Inverters are expensive and often bulky devices, especially the ones for high-power solar panel systems.
Nevertheless, inverters are neither mysterious nor that difficult to understand.
To be in clear what solar inverter to buy for your solar system, you have to know how solar inverters work.
Furthermore, to know how to compare solar inverters, you have to know their essential specifications and features.
This would help you not only in selecting the right kind of inverter but will also make you confident upon receiving an offer from a solar vendor or installer.
Apart from the well-known solutions for grid-tied and off-grid solar panels systems, this article also presents some modern trends in solar inverters, such as micro parallel inverters and inverter/chargers which are gaining popularity in the recent years.
Read on to find out how to select the right inverter for your solar panel system, matching best your energy needs and available budget!
What are solar power inverters?
The solar inverter is a device capable of converting DC into AC electricity.
Inverters are typical components of solar electric systems since solar panels generate DC electricity and most devices used in homes or offices operate on AC voltage.
Depending on its size a photovoltaic system could comprise either a single inverter or multiple ones.
There are two main types of solar systems – connected to the grid (grid-tied) and disconnected from the grid (off-grid).
Although the inverter’s main function is always the same – converting DC into AC electricity – these two kinds of solar power systems use different kinds of inverters.
Inverters for grid-tied solar panel systems without battery backup
The inverter is the heart of any grid-tied solar system since any grid-tied system must have an inverter.
A grid-tied inverter converts the DC voltage from the solar array into AC voltage that can be either used right away or exported to the utility grid.
As a rule, grid-tied inverters without battery backup are highly efficient and straightforward to install.
A grid-tied inverter only operates when the utility is on.
When the utility goes down, the grid-tied inverter turns off immediately.
Most grid-tied inverters are based on Maximum Power Point Tracking (MPPT) – a feature ‘squeezing’ maximum possible amount of power from the PV array.
The inverter is connected to the utility grid either directly or via the building’s electrical system.
In case of a direct connection, the generated AC electricity is sent towards the utility grid.
In case of a connection via the building’s grid, the AC power generated by the PV system is first consumed by your appliances, and what remains unused, is directed to the utility grid.
A grid-tied inverter must strictly comply with the utility grid’s requirements and regulations.
For example, grid-tied inverters must generate AC voltage of a strictly sinusoidal form.
One of the main features of a grid-tied inverter is that it stops operating in case of a grid failure.
Thus technicians doing any repair works on utility network are prevented from getting an electric shock.
The solar inverter can be either an individual block located outside the solar array or physically integrated into the solar panels.
Since every grid-tied inverter stops working during the grid outage, you do not have any electricity during this outage as well.
This is the so-called ‘anti-islanding protection’.
There are generally two types of inverters for grid-tied systems without battery backup – string inverters and micro inverters.
What are string inverters?
In small home PV systems the solar array is usually connected to a single (central) inverter:
String inverters are also known as ‘central inverters’.
They treat the solar array as one single solar panel.
Their main advantage is cost-effectiveness and simplicity to install.
String inverters, however, suffer from a notable drawback – if one solar panel of the array gets shaded or otherwise degrades in performance, the performance of the whole solar array deteriorates, which in turn results in a reduced inverter’s DC input power and hence – in a reduced inverter’s AC output power.
Using multiple inverters, also known as ‘inverter stacking’ is an approach to get a higher voltage or power.
This is done as a result of either adding more solar panels or increasing the total AC loads wattage to be handled by the inverter.
Provided that you have two compatible inverters, you have the option either to wire them in series and get a higher output voltage, or wire them in parallel thus increasing the output power.
For example, connecting two 120 VAC inverters in series doubles the output voltage to 240 VAC, while the total output power remains the same.
Alternatively, if you wire two 2,000 W/120 VAC inverters in parallel, the total power you get is 4,000 W, while the output voltage is still 120 VAC. A couple of stacked inverters, however, suffer from the drawback of reducing the inverter’s efficiency.
What are solar micro inverters?
