Dec. 09, 2024
There are three main types of inverter technologies available for your solar installation: string inverters, power optimizers, and microinverters. Both string inverters and power optimizer systems use a central inverter to convert the electricity from all of your panels from DC to AC, while microinverters convert DC electricity to AC electricity at each individual solar panel.
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String inverters aggregate the output of groups of solar panels in a system into &#;strings&#;, which are then connected to a single, central inverter where electricity is converted from DC to AC electricity. With a string inverter, you can connect multiple &#;strings&#; of panels to the same central inverter, allowing some flexibility with your solar panel system design. However, string inverters do not provide panel-level optimization, which makes them less suitable for installs where you expect shading throughout the day or with complex roofs. String inverters are typically the least expensive inverter option, and are a great option for homes that receive consistent sunshine and with straightforward rooftops.
Power optimizers are located at each individual panel and are paired with a central inverter to complete the conversion from DC to AC electricity. Power optimizers don&#;t convert electricity to alternating current, rather, they &#;condition&#; the DC electricity to optimize its voltage before sending it to a string inverter to be converted. This results in a higher overall system efficiency compared to using string inverters alone in shading scenarios. In situations where you anticipate individual panels to experience a drop in production throughout the day due to shading or other factors, power optimizers can help reduce overall system inefficiencies.
Microinverters are also located at each panel site, but are not paired with a string inverter. Unlike power optimizer systems, microinverters convert electricity from DC to AC right at the panel. This approach also leads to optimized electricity production when compared to using a string inverter alone in shading scenarios or on complex roofs. By converting DC electricity to AC electricity right at each panel site, microinverters can also minimize the system-wide effects of a single panel under-performing.
Microinverters and power optimizers are often collectively referred to as Module-Level Power Electronics, or MLPEs for short. As a group, MLPEs offer several specific benefits when compared to traditional string inverters, including shade tolerance and individual panel monitoring options. However, MLPEs typically come with a higher price tag than string inverter options, so it&#;s worth comparing all of your options to see what is best for your specific solar installations.
An inverter converts DC (direct current) into AC (alternating current). Most home loads and appliances use AC power. However, solar panels output DC power. The inverter takes in DC power that is produced by solar panels or stored in a battery and converts it into AC power in order to run your loads.
A straight grid-tied system is usually one of the simplest and most cost-effective ways to incorporate solar into a home that is already connected to utility electric power. A basic grid-tied system really only requires two main components: the solar panels and the inverter. A grid-tied inverter will use available solar energy to power loads in the home and then export excess solar production to the grid/utility whenever it is available. Grid-tied inverters are not allowed to back-feed the grid during a power outage, so most are required to shut down during a grid outage to satisfy anti-islanding requirements. Because of that, a straight grid-tied system as described above will not be usable during a grid outage, even if it&#;s sunny.
An Off-Grid inverter would be used in an application where there is no, and may never be, grid power. Most Off-Grid inverters require a connection to a battery bank to operate. These do not accept solar power directly like a grid-tied inverter but instead rely on an external charge controller (separate component) to regulate energy flow from the solar panels to the battery bank. The Off-Grid inverter then discharges the battery bank to power your loads. An Off-Grid inverter provides maximum independence from the grid or utility company and allows a solar system to be active regardless of the presence or status of the grid.
A hybrid inverter, also referred to as grid-tied with battery backup or ESS (energy storage system), incorporates the best of both grid-tied and o- grid type inverters. A hybrid inverter is capable of selling excess solar production back to the grid like a grid-tied inverter but can also easily be connected to a backup battery bank like an o-grid inverter. Some hybrid inverter systems are all-in-one, while others are configured with individual components (i.e., separate inverter, charge controller, battery monitor, etc.). There are some hybrid inverter options which do not require batteries to operate, meaning one can often start with a grid-tied system and then add batteries later on. This is usually the preferred inverter type for anyone that wants battery backup during a grid outrage or to do things like peak load shaving, self-consumption and time of use.
At their core, inverters used in the RV/marine industry on mobile applications perform the same function as the inverters described previously, just on a smaller scale. Most of the time a mobile inverter will have a lower output power rating than a grid-tied or o-grid inverter. This is because the power and energy needs for most mobile systems are usually much lower than a typical residential home. Mobile inverters are not designed to sell excess solar production to the grid, but some are able to pull power from the grid (shore power) to run loads and charge batteries in the vehicle. Mobile-specific inverters which are designed to accept shore or generator power also have a switching feature between AC neutral and ground which is not found in the other inverter types.
