The solar inverter is arguably the most important part of your system. For this reason, picking your inverter is something you want to spend a bit of time doing, particularly if you want to interact with your system after it's installed. What makes a good inverter though?
Note: This blog post is a continuation from our Solarpedia article on solar inverters. This is summarised in the first section below, but we recommend you read that first.
Three different types of inverters
There are three main types of solar inverters. They are:
- String Inverters
- Micro Inverters
- Inverters with Power Optimisers (Optimised Systems)
Inverters have two key functions, to maximise energy harvest from the panels and to convert the DC power from your panels into AC power for your home. String inverters are the most common. They are mounted on the wall and connect to all the panels on your roof, in a string. They perform both these key functions on the wall. Micro inverters work in a similar way, but rather than mounted on the wall, they are much smaller and you mount one under each panel. Each panel has its energy harvest optimised independently (referred to as panel level optimisation), which presents numerous benefits which we'll discuss later. Both the key functions are performed under the panel with micro inverters. Optimised systems combine a string inverter on the wall but have power optimisers under the panels. This enables you to still have the DC to AC conversion on the wall and still maximise energy harvest at the panel level.
If you want a more detailed overview of the three different types of inverters, see our Solar Inverter article in our Solarpedia.
We’ve mentioned panel level optimisation, which is an important feature that both micro inverters and optimised systems come with. To understand the benefit of this, we must first understand how the Maximum Power Point Tracker, or tracker, on an inverter works.
Maximum Power Point Tracking
A solar panel outputs as much voltage and current as it can. However when the panel voltage gets too high, the current can drop. Remember though that power equals voltage times current, so too low a voltage results in lower overall power. So there’s an optimal voltage point to achieve maximum power, and an inverter has a Maximum Power Point Tracker (MPPT), or tracker for short, which is responsible for finding this. Better inverters do this faster and more accurately, resulting in higher yields - datasheets won’t tell you this. Any variation between the panels, like shading, stronger or weaker performance, different orientation, pitch or panel type, will disrupt this process. Panels producing lower currents, due to shading, different orientation, pitch, type etc, can also create a bottleneck on the string, so this needs to be avoided completely.
When designing systems, solar consultants will ensure that panels on a tracker are free of variation. Where this is not possible or more flexibility is required, different panels can be put on a separate tracker or grouped together on one tracker but in separate strings of equal length (commonly done with different orientations). A dual tracking string inverter also provides some flexibility and increased performance. String inverters however are no match for micro inverters and optimised systems here, as they offer panel level optimisation for ultimate performance and flexibility.
Panel Level Optimisation
Panel level optimisation refers to a system that has a tracker per panel, which can be achieved in micro inverter or optimised systems. Panels can have small variations due to manufacturing, differences in dirt build up, inconsistent cloud cover, birds and their excrement, the neighbourhood cat, shading from flues, antennas etc. Because of this, panel level optimisation will provide a 5% gain, even on an ideal roof. In partially shaded, non-ideal conditions, this can get much higher. An excellent analogy would be having a teacher per student in the classroom, compared to one teacher per class. Panel level optimisation can also enable other features, including:
- Panel level monitoring - Allowing you to see the performance of each panel and not just the system on the whole.
- Complete design flexibility - Panels can face different directions or be on different pitches.
- Better shading and fault management - The bottleneck created by shaded or faulty panels won’t impact the other panels.
- Better mismatch management - Minor differences, good or bad, are managed at the panel level to increase yield.
Note that in addition to the above, some manufacturers of micro inverters, optimisers or optimised systems may have extra features unique to their products. While panel level optimisation is not essential, it’s becoming cheaper and easier to achieve, and thus more common. Also note that panel level optimisation only helps shading when some panels are shaded and others aren’t. If the panel you plan to install is going to spend more than 50% of it’s daytime in the shade, it might be best not to install it.
Now we’ve covered the fundamental aspects of the inverter types and how they work, now it’s time to look at some of the other features of the inverters.
Reliability, service and warranty
Ask any solar installer, the inverter is the most common component fault in solar. Choice also came to the same conclusion. Cheaper inverters commonly chosen are a high risk to fail in the first 5 years - the minimum expected lifespan. Many get replaced, but some customers have been stranded by vanishing inverter manufacturers and solar companies, so be carefull assuming your warranty will always cover you. Since solar is a long term investment, we recommend a good inverter from a reputable manufacturer. Reputable manufacturers test their inverters more rigorously, use proven, better quality components, have superior manufacturing facilities and use refined manufacturing techniques - all learned from years of manufacturing inverters and studying where previous models have failed. Reputable manufacturers also respond quickly to issues as they have the correct servicing centres and processes in place.
Don’t be fooled by looking at a datasheet of two inverters and assuming that the one with the higher max efficiency will produce more. This figure just looks at converting DC into AC at its most efficient DC input. Since that DC input is changing all the time, it’s important to look at other figures like Euro or CEC Efficiency, which measure efficiency across a wider range of inputs. Also note that inverter efficiency doesn’t take into account the benefits of a better MPPT tracker or panel level optimisation, which can account for over 5% more yield.
Like the odometer on your car, you can usually get your overall kWh production from your inverter. However if you want a little more, or your inverter does not have an informative display, we highly recommend an inverter with a good monitoring portal. Whether you check it once a day or once a year, you and/or your solar retailer can access the data and make sure everything is working. Fault alerts can also be set, so you’re notified of any issues sooner. Web based monitoring programs like Solarweb from Fronius, Enlighten from Enphase and SolarEdge’s online platform, are easier to access than USB or Bluetooth based applications, have a full suite of apps and enable access from a web browser. They are also beginning to communicate with other household products. Be mindful that these monitoring systems require internet access, which can be provided by a WiFi connection to the inverter, or Ethernet cable if you want added reliability and are happy to pay extra. Once connected, monitoring setup is easy enough for the tech savvy, but a little beyond the layman. Once done though, they usually work fine unless the WiFi signal is weak or you change your home router or password.
Some inverters are now coming without screens, relying solely on the WiFi connection to monitor the system. If WiFi and the internet is not something you use regularly, bring this to the attention of your solar retailer.
A consumption monitoring device tells you how much power you’re consuming and where the power is coming from. This ties in nicely with production monitoring and adds meaningful data to a monitoring portal, like accurate financial savings, percentage of grid and solar power you’re using and how much you're returning to the grid. Monitoring portals stores this data and display it in real time. This information helps you identify big consumers and uncover ways to save more money. It is also extremely valuable for correctly sizing any future battery storage. We highly recommend consumption monitoring for enthusiasts, the energy conscious or those planning for batteries in the future.
The inverter is the main solar component you'll interact with once your system is up and running. It's also the component you're most likely to replace first. Choosing wisely when it comes to your inverter will ensure that the ongoing use of your solar system is pleasant, the system generates as much yield as possible, you'll have excellent service if anything goes wrong and you'll be less likely to replace it in the near future.