Micro inverters (also known as ‘module inverters’) have a relatively low output power of less than 250 W.
A micro inverter is a part of a PV module and operates as a central inverter, but only for the solar panel it is connected to.
Micro inverters are installed on each panel of the solar array.
Thus both the installation cost and the overall system cost tends to increase.
Micro inverters, however, overcome the drawbacks of string (central) inverters.
If one solar panel gets shaded or otherwise degrades by performance or even fails, this in no way can affect the performance of the other panels of the solar array.
Moreover, micro inverters use Maximum Power Point Tracking (MPPT) to constantly monitor the performance of a single solar panel and ensure that the maximum power is obtained from that panel.
Also, if a solar panel fails, the micro inverter’s structure makes much easier to localize the point of failure.
Typically, a solar system based on micro inverters is reported to generate 16% more solar power compared to string(central) inverters.
Furthermore, the inverter efficiency of a system with micro inverters can reach 96.5% compared to the maximum 92% efficiency of a string (central) inverter.
It has also been proved that, when connected to a central (string) inverter, shading of 9% of the solar array results in about 54% decrease in its power output!
Here are some more advantages of micro inverters as opposed to central inverters:
- Safer, both to install and maintain, since avoiding wiring lots of panels in series (when DC voltage can increase up to hundreds of volts) eliminates the need of high voltage DC wiring.
- Allow mounting PV modules on different surfaces and facing different directions.
- Much lower heat dissipation and hence, no need for active cooling which, in turn, means that each inverter operates silently.
- Expanding your solar panel system is easier – you don’t have to worry about buying and installing a second central inverter and redesigning the whole system.
Micro inverters are noted for their much longer durability than central inverters due to the fact that they are not exposed to such a high power and heat like central inverters.
For this reason, micro inverters come with a more extended warranty – 20-25 years – compared to the typical 10 years guarantee of their string counterparts.
The drawbacks of micro inverters are that their higher cost and lower resistance to heat.
Micro inverters are a good option also for larger solar power systems, as:
- There is no need for DC cabling.
- Shading of a module and/or inverter failure cannot affect the rest part of the PV array.
- The voltages used are lower than 120 VAC and thus are less dangerous.
What are micro parallel inverters?
Micro parallel inverters are a relatively recent achievement that combines the benefits of string inverters and micro inverters.
A micro parallel inverter is a smart device containing four individual channels that can be connected to four separate solar panels. Each channel acts as a single micro inverter and can track the performance of its solar panel by using MPPT.
Also, a micro parallel inverter acts as a string inverter for all the four solar panels, with the exception that if one panel fails, the performance of the other channel panels remains unaffected.
Indeed, this is also valid if three of the four solar panels fail.
Furthermore, one micro parallel inverter is easier to install than four micro inverters, concerning both labor cost and wiring cost.
Maximum Power Point Tracking in Solar Inverters
Most grid-tied inverters are capable of providing up to 20-30% more solar generated power due to MPPT, delivering the optimal load to the solar array.
Constantly changing solar irradiation and ambient temperature causes constant changes in electrical characteristics of the solar panels.
Therefore, the solar array needs different optimal loads to deliver optimal power to the other solar system components.
MPPT inverters are more expensive than non-MPPT ones.
The extra 20-30% of the solar power provided by the array however not only compensates the higher price in the long run, but can bring you more savings as well.
If you use an MPPT inverter in your grid-tied system, however, you must be sure that the maximum and minimum output voltages of the solar array fall within the inverter’s maximum power point tracking window:
Such a tracking window is defined by the inverter’s maximum and minimum input tracking voltage.
A solar array reaches its maximum output voltage at the lowest ambient temperature and its minimum output voltage at the highest ambient temperature.
By not matching the inverter’s tracking window, at extreme ambient temperatures, an MPPT inverter will stop tracking and the efficiency of your solar system is going to degrade.
It is important to ensure that:
- The minimum output voltage of the solar array does not fall below the inverter’s minimum input voltage. Otherwise, the inverter will not be able to operate properly.