String inverters are typically the easiest and most cost-effective option for grid-tied solar systems. Solar panels are wired together in groups to create high voltage DC, and these feed into the string inverter&#;which converts the DC to usable AC. There is often not much additional equipment required for a system like this other than the solar panels, mounting and the inverter. Most modern string inverters have a built-in DC disconnect along with multiple PV inputs to accommodate solar panel strings of different lengths or in different physical locations.
Examples: SMA Sunny Boy, Fronius Primo
Some grid-tied string inverters require the use of DC optimizers. These are module-level electronic devices which are installed on every solar panel in the system. From there the rest of the installation and connections would follow that of a standard grid-tied string inverter, but the optimizers do oer some added benefits. Generally speaking, the addition of third-party optimizers with a string inverter system better mitigates shading losses from the solar panels and allows for module-level monitoring. Most optimizers inherently satisfy module-level rapid shutdown requirements. In some cases, the optimizers are required for a particular inverter (SolarEdge) and are used to maintain the voltage of each string of solar panels at a &#;sweet spot&#; where power conversion from the inverter is most ecient.
Examples: SolarEdge, SMA + Tigo TS4-A-O
Microinverters, as the name suggests, are small inverters designed to work with a single solar panel at a time. These are similar to DC optimizers in that you have a microinverter installed on every solar panel in the array, but there is no central string inverter downstream which they feed into. Instead, the microinverters produce AC power right at the solar array, so there is no high voltage DC present. This can potentially allow for more freedom and less strict electrical code when it comes to wiring microinverters through the roof and into the home. Microinverters also provide integrated rapid shutdown, shade mitigation and module-level monitoring. Because microinverters can produce usable AC power, these are a good option for future expansion as there are no minimum input voltage requirements. In theory, one could have a fully functioning grid-tied system with just a single solar panel and microinverter.
Examples: Enphase
Rapid Shutdown (RSD) can be required for some grid-tied systems as an additional safety measure for first responders. It reduces the solar array voltage to a safe level within a defined array boundary. This must be achieved within a set amount of time after rapid shutdown initiation. Ultimately the local authority having jurisdiction (AHJ) will specify the correct national electric code (NEC) and RSD requirements for the site. There is no NEC for the entire country, so RSD requirements can vary by state and even by county level. Older systems or code years can sometimes get back with string level rapid shutdown. However, most newer grid-tied systems require module-level rapid shutdown. DC optimizers and microinverters satisfy module-level RSD requirements by default, so there is no additional equipment needed for those types of systems. A basic string inverter system, however, may require module-level electronics to be added in order to comply with RSD requirements.
Examples: OBFRS, TS4, SolarEdge optimizers, Enphase micros, MNSOB
Sizing a grid-tied inverter or microinverter system ultimately depends on the mount of solar panels required for the project. The size of the solar array is determined by energy requirements at the site, percentage of energy oset desired and the available solar potential at the location. Other details such as available roof space and local code requirements may also determine PV array size.
Once the solar array size is determined, inverter input specifications must be considered. With string inverters, there are minimum and maximum input requirements for both solar voltage and current. DC optimizers and microinverters also have minimum and maximum input specs, but these are on a module level for an individual solar panel, rather than string level. Most DC optimizers and microinverters also have limitations regarding how many can be connected together in a string, as well as how many can be used in a single system without requiring additional equipment.
Inverters used for o-grid or standalone applications are designed to work independently of the electric grid. Since these types of inverters do not require a connection to the grid, most require a connection to a battery bank in order to operate. For o-grid type applications, things like output wattage, surge capacity, and sine wave type are factored in when choosing an inverter, as this is often the primary source of AC power for that system.
O-grid inverters can be found with two types of output&#;pure sine wave and modified sine wave. Without getting too complicated, the AC output waveform of an inverter resembles a repeating pattern due to the alternating nature of the current. Modified, or square, sine wave inverters produce an output waveform with chunky steps which reduces overall efficiency and quality of output power. Modified sine wave inverters are capable of powering a wide range of devices but are not the best choice for appliances with motors or high-end electronics. Sensitive equipment or newer electronics like computers, TVs and even some newer refrigerators and dishwashers are not compatible with modified sine wave inverters and may not function properly. Pure sine wave inverters, on the other hand, have a smoother and more efficient output waveform. The electricity found in your home from the grid is pure sine wave. Inverters with this type of output will not have any issues running specific loads and are compatible with everything. All things being equal, pure sine wave inverters are usually more expensive than modified sine due to their increased efficiency.
