Comparing Different Inverters: Choosing the Best Option for Your Solar System

Are you considering installing a solar PV system? One crucial component you’ll need to decide on is the type of inverter.

What is an inverter?

The basic function of an inverter is to convert the direct current (DC) electricity generated by your solar panels or drawn from your batteries into alternating current (AC) electricity, which is suitable for use in your home or business.

Inverter types

String inverters versus microinverters

Let’s start with the clearest delineation – string inverters versus microinverters.

String inverters – receive DC output from multiple solar panels (often your whole solar array or at least a ‘string’ or two) and convert it to AC electricity. It is a single large(ish) component usually mounted near your distribution board (although certain systems will call for multiple string inverters).

Microinverters – a small inverter mounted on each PV solar panel that directly converts the DC output of the panel into AC and then combines it with the AC output of the other microinverters in the system to provide AC electricity directly into the distribution board for household use.

Differences between string inverters and microinverters

Tracker (MPPT) performance

The commonly held view has been that microinverters, when compared to string inverters, generally perform better, particularly if one or some of the panels are shaded, last longer and are safer. However, the latest string inverters, such as the Fronius Gen24 range, have very high performing maximum power point trackers (MPPT), which were found by a recent French study to perform as well as microinverters. These modern MPPTs even minimise the power loss across an array if there is shading on some of the panels to the extent that there is little to chose from between a microinverter system and a string inverter system.

Longevity and impact of failures

The argument that they last longer is also generally discounted nowadays on the basis that they are both inverters and it really just comes down to the quality of the built by the manufacturer. Therefore, there is no particular reason string inverters from reputable manufacturers such as Fronius and Victron shouldn’t last as long as an Enphase microinverter. However, if a microinverter fails, it won’t take out the whole system, whereas a string inverter likely will. However, there is still a high likelihood it is simpler to get a string inverter repaired as it is usually at ground level, easily accessible and may only need a component replaced. With a microinverter, you will need to locate the problematic unit in the array, potentially requiring removing part of the array, and then install a new unit.

Warranties

In New Zealand the warranties for Fronius string inverters (provided you register the inverter with Fronius) and Enphase microinverters (by far the most common microinverter brand) are both 10 years.

Expandability

If the ability to expand your system is a key design requirement, microinverters are likely worth considering. As each panel effectively becomes its own little power station, feeding AC electricity, expandability can be more straightforward. However, as is usually the case, it depends on the situation. There can still compatibility issues with microinverter controllers and the like, and in many scenarios, especially with a bit of planning when the first phase of the installation is designed, it may be easier and cheaper to expand a system using a string inverter.

Safety of DC versus AC transmission

One notable upside to microinverters is that the electricity moving around is generally AC electricity, rather than DC electricity, which is safer. This is a consideration although modern string inverters do limit this risk through systems like Fronius’ Arc Fault Circuit Interruption (AFCI). There are also regulations in place to make sure the cables carrying DC cable are well protected and this part of the installation is a focus of the independent electrical inspector, who is required to inspect all solar installations.

Panel layout

Another advantage of microinverters is that the panels do not all need to be aligned the same way. A single MPPT requires all panels to be in the same alignment (both angle and pointing in the same direction). The likes of a Fronius inverter only has two MPPTs built into it so you can only arrays in two different alignments feeding it. Other manufacturers such as Victron, while they use string inverters, do not generally build in the MPPTs, producing these separately so they can be sized for the number of panels in a given array/on a given aspect. However these can multiply up quickly and create a complex power wall. Microinverters incorporate individual MPPTs so are a great solution if a installation calls for small numbers of panels (say 1 to 4) on lots of different aspects (three or more).

The major advantage of string inverters is price and simplicity. The reality is that for the vast majority of installations undertaken in New Zealand, the advantages of microinverters are very limited when compared to modern string inverters (and MPPTs) and do not justify the additional cost which can result in the system costing as much as 50% more.

Other inverter types

Inverter

A basic inverter function is to simply convert a particular voltage of DC electricity (usually 12V, 24V or 48V) to AC electricity (230V for regular NZ usage). It does not have a MPPT and therefore needs a separate solar charge controller, such as the Victron SmartSolar Solar Charge Controller above, to manage the ‘raw’ output of the PV solar. It will then take the DC output of the solar charge controller or the battery and convert it to AC for use in your household or business. It only has an AC output, not an AC input, and therefore cannot charge a battery from an AC power source such as the grid or a generator.

An example of this type of inverter is the Victron Phoenix range of inverters.

Inverter Chargers

An inverter charger is similar to the basic inverter above in that it doesn’t have an in-built solar charge controller, but it will have at least one AC input. This allows it to take power from an AC source such as a generator or the grid and charge the battery. This is a requirement in most off-grid systems with a back-up generator and in grid-tied peak shaver systems that charge the batteries from the grid to provide back-up capability from battery, shift import of power to times with cheaper tariffs or provide additional capacity to a system for high demand periods (i.e. if power demands at certain times are higher than the the supply current, in NZ most single phase residential systems have a supply limit of 63A).

Examples of inverter chargers are the Victron Multiplus, Mulitplus-II and Quattro ranges.

Hybrid inverter

A hybrid inverter is a string inverter (i.e. an inverter that has the MPPT to allow it to manage the PV solar) that can also integrate with a battery. Hybrid inverters are often quite particular about the batteries they will integrate with but, for many people, it gives a great, streamlined system.

A hybrid inverter will also synchronise with the grid and meets the regulations to be compliant to be connected to the national power grid – the relevant regulation for whether you can have an inverter connected to the grid in New Zealand is AS/NZS 4777.2.

Examples of hybrid inverters are the Fronius Gen24 Plus inverters and the Victron EasySolar range.

Making an Informed Decision

Ultimately, the best inverter for your solar installation depends on your specific needs and situation. For the majority of grid-tie installations, a string inverter or hybrid inverter will be the best, most cost-effective solution but there is definitely a place for microinverters and inverter chargers in certain systems. In off-grid systems, inverter chargers come into their own, but there are significant loads while the PV solar is producing (e.g. a spa pool or EV charging), including an efficient string inverter such as a Fronius Gen24 inverter in the system may still make sense.

By carefully weighing the factors and consulting with a professional solar installer, you can choose the inverter(s) that best meets your requirements and optimises the performance, cost-effectiveness, and longevity of your solar system.

Investing in solar is a significant decision, and selecting the right inverter is crucial to ensuring that your system performs at its best.