- The maximum output voltage of the solar array is always below the inverter’s maximum input voltage. If the maximum input voltage is exceeded, the inverter can get damaged.
If the inverter is either overloaded or underloaded, its efficiency decreases.
Therefore, in the long run you will lose both solar power and money saved on electricity bills.
If the inverter’s power rating is significantly lower than the power required by your loads, the inverter will shut down as a result of the excessive power requests from these loads, even though your solar panel might be capable of providing such power at that moment.
The inverter tries to handle the excessive power by dissipating it in the form of heat, which leads to overheating.
If such dissipation is not enough, the inverter will shut down.
It should be noted that, as a result of such frequent shutting down and overheating, the inverter’s lifecycle will decrease.
Therefore, it is vital to mount the inverter in a well-ventilated place to ensure good cooling.
Otherwise, the inverter will try to deal with self-heating by reducing the generated AC output power until overheating reaches the shutting-down point.
Solar inverters types based on the produced type of output wave
Three different types of inverters are currently available on the market – sine-wave, quasi (modified) sine-wave and square-wave ones.
Certain electronic devices, such as mobile phones, microwave ovens, computers, vacuum cleaners, etc., might have problems while operating with a quasi sine-wave inverter.
Furthermore, quasi sine-wave inverters may create additional noise to audio and TV equipment.
Inductive loads, such as fridges, pumps, drills, etc., must be powered by pure sine-wave inverters.
A square-wave inverter is of less quality than a modified sine-wave inverter.
Although the most expensive ones, sine-wave inverters are the only possible choice for any grid-tied system – not only because they are suitable for any applications, but also because they comply best with the applicable regulatory requirements.
Grid-tied inverter specifications
- Rated input DC power – usually selected 20% lower than PV array peak power, due to solar array losses.
- Rated input DC voltage – typically between 75 V (minimum value) and 750 V (maximum value) for most inverters used in residential grid-tied systems. The PV array’s output voltage should fall within this voltage window.
- Maximum input DC current – should always be higher than the short-circuit current of the solar array.
- Output voltage – 120 VAC or 240 VAC.
- Output frequency – 50Hz in Europe, 60 Hz in the USA.
- Efficiency – the percentage of losses resulting from the DC to AC conversion.
Selecting the inverter for your grid-tied system
1) The output power of the inverter should be (0.9?0.95) of the solar array peak power.
2) The solar array’s maximum voltage should be lower than the inverter’s maximum input DC voltage.
3) The minimum allowable voltage of the inverter should be less than the minimum DC voltage of the PV array.
4) The working voltage range of the PV array should be within the inverter’s MPPT voltage range.
5) The maximum current of the PV array should be below the inverter’s maximum input DC current.
Inverters for grid-tied systems with battery backup
Battery-based inverters can be used in systems with energy storage – either grid-tied battery-based systems or stand-alone (off-grid) systems.
A battery-based inverter for a grid-tied battery-based system:
- Converts the DC power into AC power for meeting the energy needs of the household devices, and
- Converts the AC electricity from the grid into DC electricity to charge the battery.
If the battery is fully charged and no loads are plugged to the system, the whole power generated by the PV array is sent to the grid. If the PV array produces more power than the inverter can handle, the remaining power is not going to be used and the inverter is going to operate with less efficiency if this additionally provided DC power is not used.
Grid-tied battery-backup inverters are more complex and more expensive than grid-tied battery-less inverters since apart from sending power to the grid, they are also expected to:
- Charge the battery bank after outage.
- Provide power to all the backed-up loads during outage.
Indeed, such inverters have the features typical for grid-tied battery-less inverters and stand-alone inverters.
Specifications of grid-tied battery-backup inverters
- Rated input power – compared to grid-tied battery-less inverters, here the inverter should be able to handle not only the DC power delivered by the PV array but all the backed-up loads operating simultaneously.
- DC input voltage accepted from the battery bank the most typical voltages are 12V, 24V, and 48V.
- Output voltage – 120 VAC or 240 VAC.
