Inverter Selection
You will need to make a choice among microinverters, string inverters, and string inverters with optimizers. Once this choice is made, decisions need to be made on the specific inverters and associated equipment.
The photovoltaic (PV) panels on your roof convert sunlight (photons) into electrical energy. However, the direct current (DC) electricity that the panels produce isn’t yet useful to you (not counting some off-grid situations). It can’t run your refrigerator or cool your home until it’s converted into alternating current (AC). This is where the solar inverter comes in. Solar inverters convert the direct current (DC) electricity produced by solar panels into the alternating current (AC) electricity used by everything powered by your home's wiring. Processing “intelligence” within the inverter also relays critical information to you about how well your solar setup is producing energy for your home. This data allows you to monitor system and/or panel efficiency. Inverter selection is driven by panel selection, batteries/no batteries and backup generators along with decisions on optimizing price/performance vs. maximum energy production.
What are the major types of inverters for home solar?
String inverters
Microinverters
String Inverters with Optimizers
A more detailed discussion of each follows. A quick summary is:
String inverters (without Optimizers) are best suited for solar arrays with no shading and that are easily accessible for diagnostic. String inverters are central inverters connected to large groups of panels by high voltage DC wiring. They are typically used in large scale ground mount installations. This is the lowest cost option, but, if there is any shading or degradation of a single panel, the performance of all panels on the string (group of 10-20 panels) is reduced to that of the poorest performing panel. The inverter is still a single point of failure that can bring down all optimizers/panels connected to it.
Microinverters have an inverter for every panel. This optimizes the performance of each panel. If a single inverter fails, only one panel is offline/degraded. Microinverters historically have been more reliable and offer a longer warranty. Common AC wiring is used. Microinverters are not a good fit for larger power panels typically found in ground mount arrays or when there is a long distance between the panels and the connection to the grid. Microinverter-based systems are more expensive than string inverters. Most rooftop installations use microinverters. Microinverters generally do not support higher power panels (e.g. 450w and above).
String Inverters with Optimizers offer many of the advantages of Microinverters and string inverters. They add an Optimizer per panel allowing power optimization and monitoring at the panel level. They also better support higher powered panels and long wire runs from the panels to the grid connection. The use high voltage DC wiring, which is required to be in metal conduit, when in the house or outside (but not on the roof when specialized wire is used or underground where PVC conduit is adequate). The inverter is still a single point of failure that can bring down all optimizers/panels connected to it. String Inverters with optimizers are most commonly used on ground mount systems and some larger rooftop installations. The cost of string inverter with optimizer systems is close to that of microinverter based systems.
Most inverter product lines integrated or add-on hardware to support battery charging and offline (when grid is down) energy use from solar, and generators.
Specific inverter selection will be done in the detail design of your system.
Why is the selection of a solar inverter so important?
Since 85% of all solar system failures in the first 10 years of operation are caused by the inverter, it’s essential that you choose a reliable one that best matches your needs, make sure it’s installed correctly, and monitor its functioning regularly. Section 5 below provides a summary of 15 characteristics that would factor into a decision and concludes with a table showing the strengths and weaknesses of each inverter type.
String Inverters
A string inverter is a standalone box that is typically installed close to your main service panel and electricity meter. We’ll initially consider the case of a string inverter without “optimizers,” and will subsequently look at how optimizers can address some of the shortfalls of this technology.
There is typically only one single inverter, or possibly two string inverters on each residential solar installation; it really depends on the overall solar power system size.
A string inverter functions in a series circuit, with there usually being 6 to 12 individual solar panels in what is known as a "series string."
Most of the disadvantages of string inverter noted below are eliminated when used with optimizers. This option discussed further down the page.
String inverters (without Optimizers) generally offer a lower cost solution where there is no shade and panels are easily accessible.
Advantages of String Inverters
Easy to troubleshoot - The main advantage of a string inverter is that you only need one of them to convert the DC electricity coming from your solar array to AC power. If anything is going to fail in a solar system, it is likely to be the inverter - making troubleshooting relatively easy when things go wrong.
Purchase Price - Installing a single string inverter on a solar panel installation is cheaper than installing microinverters. Fewer labor hours are required, and string inverters are more affordable than many microinverters.
High voltage DC wiring reduces wire cost - The wiring between panels and from the panels to the inverter is DC and can be high voltage. The high voltage means lower current. That results in smaller wire sizes which can be less expensive than 240V AC runs used with microinverters, particularly for longer runs. All DC wiring in the house and above the ground must be in metal conduit.
