The primary role of a solar inverter is to convert direct current (DC) produced by solar panels into appliance-friendly alternating current (AC) electricity. However, its role has expanded over the years, particularly with the development of newer and more complex types of solar inverters.
There are solar inverters which are able to monitor the performance of a solar power system and provide diagnostic information of the system. As a result, these inverters help to improve solar grid stability and efficiency. With the advent of solar+storage, solar inverters are also able to take on responsibility for battery management.
Solar inverters can be broadly categorized into 4 types according to the set-up of the solar energy system and nature of the inverter. We explain what these 4 types are, how they work, their pros and cons, as well as the context for their usage in this article.
The name for this type of solar inverter comes from its set up. The inverter is connected with a ‘string’ of solar panels. In other words, a number of photovoltaic panels are connected by a wire which is then connected to the inverter.
One string inverter can be hooked up to just one or a few strings of PV modules. This setup is normally used for smaller-scale solar power systems such as home solar installations.
Historically, it is the oldest type of solar inverter and it is the one used most commonly worldwide. At present, it is the most cost-effective option available in the U.S.
How do string inverters work?
Here is an example to explain string inverters more clearly. Say a large house has 6 solar panels. 3 solar panels are set up on one side of a roof gable, strung together by one wire. Another 3 are placed on the other side of the gable, also connected by one wire.
These two wires are then linked to one inverter. Each wire or ‘string’ carries the DC power of 3 solar panels. The combined DC power from each string is then channelled into the inverter to be converted to AC electricity.
The pros and cons of string inverters
Since string inverters have been around the longest, they are also the cheapest and most durable inverter technology.
They are also the easiest to maintain because string inverters are usually installed in an easy-to-access location, and not on the roof. The typical spot for string inverters is close to the fuse box and electricity meter.
Having solar modules connected in a few strings makes it a little easier to detect issues than having all the panels connected to just one wire. If one of the panels is malfunctioning, that string would indicate the problem.
String inverters are only able to convert the aggregate power generated by each string. If one of the panels of a string is mostly shaded from the sun throughout the day, the total aggregate DC output will be affected. Therefore, a drop in the performance of a single solar module can impact the output of all of the panels on the string.
Best situations for string inverters
Solar inverters are ideal for residential and commercial properties where the PV panels get consistent sun throughout the day. To overcome the issue of DC power becoming aggregated, string inverters can be paired with power optimizers, which are installed at module level.
Power optimizers mitigate the effects of shading that string inverters cannot do. This gadget conditions DC electricity before sending it to the inverter; thus resulting in a higher overall efficiency than using a string inverter alone.
Central inverters are similar to string inverters. However, it is many times larger and can support more strings of panels. Central inverters are often confused with string inverters and these two terms have been used interchangeably, albeit incorrectly.
How do central inverters work?
In a solar system with central inverters, wires or strings do not run directly to the inverter. Instead, they are connected in a combiner box first. The DC power from the entire array is sent from the combiner box to the central inverter where DC is converted to AC.
The combiner box and central inverter are relatively large electrical items. As such, they are usually placed on the ground and in a protected area, free from any harsh weather.
Pros and cons of central inverters
Only one central inverter is needed to handle any number of PV arrays. This makes it a cheaper option than string inverters for very large-scale solar installations. And because it is a large piece of equipment, it is usually installed on the ground where it is easy to access and fix if there are any issues.
Solar installations with central inverters are usually connected to the power grid. When there is an outage, the inverter will also stop working.
Best situations for central inverters
Central inverters are best suited for very large installations with consistent production across the PV array. For example, solar power stations or solar installations for industrial use.
As its name suggests, a microinverter is a miniature version of a solar inverter. It is fitted directly into a solar panel. Only one microinverter is installed per PV panel. Because microinverters are installed in all the photovoltaic modules of a solar array, they are sometimes called ‘distributed’ inverters.
Microinverters are comparatively more expensive than string inverters. The reasons for the higher cost of a system fitted with microinverters: every panel has its own inverter, and it is a newer photovoltaic technology than string inverters.
Nevertheless, microinverters are gaining popularity for residential and some commercial solar installations as their cost declines and the demand expands.
How do microinverters work?
In a solar power system using microinverters, the microinverters are mounted either on the back of the solar panels or next to the panel racking system. Solar energy is converted from DC to AC right at the PV panel.
AC electricity then flows through a shared connection directly to the output source. There is no need for any other intermediary or central inverter in this type of system.
Pros and cons of microinverters
Since microinverters make the DC to AC conversion right at each solar panel, this allows you to monitor the performance of individual solar panels. Also, should there be a malfunction of one panel, the energy output of the other panels will not be affected.
Microinverters are also advantageous for systems with a more complicated design or set-ups which experience shading. They optimize the output of every panel at the panel itself, thus mitigating the impact of shading.
They are also able to take a beating from harsher elements since they are created to be mounted on solar panels, which are exposed to the sun, rain and snow.
Aside from their higher cost, microinverters can be harder to maintain or repair in the event of a problem because they are located on the roof.
Best situations for microinverters
Whether it is for residential or commercial usage, microinverters are better suited for panels affected by shading or are facing different directions. Microinverters are also great for maximizing solar production in a small space.
String inverters, central inverters and microinverters are usually used for grid-tied solar power systems. For off grid solar power systems, hybrid inverters are the go-to device but they can also run in a grid-tied system.
They are designed to be multi-functional and multi-mode. For these reasons, they are particularly favored for mobile homes such as tiny homes, caravans, RVs, vans, buses and trucks.
How do hybrid inverters work?
A hybrid inverter can connect solar panels with a solar battery as well as AC appliances and the power grid. Any excess converted AC electricity can be channelled back into the grid.
Conversely, DC power generated can be stored in the solar battery. Hybrid inverters can also work with AC batteries.
When the need arises, such as in situations of a power outage or lack of sunlight, a hybrid inverter can draw power from either the grid, DC battery or AC battery. If it is a DC battery, the inverter converts it to AC for use with appliances. If the hybrid inverter is connected with a power grid, it can also convert AC to DC to be stored in a DC battery.
Pros and cons of hybrid inverters
When there is no sunlight or in the event of a power grid outage, hybrid inverters can draw DC electricity from the solar battery and convert it into AC for use. Should any problem arise, a hybrid inverter can go to a standby mode instead of turning off completely.
Not all hybrid inverter models in the market have the same capabilities mentioned above.
Best situations for hybrid inverters
They are great for buildings that are grid-connected and not grid connected, as well as mobile homes.
Each of the 4 types of solar inverters has its advantages and disadvantages. Choosing the right type hinges on the situation and the size of the solar power system. For smaller installations, the more suitable types are string inverters, microinverters and hybrid inverters. For large-scale installations, cost is always a major concern, which makes central inverters a more financially viable option.