How to choose a solar inverter
There are plenty of options when it comes to inverters, we’ll help you choose the one that is right for you.
With so many choices when it comes to solar panel inverters, it can be difficult to find the best one. Essentially, there are two main types of classifications for inverters. One references their position in the photovoltaic system: micro-inverter, string inverter or central inverter. The other is about the topology of the system where it will be connected: grid-tied or battery-based inverter. So, how do you choose a solar inverter?
Let’s start with types of inverters according to the position in the photovoltaic system:
String vs. central solar inverter
This topology has two variations:
- Consider an inverter per string of panels or
- Consider a single central inverter for the whole system. Under this scheme if one module is shaded, then the whole string is impacted.
Here are advantages and disadvantages of string and central inverters.
String solar inverters
|Mismatch losses are lower as each string does not affect the other one||Higher costs compared to central inverters|
|Shading affects only a string||Higher complexity in wiring and connection|
|Performance on several strings with different orientations is high||Multiple maximum power point trackers (MPPT) charge controllers need to be sized according to the number of inverters|
|Combiner boxes are not needed|
|Reliability is increased|
Central solar inverters
|Simplicity in design and connection||Mismatch losses are higher|
|Lower costs||Shading affects the whole array|
|Single MPPT controller can be sized||Low performance for strings with different orientations|
|Needs a combiner box to mix all the strings|
Each PV module has a micro-inverter to convert DC-AC. In this configuration AC energy is directly injected from the modules to the load panel as no string inverter is needed.
|Shading affects individual modules only||Higher system costs among all system configurations|
|Highest efficiency regarding several orientations in each string||High number of total components|
|Monitoring of performances in each module||Low reliability under harsh environmental conditions for complex electronic equipment (temperature and relative humidity)|
|No DC power loss|
|Module mismatch loss reduction|
|No rapid shutdown configuration is needed as DC side is de-energized as soon as the grid is interrupted|
|Junction box is not needed|
|Maximum power point tracking can be accurately achieved|
|Expansion is easy just by adding more panels with micro-inverters|
Now we’ll talk you through types of inverters according to the topology of the photovoltaic system:
Grid-tied solar inverters
This one is conventional inverter type.
The aim of this inverter is to transform the DC current from the solar panels into AC current that is consumed by the loads of the house.
It also injects sinusoidal currents into the grid through a bidirectional meter. This means that the inverter has a single DC input (or in any case, two MPPT inputs for solar panels) and a single AC output with a pre-established AC voltage, power factor and maximum current that is connected to the switchgear (in utility scale projects) or to the main panel of the house (residential scale).
These type of inverters are usually related to the central/string inverter topologies with PV modules connected in series to build a string and to match the minimum and maximum DC voltage input of the inverter.
Usually if the number of strings is large enough to match the required power amount, then they are connected in series towards a junction box from which two higher-size wires will be directed to the inverter’s DC inputs.
Grid-tied inverters under central topologies offer the option of either selecting a single high-size inverter (unique central inverter) or selecting several smaller inverters with a master-slave configuration (several central inverters).
Here is a grid-tied inverter with master slave configuration:
Under this topology, a single inverter (Master) performs the maximum power point tracking on the array and transmits the information to the other inverters (Slaves).
If a master/slave configuration is to be established, then it is important to know that all inverters MPPT inputs must be wired in parallel, as all of them need to see the same size of the array and receive the information for such array.
Grid-tied inverters are also coupled with the string scheme under which each separated string has its own inverter (string configuration).
Here the MPPT inputs of each string inverter must not be wired in parallel as there will be MPPT tracking conflicts:
The main concern regarding grid-tied inverters is related to their independence from the grid.
Grid-tied inverters are neither capable, nor recommended to operate on islanding mode (grid-disconnected mode), meaning that if your distributed utility grid goes off (there is a blackout), then so will your grid-tied inverter.
The reasons for this shutdown are related to stability and security.
The stability reason is related to the fact that the PV system uses the grid as a large reserve for power when needed. This means that when the sun is low and your panels are not capable of supplying the entire load of the house, the grid acts as a supplier to balance the system.
If the grid goes off, then there is no way to balance the system if the panels are not capable of supplying all the load.
If such imbalance occurs, the voltage or frequency values on the output of the inverter could be different from the ones that your electrical appliances use, which would cause flickering and in the worst case could damage your appliances or the inverter.
The second security reason is strictly related to the operation of the grid.
When there is a failure in the power service, it is highly probable that the utility company will send a team of technicians to repair the failure.
If your PV system is generating and is connected to the grid, then it is possible that the technicians will receive an electric shock while working. Therefore, before connecting yourself to the grid, the utility company will verify that your inverter deactivates when the grid goes off.
So, if you’re considering installing a grid-tied inverter, remember that you won’t have it available when there is a blackout.
Battery-based solar inverters
Battery-based inverters are applied in many configurations where a battery bank is needed to provide backup to specific loads (grid-tied case) or in order to work independently (off-grid case).
Depending on the system topology, battery-based inverters are able to work under different schemes:
- Grid-tied with battery backup (hybrid).
Off-grid solar inverters
The main application for these inverters is the stand-alone case.
Here the off-grid inverters are used to transform the DC power from PV arrays or wind generators, isolated from the grid into AC power.
Under this scheme, it is necessary to establish a charge controller to adjust the voltage from the PV array to the battery bank in order to charge it. The inverter does not perform the charging of battery; it only transforms the DC power needed from the battery bank into AC power for the loads.