O-grid inverters usually have fixed input and output voltages. Since nearly all o-grid inverters are battery based, they have a specific battery input voltage they are compatible with (most commonly 12V, 24V and 48V). A 12V inverter, for example, would not have a DC input voltage range wide enough to accommodate a 24V or 48V battery and could potentially be damaged by a higher voltage battery. A 48V inverter, on the other hand, might not even power up if connected to a 12V battery. Input voltage should be considered during the initial design, as most o-grid inverters cannot change or adjust their input voltage.
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The output voltage of the inverter is just as important. In North America most household appliances use 120V AC. Some larger appliances like dryers, HVAC, water heaters, etc. require 240V AC. All o-grid inverters output 120V, but some can output 240V to power heavier loads. Also known as a split phase, there are o-grid inverter options capable of outputting 120/240V AC so that loads requiring either voltage can be operated, just like in most homes. Some inverters which are only capable of 120V output can be stacked with another unit to produce 120/240V. Inverter output voltage is also usually fixed, so if heavier loads are added later, a split phase inverter can make things easier.
Some o-grid inverters are actually inverter/chargers, meaning they can utilize an external AC power source for pass through to loads and/or battery charging during times of high consumption or insucient solar potential. In order words, an inverter/charger can pull power from the electric grid or a generator and use that to run the loads plus charge the batteries if they are low. That being said, inverter/chargers have both an AC input and AC output. As soon as an inverter/charger qualifies incoming AC power from the grid or generator, an internal transfer switch within the unit engages. The inverter/charger switches to pass-through mode and lets power flow directly from its AC input to the loads on its AC output. In this state, the unit does not invert or discharge the batteries. Rather, the batteries are charged using the leftover incoming AC power which is not being occupied by the loads. Most inverter/chargers allow you to adjust how much AC power they can pull from grid or generator, along with the charge settings for the batteries (charge voltages, timers, current, etc.). Some inverter/chargers even have two separate AC inputs (one for grid, the other for generator). An external transfer switch may be needed if multiple AC sources are to be used with an inverter/charger that has a single AC input. There are some inverter/charger options that will actually supplement incoming AC power with inverted power from the batteries to deliver more overall power to the load&#;such as in the case of an undersized generator.
Within the world of battery-based inverters, there are two unique types based on the size and capabilities of their internal components. This, combined with the speed at which the internal components operate during the inversion process, are used to classify an inverter as high frequency or low frequency. High-frequency inverters make up a large portion of the retail market. These are usually smaller and less expensive than low-frequency inverters, however they can struggle with heavier loads like pumps, motors or other appliances with high startup surges. These can be a cost effective and simple solution for many different types of applications, including light duty electronics and smaller appliances. Low frequency-inverters, on the other hand, are more robust and heavy duty than high frequency types. These are better suited for industrial loads and start up surges. They are often more feature rich, including options for custom programming. In addition, higher quality components and longer warranties can also be found in low-frequency inverters.
Rapid shutdown (RSD) is not as common in o-grid systems as it is with grid-tie, but it can still be required for some applications. RSD ensures that the solar array voltage reduces to a safe level within a defined PV array boundary in a set amount of time. Ultimately, the local authority having jurisdiction (AHJ) will specify the correct National Electric Code (NEC) and RSD requirements for the site. There is not yet a uniform NEC for the entire country, so RSD requirements can vary by state and even by county. Inverters with integrated auxiliary outputs or relays can sometimes be used for power and/or control to the RSD components in an o-grid system.
Examples: OBFRS, MNLSOB, MNSOB
The most important metric in sizing an o-grid inverter is the unit&#;s output power rating. Calculating the maximum continuous AC load in the home or system is a good starting point in choosing the right size inverter. The continuous running wattage of all AC loads in the system which could be in use simultaneously must be calculated in order to determine the size of the inverter. In other words, think of any appliances, lights, devices, etc. which may be turned on at the same time and make sure the inverter can supply that amount of AC power continuously. Any potential startup surges from heavy loads like motors or pumps must also be considered. An unspoken rule in all o-grid systems is to manage your usage so all your appliances aren&#;t running at the same time. This can allow for a smaller and more cost effective inverter to be used as the instantaneous power demand is lower.