- Output frequency – 50Hz in Europe, 60 Hz in the USA.
- Surge capacity – allows an inverter to supply much more output power than its rated value within a short period of time, in order to provide high starting current to motors (in refrigerators, water pumps, etc.)
When selecting the inverter for an off-grid system, the power output of the solar array doesn’t need to be considered, since the battery bank is placed between the inverter and the solar array.
In a grid-tied system with battery backup, however, both the solar array power output and the battery power output should be considered.
If the batteries are fully charged and there are no loads plugged, the inverter must be able to send all the solar-generated power to the utility grid.
In this situation, if the PV array provides more power than the inverter can process into AC power and send into the grid, the additional power will not be used, and the PV array will not be able to operate at its maximum efficiency. As a result, your system is going to send less energy to the grid.
You can enjoy the comprehensive guide on inverter selection and sizing both for grid-tied and off-grid solar power systems in our book ‘The Ultimate Solar Power Design Guide: Less Theory More Practice’.
Inverter/chargers are used in battery-based grid-tied systems:
In an off-grid solar power system, solar panels are typically used to charge the battery bank.
In winter, however, when days are shorter and there is less solar irradiation, solar panels alone might not be capable of charging the batteries.
In such a case, an additional power source is needed to keep the battery bank fully charged.
Such a power source could be a diesel generator that generates AC power rather than DC power.
Therefore, you need a device to convert the AC generated power into DC power to charge the batteries.
Also, if you have AC appliances in your household, you need an inverter to convert the DC power from the battery bank into AC power.
Often a single device can unite these two reverse functionalities and such a device is called ‘inverter/charger’.
It acts as a bidirectional power converter – both AC to DC and DC to AC – and can also offer some additional features, such as staring the diesel generator in case of insufficient sunlight and low battery state of charge.
Apart from residential off-grid, inverter/chargers are also used in marine solar panel systems, where shore power is the ‘additional power source’ intended to charge the batteries in case of poor sunlight.
Another use of inverter/chargers is in battery-based grid-tied systems where batteries are used as a backup power source in case of a grid failure.
So, in case of insufficient sunlight, not capable of keeping the batteries fully charged, the inverter charger can charge the batteries by using the AC power from the grid.
Some inverter/chargers, also known as ‘solar hybrid inverters’, have two AC inputs.
The first one is connected to the utility grid, while the other can be connected to an additional AC generator (e.g. a diesel one).
When the sun is shining, the solar generated power is charging the batteries.
In case of poor sunlight, and if the grid is up, the hybrid inverter charges the battery from the grid.
If the grid fails, the hybrid inverter switches on the AC generator and starts charging the batteries from the other AC input.
When the grid is still down but the sun starts shining, the inverter switches off the generator and the batteries start getting charged by the solar array.
An inverter/charger cannot replace the solar charge controller, since inverter chargers can only manage the battery charging through an AC power source – an AC generator (e.g. a diesel one), the utility grid (for residential solar panel systems) or the shore power (in case of mobile/marine off-grid solar panels systems).
The solar charge controller is a must if the batteries are getting a charge from the solar panels.
Smart grid feature
‘Smart grid’ is another feature of inverter/chargers.
This feature can program the device to connect the AC loads to the grid at a suitable moment, for example at night where the billing rates are low.
Apart from connecting to the grid, this feature might also start recharging the batteries in a grid-tied system with battery backup.
When the billing rates are high – usually in daytime hours – the loads get disconnected from the grid and operate on solar-generated electricity instead.
The main advantage of the ‘smart grid’ feature is that enables to reduce power consumption from the grid thus lowering your electricity bills.
The ‘peak shaving’ feature can act as a complementary to ‘smart grid’.
During ‘peak shaving’, the energy from the grid is initially used when starting some surging loads (among the other AC loads) and then all the loads are switched to getting power from the solar array.
Such a smart feature eliminates the need for an expensive inverter of high surge power.
Some inverters are provided with a feature of automatically reducing their AC input current which is the output current of an external generator if any.