As the inverter is isolated from the grid in this case, it is necessary to sum up all the AC loads to establish the battery bank and panel’s size (the system should be able to work independently).
If there are induction motors, it is important to make sure the inverter is able to provide the surge current needed for the start-up of these machines.
Off-grid inverters will output either sine-wave or square-wave forms AC, although sine-waves are preferred, as they are similar to the ones of the grid.
As the typical use for these inverters is for small system sizes, then the output of these inverters are generally between 120V/240V for single-phase and 208V for three phase inverters.
You must be aware that off-grid inverters can cause interference with radios, television and phones. To avoid these issues, choose an off-grid inverter with a sine-wave power output.
Grid-tied with battery backup solar inverters – Hybrid inverters
Unlike the off-grid inverters, the hybrid inverters (also called Inverter/Chargers) allow interaction with the grid in order to provide energy to the loads when needed or to sell the excess energy to the grid when is possible. At the same time, they provide backup to critical loads through a battery bank.
Most hybrid inverters are used under two configurations:
It is important to understand that hybrid inverters are battery-based inverters with the capability of interacting with an existing grid.
On this configuration the hybrid allows the option of connecting a battery bank. They interact in a bidirectional way to charge the batteries while at the same time provide the DC current from the batteries and transform it into AC for the loads.
Grid-connection is also performed, which is why these inverters are also used for grid-tied with battery backup systems on an AC-coupled configuration.
Under this configuration, there are two inverters:
- the grid-tied (directly transforming DC to AC current towards the sub-panel)
- the battery-based (used to charge the batteries with the excess energy from the panels or to feed the sub-panel from the batteries stored energy).
Here is a grid-tied with battery backup – AC coupled configuration:
An example of this configuration can be the combination between the Sunny Boy (grid tied inverter) and the Sunny Island (battery based inverter) from SMA.
The Sunny Island is a bi-directional and sine-wave battery based inverter/charger that can be used for stand-alone purposes or for grid-tied with battery backup systems under an AC coupled configuration.
The Sunny Boy inverter is connected to the sub-panel to directly provide energy from the panels to the sub-panel, which is also connected to the AC output of the Sunny Island inverter.
If the grid is up, the power from the panels passes through the Sunny Boy and goes into the sub-panel and to the Sunny Island’s built-in transfer switch towards the grid with no efficiency loss.
Power can also flow in the other direction if needed. The Sunny Island Inverter uses the power from the grid or the Sunny Boy to charge the batteries through a conversion AC/DC.
If the grid goes off, the Sunny Island disconnects itself from the grid and starts providing AC energy to the sub-panel, extracted from the batteries.
At this moment, the Sunny Boy inverter will disconnect itself as well, but will be back on after nearly 5 minutes when the Sunny Island has already established the frequency of the system.
That way both inverters will work at the same time to provide energy to the sub-panel (Sunny Boy from the panels and Sunny Island from the batteries).
If the Sunny Boy is able to provide all the loads of sub-panels, the energy will be used to charge the batteries as well, through the Sunny Island.
If the batteries are full and the Sunny Boy keeps producing excess energy, then the Sunny Island will communicate to the Sunny Boy to reduce its power output.
Finally, if there are no loads and the batteries are full, then the Sunny Island will shut down the Sunny Boy inverter to prevent overcharging of the batteries.
It is recommended that if selecting this AC-coupled system, both inverters be from the same manufacturer, as coupling and communication between different inverters might not be simple.
Under the DC coupled scheme the hybrid inverter works together with a charge controller in order to track the maximum power point and charge the batteries.
Here the input is on DC from the charge controller that also goes to the battery bank and then to the AC output that goes to sub-panel. There is also an AC input to connect the grid to the system.
The main difference here with the AC coupled scheme is that in this case the inverter does not charge the batteries. Instead, a charge controller is needed to perform this task.
Remember that the inverter/charger scheme used in AC coupled uses as input an AC source and converts it into a DC source to charge the batteries, however, in a DC coupled scheme the charging is done in DC directly by the charge controller (inverters cannot charge from DC to DC).
As an example of this configuration let’s take a look at the Schneider topology for DC coupled systems using an XW+ battery based inverter and the MPPT-80 charge controller from Schneider:
Advantages and disadvantages of grid-tied and battery-based solar inverters
Let’s compare grid-tied and battery based inverters and take a look at their advantages and disadvantages.
|Simplicity||Disconnection of the system when the grid goes off|
|Can be used for big system sizes||Does not allow backup of critical loads|
Battery based solar inverters
|Multiple system configurations||Higher cost|
|Includes battery backup||Complex system designs|
|Active with or without the grid||Are not usually applied for commercial/utility size systems|
|Charges the batteries (AC coupled case)||Lower availability of brands and models|
As you can see there is a wide range of solar inverters, the possibilities allow you to evaluate several scenarios and analyse the best performance between power outputs, efficiencies, simplicity and wiring.
There is some technical criteria that must be evaluated to select the appropriate inverter for your case:
- Type of PV system to be implemented: for off-grid or grid-tied you must select a battery-based inverter (and a grid-tied inverter if the system is AC coupled).
- Available power outputs of each inverter that match your demand. That will establish if it’s better to use several small inverters or a bigger central inverter.
- Efficiencies of each inverter.
- MPPT range of the inverters must be wider than the MPPT range of the PV array (under high and low temperatures).
For more about solar inverters, visit our product page.
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