With extra hardware, in a brief grid outage the system can continue producing power so long as the sun is shining - However, the utility of this feature is questionable unless you have batteries. If one does have batteries with sufficient capacity (~ 30 kWh), one can power backup loads for a day or so with or without sun. For longer term coverage of grid outages, a generator is the more economical solution.
Disadvantages of String Inverters
Efficiency in partial shade - Since string inverters require solar panels to be wired in series, if one solar panel's output is affected, the entire series of solar panels is affected in equal measure. This can pose a major issue if some part of a solar panel series will be shaded for part of the day.
More difficult system expansion - To get optimal performance from a string inverter, it needs to be working near its peak capacity. So, if you want to increase the size of your solar array at some point down the road, those panels will need to be routed to a separate string inverter, adding in additional complexity and cost.
Shorter lifespan - String inverters typically have an 8 and 12-year warranty, whereas microinverters have a 25-year warranty.
System monitoring - Panel-level insight is not possible with a string inverter system since there are no components affixed to the back of each panel to do the job. While aggregate solar production is viewable, you won’t be able to see if there are individual panel performance problems that could be caused by a crack, defects, or debris.
The solar array shuts down when the Grid goes down - Unless you have batteries or some other energy storage capability and the additional system to transition from grid to batteries/solar, your solar system will stop production when the grid goes down.
Problem Isolation - A problem can only be isolated to a string of panels, not at the individual panel level. Further problem diagnosis requires testing tested each panel on a string.
Single point of failure - if a sting inverter goes down, all connected panels are offline.
High voltage DC wiring - The wiring between panels and from the panels to the inverter is DC and can be high voltage. This is more dangerous. All DC wiring in the house and above the ground must be in metal conduit.
Summary - String Inverters
Most of the disadvantages of string inverter noted below are eliminated when used with optimizers. This option discussed further down the page. String inverters (without Optimizers) generally offer a lower cost solution where there is no shade and panels are easily accessible.
Microinverters
Microinverters perform the same basic function as string inverters, except they are installed underneath each solar panel on your roof. Each of these microinverters is about the size of an internet router.
While standard string inverters will cap the electricity production of each panel by the lowest-producing panel on your roof, microinverters don’t have this problem since they function in a parallel circuit. A microinverter will take full advantage of the production of each individual panel. It will convert the power generated by each panel to the grid voltage. Each solar panel and microinverter combination can “do their best” and contribute as much power as they can.
Microinverters offer a simpler and more flexible solution, albeit at a marginally higher cost. They also use conventional AC wiring.
Advantages of Microinverters
Rapid shutdown capability - New electrical codes require rapid solar system shutdown so first responders or firefighters are safe from high voltage when they need to be on rooftops or servicing power lines. Microinverters comply with these rapid shutdown requirements and have this capability embedded into each module.
More electricity - The core advantage of using microinverters is that theoretically, you can yield more solar electricity. The reason for this is that there are slight differences in currents between solar panels. When solar panels are in a string, the current is reduced to that of the least-producing panel in the string.
Suitability for challenging installation conditions - If a solar system is facing multiple angles, meaning some panels are facing south, some east, and some west, then microinverters are the way to go. Or, if you have shading issues from trees or a large chimney, again microinverters would be best. In these situations, the solar panels will be producing different amounts of electricity at different times of the day, but microinverters will ensure you harvest all of the energy, while with a standard inverter you will lose some of this production.
Lifespan - Microinverters typically have 25-year warranties, while standard inverters typically have 8 to 12-year warranties. The reliability of microinverters was in question several years ago, but the technology now has caught up with the industry, and the long warranties on microinverters show the confidence the manufacturers have in their products.
Panel-level monitoring - Microinverters have the ability to track the production of each individual panel, while with a standard inverter you only can track the production of the whole system.
No single point of failure - The entire system does not go down when one microinverter fails.
System expansion ease - If you were to expand your system in the future, microinverters are simple to add one at a time. Each panel and microinverter pair can be easily added to your existing solar array without needing to worry about purchasing, siting, and installing additional string inverters.
With extra hardware, in a brief grid outage the system can continue producing power so long as the sun is shining - However, the utility of this feature is questionable unless you have batteries. If one does have batteries with sufficient capacity (~ 30 kWh), one can power backup loads for a day or so with or without sun. For longer term coverage of grid outages, a generator is the more economical solution.