This prevents the generator from being overloaded should the total current of AC loads connected exceed the maximum output current of the generator.
Powering some 240 VAC loads (such as water pumps) is possible if you have an inverter/charger with 120/240 VAC output but this is not the only way to do that.
While the option of stacking two single-phase 120 VAC inverter/chargers in series to obtain a total output voltage of 240 VAC was already mentioned above, another good way is to connect an autotransformer to the inverter’s output.
This is a cost-effective way to double the output voltage if you have mostly 120 VAC loads available but need occasionally to power also 240 VAC ones.
Inverter/charges can be provided with a wide range of monitoring features related to all possible devices connected – the battery (state of charge and temperature), the additional power generator (Automatic Generator Start when needed) or loads (tracking and disconnecting all of them or just non-critical ones only thus preventing the battery or the external generator from damaging).
Off-grid solar inverters
Off-grid inverters are different from grid-tied inverters.
An off-grid solar system might not contain an inverter if DC loads only are to be powered.
Since off-grid systems are disconnected from the utility grid, off-grid inverters need not match the utility grid requirements and regulations.
The main function of an off-grid inverter is converting the output voltage of either the battery bank or the solar array to AC voltage.
Not every off-grid solar system needs an inverter. An inverter is not needed, if power is to be provided to DC loads only:
1) Inverter-less off-grid photovoltaic system with a battery bank:
2) Inverter-less off-grid photovoltaic system without a battery bank:
Grid-tied and off-grid photovoltaic systems use different kinds of inverters.
Since inverters for stand-alone systems are disconnected from the grid, they do not need an anti-islanding protection.
There are two types of inverters for off-grid systems.
1) An off-grid inverter directly connected to the solar array, thus providing AC power directly to the AC loads:
2) A battery back-up inverter that is connected to a battery, either directly:
or by a DC breaker:
To effectively convert the battery DC power into AC power, the inverter’s input voltage range must match the voltage range of the battery bank.
The voltage of the battery bank reaches its lowest value when the batteries are discharged, and correspondingly its highest value when the batteries are fully charged.
Off-grid inverters are produced in various power outputs, depending on the type and size of the PV systems.
There are 100 W inverters for a small off-grid system, and there are 5 kW inverters for providing power to all the possible loads in a household.
Another essential feature of off-grid inverters is that their DC input is available just for a limited number of DC voltages (12V, 24V, and 48V), due to the reason that the inverter input is connected to the battery output that comes in these DC voltages.
With grid-tied inverters the situation is different – the inverter’s input is the solar array’s output.
The output voltage can vary greatly in voltage due to opportunity for connecting a number of solar panels in a string.
If you use both the utility grid and a generator as an AC power source, you need a grid-tied battery-based inverter supporting multiple power sources.
There are three different types of stand-alone inverters currently available on the market, with regards to the produced type of voltage wave – sine-wave, quasi (modified) sine-wave and square-wave ones.
Modified sine-wave inverters are used in simpler and less expensive solar panel systems.
They work well with lights, motors, fridges, and other non-electronic equipment.
Generally, however, they cause interference to most radio- and electronic equipment, such as TVs (especially the newest models), audio systems, computers, laptops, and digital clocks.
Therefore, such inverters are a fair choice for emergency solar kits or off-grid solar systems with quite simple electrical needs.
Such inverters are the most used ones in off-grid residential solar panel systems and are also often used in mobile solar applications.
They are more expensive than their modified sine-wave counterparts but they also of higher quality.
Pure sine-wave inverters are compatible with any electronics.
Pure sine-wave inverters are a must in larger off-grid solar panel system as well as in grid-tied systems.
The prices of pure sine-wave inverters are continuing to fall.
These inverters come in various sizes – from 100W up to 7,000W – and can easily match to your specific case of electrical loads combined.
Also, pure sine-wave inverters can be stacked to handle larger electrical loads in more complex solar panel systems.
Off-grid inverter specifications
- Rated input power – usually selected to be of 20% less than the PV array peak power, due to the various losses in the solar panels.