Since almost all o-grid inverters are battery based, the voltage of the battery bank does determine the input voltage of the inverter, most commonly 12, 24, or 48 volts DC. Most inverters have a fixed battery input voltage, so a 12V inverter is and forever will be a 12V inverter. If a small 12V starter system is eventually upgraded to a 24V or 48V system, the inverter would also need to be changed as well to match the new system voltage. One very important note regarding o-grid inverters is that their power rating has nothing to do with the size of the solar array or battery bank. Most o-grid solar systems utilize charge controllers to manage energy flow from their solar panels to batteries, so the number of solar panels one can have in an o-grid system is not related to the size of the inverter, or vice versa. It is common to oversize an o-grid inverter past the initial demand to account for a future increase in power needs.
If you go the inverter/charger route, then it&#;s especially important that the unit be compatible with whatever type of battery is used in the system. Most battery manufactures have recommended charge settings, timers, low-voltage disconnect points and more for their batteries. Ideally, the inverter/charger needs to be adjusted to match these parameters. This is critical with lithium batteries which rely on a battery management system for safe operation. Although most modern inverter/chargers will allow for adjustment of their settings, older style inverter/chargers or ones which cannot be adjusted are often.
A hybrid inverter system utilizes the best of both grid-tie and o-grid worlds for a very custom and flexible solution. These types of systems are commonly referred to as grid-tied with battery backup or ESS (energy storage system). These inverters are used in applications where there is existing grid power. Hybrid inverters can sell excess PV production back to the utility company, like a grid-tied inverter, but will also provide backup power from batteries during a grid outage, like an o-grid inverter. Most will allow for the solar array to stay active during a grid outage as well. The majority of applications using hybrid inverters will require installation of a separate subpanel to contain the critical loads which must have power during a grid outage. Given the flexible nature of hybrid systems, this is often the best choice to maximize self-consumption or do things like peak lead shaving/time of use. Some of the newer hybrid inverters are all-in-one units, meaning there are inputs for solar, grid, loads, generator and battery all built into one inverter&#;there are no separate charge controllers or other components. Some hybrid inverters do not require batteries, making this a great option for anyone who wants to get started with grid-tied solar but have the capability to easily add batteries in the future. Most will also allow for charging batteries from the grid in certain situations. A big factor in selecting the right hybrid inverter is how you want to use (or not use) power from the grid.
A stand-out feature for most hybrid inverters is their ability to sell excess PV production back to the grid while still being able to maintain critical loads during a power outage. That being said, their mode of operation changes depending on the condition of the grid. In systems where the electric grid is stable and outages are infrequent or short lasting, a hybrid inverter will really shine day-to-day in sell back mode as a grid-tie inverter. On the other hand, if power outages happen often and can last a long time, then the battery backup and standalone elements of a hybrid inverter could be most valuable. When the grid is up, the hybrid works most similar to a grid-tied inverter, in that it feeds loads (and potential battery bank) with available solar energy, and any excess produced is sold to the grid. Upon grid loss, the inverter instantly detects the outage and ceases to export power but keeps the critical loads running o batteries and/or available solar.
The inverter&#;s maximum sell back rating and/or AC breaker size will ultimately determine how much power can be sold back to the grid. This also determines how much power the inverter can pass through from the grid to the loads or pull from the grid for battery charging. Even with stable grid, it may not be feasible to run some of the heaviest loads in the home o the load side of the inverter. Exceeding the inverter&#;s AC pass-through limitations would trip breakers or even potentially damage the unit. Best practice would be to keep heavy loads&#;like HVAC or hot tub&#;on the grid side of the system and set with solar production whenever available.
The advanced capabilities and flexibility of hybrid inverter systems can be optimized by allowing direct communication between the equipment and the batteries. Some hybrid inverter systems have a closed-loop communication profile with certain lithium battery manufacturers. Closed loop allows the batteries in the system to provide real-time status to the equipment and dynamically control its operation. In other words, the batteries tell the inverter when and how hard to charge or discharge, depending on their status. This helps to get the most out of a lithium battery bank, and eliminates the need for in-depth custom programming of the inverter. Closed loop is not a requirement for most hybrid inverters and lithium batteries, but it does take an already sophisticated, advanced solution to an even higher level.
Rapid shutdown (RSD) would be required for most hybrid inverter systems, especially if configured to sell power back to the grid. RSD ensures that the solar array voltage reduces to a safe level within a defined PV array boundary in a set amount of time. Ultimately the local authority having jurisdiction (AHJ) will specific the correct National Electric Code (NEC) and RSD requirements for the site. There is not yet a uniform NEC for the entire country, so RSD requirements can vary by state and even by county. Inverters with integrated auxiliary outputs or relays can sometimes be used to power and/or control the RSD components in an hybrid system. Often RSD transmitters or accessories can be used with a hybrid inverter to control the array located components.