Disadvantages of Microinverters
Cost - The addition of optimizers drives up cost.
Repairability - If a microinverter fails replacing the component is not as easy as replacing/replacing string inverter on the side of or in your house. Your solar installer would need to again get up on your roof, unbolt one or more solar modules, and replace the microinverter. If access to the microinverters is not an issue (e.g. ground mount), Repairability is not a disadvantage.
Voltage drops on long wire runs - Microinverters-based system require a limited voltage drop (typically 2%) from the microinverter to the grid connection. This requires the wire to be sized to limit voltage drops. Longer wires have higher voltage drops. If there is a long distance between the panels and the grid connection, the larger wire can get expensive.
Limits on panel size - The power produced by a panel/microinverter pair is limited by the power rating of the microinverter. Currently, microinverters do not support the higher output panels (>450W) typically used in ground mount systems.
The solar array shuts down when the Grid goes down - Unless you have batteries or some other energy storage capability and the additional system to transition from grid to batteries/solar, your solar system will stop production when the grid goes down.
Summary - Microinverters
Microinverters offer simplicity and relatively easy expansion, albeit at a higher cost. They are typically not an option for high powered panels typically used in ground mount system and when there is a long distance between the panels and the grid connection requiring larger wires to be used tominimize voltage drop.
String Inverters with Optimizers
The addition of optimizers Optimizer eliminate many of the disadvantages of string inverters. In many ways they make the string inverter system behave much more like a microinverter based system. Optimizers are added for each panel feeding the string inverter. The additional function optimizers add to a string inverter-based system include:
Optimizing performance at the panel level. The string is not degraded to the performance level of the lowest performing panel on the sting.
Monitoring at the panel level making problem diagnostics possible at the panel level (versus string level)
Safety - when there is not power from the grid the Inverter disconnects the AC power from the inverter to the grid. Without optimizers there is still high DC voltage. Optimizers reduce the DC voltage to safe levels when the there is no power from the grid.
Advantages of String Inverters with Optimizers have many of the advantages of microinverters
Rapid shutdown capability - New electrical codes require rapid solar system shutdown so first responders or firefighters are safe from high voltage when they need to be on rooftops or servicing power lines. string inverters with optimizers comply with these rapid shutdown requirements and have this capability embedded into fore each module.
High voltage DC wiring reduces wire cost - The wiring between panels and from the panels to the inverter is DC and can be high voltage. The high voltage means lower current. That results in smaller wire sizes which can be less expensive than 240V AC runs used with microinverters, particularly for longer runs. All DC wiring in the house and above the ground must be in metal conduit.
Suitability for challenging installation conditions - If a solar system is facing multiple angles, meaning some panels are facing south, some east, and/or some west, or if you have shading issues from trees or a large chimney, addition of optimizers to a string inverter system offers many of the same advantages as microinverters. In these situations, the solar panels will be producing different amounts of electricity at different times of the day. The addition of optimizers to a string inverter system ensures you harvest all of the energy, while with a string inverter alone you will lose some of this production.
Panel-level monitoring - The addition of optimizers provides the ability to track the production of each individual panel.
With extra hardware, in a brief grid outage the system can continue producing power so long as the sun is shining - However, the utility of this feature is questionable unless you have batteries. If one does have batteries with sufficient capacity (~ 30 kWh), one can power backup loads for a day or so with or without sun. For longer term coverage of grid outages, a generator is the more economical solution.
Disadvantages of String Inverters with Optimizers
Cost - The main disadvantage of string inverters with optimizers cost. They add $60-100 per panel over the cost of a string inverter.
Repairability - If an optimizer fails you would need to get up on your roof, work with your racking system, unbolt the solar modules, and replace the optimizer to reestablish AC conversion capability. If access to the optimizers is not an issue (e.g. ground mount), Repairability is not a disadvantage.
Single point of failure - if a sting inverter goes down, all connected panels are offline regardless of the use of optimizers.
High voltage DC wiring - The wiring between panels and from the panels to the inverter is DC and can be high voltage. This is more dangerous. All DC wiring in the house and above the ground must be in metal conduit.
The solar array shuts down when the Grid goes down - Unless you have batteries or some other energy storage capability and the additional system to transition from grid to batteries/solar, your solar system will stop production when the grid goes down.