- Rated output power – should be enough, so that the inverter should be able to handle all the loads that are to be working simultaneously.
- DC input voltage accepted from the battery bank – the most typical voltages are 12V, 24V, and 48V.
- Output voltage – usually 120 VAC or 240 VAC.
- Output frequency – 50Hz in Europe, 60 Hz in the USA.
- Surge capacity – enables the inverter to supply much more output power than its rated value within a short period to provide high starting current to motors (in refrigerators, water pumps, etc.).
Off-grid inverter sizing
Estimating what size solar inverter you need is important since the inverter is supposed to handle the DC power provided by the battery and supply it to the AC loads.
The inverter must be able to handle all the AC devices that are to be plugged into simultaneously (the AC total watts).
The inverter must also be able to handle the expected surge (the “in-rush” of current) produced by some power-hungry loads upon startup.
The most important parameter of any inverter is its continuous rating or continuous watts.
It denotes the total amount of watts the inverter should handle or, in other words, continuous watts mean all the AC appliances that the inverter is expected to power simultaneously.
Typically, the continuous power of the inverter is chosen equal to the total installed power of the solar array (the watts-peak).
The continuous rating can also be a bit higher than the total installed solar power but not too much higher since the inverter’s efficiency will decrease.
If you want to be sure whether an inverter of continuous rating equal to the installed power will be capable of handling all the loads in your household you intend to use at the same time, you should add the ratings of these appliances together.
What you get eventually must be less than the inverter’s continuous rating.
Otherwise, you should either choose an inverter of a higher rating or think of reducing the number of the AC devices used simultaneously.
The other important parameter is the inverter’s surge rating or surge watts.
This is the power an inverter can support for a very short period. Surge ratings is much more (for example, x2) than the continuous rating and is targeted to devices that have an instantaneous peak power consumption at startup – for example, motors or other inductive loads.
Upon starting, such appliances usually require much higher power than their nominal consumption.
For an inverter to be able to handle such ‘surges’, its surge rating should be at least equal to the expected surge watts of these devices.
The surge rating of such an appliance can be found on its back label.
A common assumption for surge estimation is multiplying the total AC watts by 3.
Most of the household devices, however, do not produce surges upon startup.
Typical surging devices are refrigerators, washing machines, and pumps. When sizing the inverter, do not forget to compare the inverter’s surge rating to the expected surge requirements of the system.
What matters next is the inverter’s input voltage.
Depending on whether your system voltage is 12V, 24V or 48V, your inverter should have an input voltage of 12V, 24V or 48V.
Other essential criteria when sizing the inverter are matching the inverter’s input voltage with the nominal battery voltage and selecting the desired AC output voltage (120 or 240 VAC).
Off-grid inverter selection
In off-grid solar electric systems, an inverter can be designed to power either a single AC device or all the AC loads to be plugged into.
The inverter must be sized to handle the peak electricity demand. Also, the inverter must also match the system voltage (i.e., the voltage of the battery and the charge controller).
Inverters for 12V or 24V system voltage are the most common, while 48V inverters are used in larger solar power systems.
To select an inverter for your off-grid system, you need to perform load estimation (or load analysis).
The load estimation is related to the loads you are going to use and how long you will use them.
The load estimation will also summarize what and how many AC devices you are going to use and which of them will be operating at the same time.
The inverter must be able to handle all the AC loads that are to operate simultaneously. Furthermore, the inverter must be able to handle the surge of these loads.
When buying an inverter, it is also important to consider adding new AC loads to the system.
You should also consider whether and which of them are expected to work at the same time.
Last but not least, you should buy an inverter that can be repaired.
We also advise you to investigate the possible ease of support of the inverter you are planning to buy.
In our book ‘Off Grid and Mobile Solar Power For Everyone: Your Smart Solar Guide’, you will find a step-by-step guide on how to perform a detailed inverter sizing and how to select the inverter for your off-grid solar power system, whether residential or mobile.
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