Examples:OBFRS,TS4, MNSOB
Finding the right size hybrid inverter employs tactics used in both grid-tie and o-grid inverter sizing. Once energy needs and potential system configuration have been determined and solar array size calculated, then the PV input of the hybrid inverter must be rated high enough to accept that number of solar panels. The voltage and amperage input limitations of the hybrid inverter must also be considered. Maximum sell-back capability to the utility while the grid is up should be factored in as well, especially if that will be the primary operating mode. For instances of grid down, then sizing becomes most similar to that of an o-grid inverter. The maximum continuous running wattage of all critical loads which must operate during an outage is used to determine the output power of the inverter.
The inverter must also be able to handle any potential start up surges from heavy loads during grid down events as well. Some hybrid inverters have different wattage ratings for PV input and AC output power.
Inverters used in the mobile industry (RV, marine, vans) are extremely similar to o-grid inverters, just usually on a smaller scale and with some other specific requirements. Mobile inverters come in all different shapes and sizes, but their availability in pure sine vs. modified, high frequency vs. low frequency, and inverter-only vs. inverter/charger are just a few of the ways their features are similar to an o-grid inverter. Inverter/chargers are extremely common for these types of applications, as well as inverters with AC pass through. A mobile inverter with AC pass through does not charge the batteries like an inverter/charger but still lets external AC power go straight to the loads without discharging the batteries.
One of the standout features for most mobile inverters is that they automatically connect or disconnect their AC output neutral to ground. AC neutral to ground switching is a requirement for most mobile systems and prevents having multiple neutral-to-ground bonds within the AC side of a mobile system. Generally, an entire electric system can only have a single instance where the AC neutral is bonded to ground. This is done for safety purposes, and in a normal home this bond is typically found in the main breaker panel. In mobile systems, the inverter creates this bond internally while it is inverting. However, when a marine/RV system is connected to shore power or a gas generator is fired up, the mobile inverters break their internal neutral-to-ground bonds and rely on that same bond being made in the external AC source (shore or gen). This is done so that only one single instance of neutral-to-ground bonding is present in the system at all times. When the external AC source is disconnected, the inverter then reestablishes its own internal bond before it starts inverting again.
Many mobile applications rely on occasional hookups to shore power or the use of a small generator for their power needs. If an inverter/charger or pass through inverter is used, it&#;s important that the unit can properly utilize the external AC power from shore or generator. Common campsites and boat docks provide 30A or 50A shore power, but &#;moochdocking&#; at friends of family with just a 15A or 20A plug available can be a reality as well. Most inverter/chargers and pass through units have an adjustable AC-input setting, so as not to overload the external source. So the inverter would be adjusted to connect to a 30A service, and then adjusted again if the next campsite is 50A. The inverter&#;s transfer relay does need to be rated to match the size of any potential shore power or generator connection. Most inverter/chargers have just a single AC input, so an external transfer switch can be used to automatically select between available shore power or generator before connecting to the inverter.
Distributing a mobile inverter&#;s AC power within a vehicle or boat is also part of the design process. There aren&#;t many cases where split phase 120/240VAC power is required for mobile systems, so often the AC output is 120VAC single phase for most of North America. Systems that plan to travel internationally may utilize a 230V 50Hz single phase rather than the 120V power we use in the States. For most mobile 120V applications, a single inverter can be used to power a 30A breaker panel. In systems with a 50A breaker panel, that run o two separate 120VAC legs, a single inverter typically only provides power to one side of the 50A panel. In that case, day-to-day or boon-docking loads would be moved to the inverter side of the breaker panel, while heavier loads reserved for shore or generator power remain on the other side. Multiple inverters can be connected in parallel, or a transformer can be used with a single inverter to potentially power up both sides of a 50A RV breaker panel for instance.
Perhaps one of the most unique constraints related to mobile inverters is their physical size and weight. Often there are times with mobile systems where equipment needs to be mounted in confined or difficult-to-access areas. Places like cabinets and storage compartments, under beds, in closets, etc. can all be called upon to house electrical equipment when free space is already a premium. That being said, the weight and dimensions of the inverter should be verified ahead of time to make sure it will fit in the proposed space. The orientation of the inverter is also important, as some are designed to mount only one way, vertical on a wall for example, as opposed to sideways, horizontally or upside down. The inverter must have sufficient clearance when mounting and also some amount of airflow so that it does not overheat.
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