels. This whole string will then be wired into the inverter. Depending on the overall size of your solar array, you may have 2,3,4 or more strings that make up the whole system.
For home solar systems you would usually have one string inverter, which would be installed near your main switch board, or near your sub board. For larger solar arrays you can install multiple string inverters.
Image above shows how a string inverter would work with one string of 8 panels.
A Microinverter is…well a micro inverter…so basically a very small inverter! The function is still to take your solar electricity and make it useful for your home by changing the DC to AC, but it is done without having to create strings of panels. Microinverters are typically designed so you have one inverter per panel. Instead of your panels being wired in series and adding up voltage as you add panels, the microinverters work by changing the voltage to 240VAC at each panel, and then they are wired in parallel which means the voltage doesn’t add up like it does when in series.
Effectively this means each of your panels is a separate entity in terms of how it performs. Whereas with a string inverter, your panels performance will vary depending on how the rest of the connected panels in that string are performing.
Image above shows how microinverters would work with 8 panels.
Pros and Cons of strings inverters and micro inverters
When you use a string inverter, your panels are connected in series in a string as per the above image. As a result of this type of wiring, if one panel is not performing well, then the rest of the string can suffer. Essentially, the whole string will only perform as well as the weakest link in that chain. Using the analogy if the relay runner above; in a 4-person relay race, if one person trips and falls then the whole team will likely come in last, even if the other team members are by far the fastest. In a solo race, one person falling over will only affect their race. As another example, you may remember that some Christmas lights were wired in series, so if one lamp blew, then a whole string of lamps turned off. This would mean the whole chain of lights wired in series would turn off, until you replaced the one broken lamp.
What this means in solar terms is that if one panel fails, or produces less energy due to shade debris or other degrading factors than your whole string will be affected which can result in major losses in efficiency. With a microinverter you have a much less severe issue. The panel that is affected will still not perform any better, but it will not drag the rest down with it.
Image above shows how your panels can be affected with a string inverter.
Image above shows how your panels will operate under the same circumstances with microinverters.
The second biggest benefit for microinverters is system design flexibility. When using a string inverter, you will need to have a minimum amount of solar panels connected to make each string work properly. Depending on the inverter chosen usually this is a minimum of 6 solar panels. If you connect less than this, the voltage will not be high enough to make that input on the inverter work properly. In addition, each panel in the same string needs to face the same way, at the same angle to make up the string effectively. So, if you have a complex roof design or want to maximise the overall system size by putting panels in places where only a small number can fit, you won’t be able to use a string inverter.
A microinverter solves this issue as each panel operates individually and the conversion to 240VAC happens on the roof.
This first image shows a solar array installed with a string inverter:
This second image shows the same roof and what you could have installed with microinverters:
Modern solar systems can always be monitored with Wi-Fi connectivity which means you can see how your solar is performing on your phone or laptop, rather than having to check the inverter screen. Nearly all inverters have this capability built in.
The difference with microinverter monitoring is you can now monitor the performance of each individual solar panel. Traditional string inverters you can only measure the overall system performance, or sometimes each MPPT (inverter input) performance. You can purchase inverter agnostic monitoring which can measure each string of your solar array, but this is not usually a cost that people want to entertain.
This can be very handy if you are really interested in seeing how your solar panels degrade over time. Solar panels in Australia come with long warranties and if they degrade quicker than they are meant to, you should be able to lodge a warranty claim to have them replaced. Having monitoring to keep an eye on individual panels will alert you if one solar panel has performance related issues. A string inverter monitoring will not necessarily do this as you will only be able to see the overall performance.
Image above shows Enphase monitoring, where you can monitor every panel individually.
All of the above means that with a microinverter system so will ultimately have a more effective solar array!
There are some other benefits too: Since 2014 the National Electrical Code (NEC 2014) included a rapid shutdown requirement for solar installs in Australia. This basically means that first responders like fire fighters need to be able to rapidly shutdown solar PV so they can safely attend to emergency situations. Microinverters like Enphase, which is the leading brand of microinverters in Australia, have rapid shutdown built into their standard capability so there is no need for extra equipment. If you install a string inverter you will need to also install a separate rapid shutdown box on the roof within 10 metres of the array, and another shutoff switch on the ground that is accessible to the responders. There is an argument that this makes a microinverter system safer, although essentially if you have the extra equipment mentioned above your string inverter system will have equal protection and will meet the standards required.
You can also increase your solar system size more easily by adding one panel at a time – if you really wanted to. As there are no constraints regarding string configuration, it is way easier to add to your existing system if you want to.
So, why doesn’t everyone install microinverters then?
At GI Energy we probably install one microinverter system for every hundred string inverter systems we install. But why… If they are so superior, is that number not much higher?
The main drawback here is cost. Installing a good quality microinverter solar system is way more expensive than installing a string inverter system. With a typical 6.6kW system for a home, you would need approximately 18 solar panels and one string inverter. With a micro inverter system, you will need 18 solar panels and 18 microinverters. While the cost of one individual microinverters is much less than one string inverter, 18 of them is still a lot more than 1! In this example, you would expect to pay 2.7 times as much for the 18 microinverters, compared to one really good quality (Fronius) string inverter. If you are installing a system much larger than 6.6kW then the gap in cost gets wider again as the more panels you install, the more microinverters you need.
The challenge here is making sure this extra cost is going to be worth it or not. If you have a really complex roof, or a roof with a lot of shade and debris, then this cost is easier to justify. If you have a roof where a sting configuration will work for the desired number of solar panels though, it is a lot more difficult to justify.
Image above shows how the extra cost adds up using microinverters.
There are also potentially additional maintenance costs to consider. Microinverters will come with good warranties but the reality is you are installing a lot more inverters which basically means you are setting up a lot more potential points of failure. Using the 6.6kW example above, you now have 18 inverters to worry about, not just one. In addition, these inverters sit up on your roof, which is not a friendly place in the Australian climate. While the leading brands of microinverters insist they are designed for this intense heat, there is no getting away from the simple fact that heat is not good for electrical appliances. While a string inverter can be installed inside your garage or on a southern wall in the shade, your microinverters are going to be underneath solar panels on your roof.
Microinverters have a lot of advantages! Ultimately, regardless of your roof area you will get a more efficient system using a good quality microinverter. However, you will spend a lot more money and frankly, they are not needed to get a great result from your solar array under most circumstances.
If you have a big budget and are not concerned by the cost vs. benefit calculation, then there is no question you should instal microinverters. Additionally, if you have a very complex roof or a roof with lots of shade or debris, then you should at the very least consider them.
However, if you have a budget or you are interested in making sure you get good value form your solar array, and you have a roof that works with string configuration then choosing a good quality string inverter will most likely do almost exactly the same job, for a lot less money.
You always need to make sure that your solar panels are good quality as well as your inverter so make sure when you do make a decision you have the right inverter and the right panels! For more information on how to choose the best solar panel for you, click here.
Side note: Power optimisers also have a say here:
A power optimiser is another option when deciding what type of inverter you need / want. The leading brand in Australia is Soleredge and they work in a very similar way to microinverters. The difference is that the conversion from DC to AC is still done with an inverter on the wall, but each panel (or sometimes every 2 panels) has an optimiser installed on the back.
What this basically means is that each panel (or sets of two) are still working individually so you get the same design and shade tolerance benefit as using microinverters. In terms of a price point they are still quite a bit more expensive than string inverters but not quite as much as microinverters.