Photovoltaic (PV) Solar panels produce direct current (DC), which is unsuitable for our electric devices and our electricity grid. So, what do inverters accomplish? They transform DC into alternating current (AC), which we commonly use in our households. The inverter also matches the voltage and frequency of the grid, so the power produced by the solar panels can flow into the network and to its intended use. Find more technical details on Wikipedia: https://en.wikipedia.org/wiki/Solar_inverter.
Grid or No Grid?
Inverters work differently for grid-tied and off-grid applications. If you intend to build an off-grid system, you will be looking for a stand-alone inverter. This type of inverter is designed to work with a solar array in combination with a backup battery, operating in isolation from the grid. The primary function is to provide electricity for use at its location.
Grid-tied inverters, on the other hand, fulfill a different role. Being connected to the grid means produced energy will primarily flow from the inverter into the grid. The connection comes with safety requirements. The inverter needs to shut down when there is a power outage to prevent electricity from flowing into the grid while there is a disruption. Generally, this type of inverter cannot supply your home with electricity while the power is down.
Hybrid inverters
However, an emerging species of smart grid inverters is closing this gap. They are also called hybrid inverters and are jacks-of-all-trades. Mainly designed to run off-grid functions, like supplying your home with power, they manage battery storage and can be used for grid-tied solutions as well. You can also run your home directly off the solar energy your panels produce, even when the grid is down. Experts see this type of inverter gaining more relevance as batteries become more affordable. Homeowners are expected to install systems that include backup batteries more frequently, even while remaining connected to the grid, creating greater independence and more flexibility.
String inverters predominant for residential installations
Inverters are designed for a variety of different purposes and array sizes. Large scale or commercial installations use so-called Central Inverters. For residential applications, we generally use String Inverters. And then, there is the family of Microinverters. They are also used in residential settings, but instead of being hooked up to the whole “string” of panels, they are attached to each individual panel, converting the power of just that one panel. They tend to be harder to maintain and replace since they are located underneath each panel. Access can be difficult once the system is mounted on the roof. However, they can be much more effective than string inverters in situations with complex panel layouts and if there is shading present. With shade covering part of a solar panel, larger sections of the panel are often unable to produce energy. That has to do with how the cells on the panel interact when producing energy.
Optimisers to counteract shading
An alternative to microinverters is power optimisers, which are equally suited to improve production with partial shading. They ensure that unshaded cells can continue producing electricity, despite the shading preventing other cells from being productive. Less costly than microinverters, the optimisers are equally hooked up to each panel and work with a regular string inverter. The Solar Edge brand has a well functioning solution of string inverter with optimisers.
List of inverter types
- Standalone Inverter – Off-grid, with battery
- Hybrid Inverter – Grid-tied, with battery
- String Inverter – Grid-tied, for the entire array (string of panels)
- Microinverter – On-grid or off-grid, on each panel
Key features of inverters
First and foremost, inverters are designed to handle different loads of kilowatt. If you intend to install a 5 KW system, you will generally look for a 5 KW inverter. In some cases, you may choose an inverter with a smaller output. That can be the case if you rarely expect your system to be producing at its peak, for example, due to midday shading in the most productive summer months. The output of your system will be limited to the inverter’s maximum kilowatt output.
Efficiency
Inverters come with an inherent energy loss, which, all things considered, is relatively small. Most inverters operate at around 96 – 98% maximum efficiency, which indicates the efficiency level at the inverter’s optimal performance point. While this feature bears relevance for a system’s energy output, efficiencies are excellent amongst all major brands.
Warranty
The longer the warranty period on the product, the better the chances that you are acquiring a high-quality product. You may want to make sure that you choose an inverter with manufacturer representation in the country. Dealing with a manufacturer abroad can be more than just inconvenient, should you ever have to call upon the warranty. The good news is that almost all the major brands have offices in Australia.
Longevity
Keeping in mind that your panels will have a useful life of 25-30 years, you will be hunting for an inverter that comes at least close to that. While the warranty is an indicator of longevity, it will not always be the sole factor. The best inverters will have a 10-year product warranty, which is extendable to 20 or 25 years against a fee. The brands Solar Edge and SMA lead the pack on this front. Most commonly, though, a 5-year warranty is the norm.
System Monitoring
With inverters being a very technical product, here is the exciting part for most of us. Inverters routinely come with a monitoring system, visualising what is happening inside your solar PV system. Apps help you track energy production and savings throughout the day or over the course of months. You can see when the system produces less due to shading or based on the season. While this has no direct impact on the system’s performance, it’s a great selling feature. Ask your installer or the manufacturer to show you how the monitoring system works and what it will show you. You will likely make your inverter purchase decision based on this feature.
Pricing
Prices for reasonable quality inverters start around the $600 mark. However, such inverters are often for small systems, i.e. 2-3 KW. If you are looking at installing a 5-8 KW system, you will see prices in the $1,500 – 2,000 range, with Fronius and SolarEdge closing in on $2,500. Unless you are on an extremely tight budget, think about spending a few extra dollars to buy a high-quality inverter. At a 10-15 years lifespan, you are only spending a maximum of $150 – 200 a year on this device, likely less, given the longevity of these higher-end brands. Well worth it in my mind.
Top residential inverters
The top two brands in Australia are SolarEdge and Fronius. They come at a premium price, however, they offer excellent quality with extended warranties. Other great brands are SMA, Fimer (ABB), Huawei and Delta. There are several emerging brands that are less well known but offering attractive pricing, among them Growatt, Solax and Solis.
Looking for more detailed information?
PVEL is an independent testing lab for solar PV products. They are an excellent resource if you want to look into the features of each inverter. They have published a scorecard in 2019 displaying the best inverters for specific technical criteria. https://www.pvel.com/inverter-scorecard/