Selecting the correct grid-tie inverter for your solar system is probably the most critical decision when designing your project. The inverter impacts daily performance and overall efficiency, and installing the correct one ensures everything you worked hard to set up will work properly.
How do you pick the right solar inverter? The short answer – you need to consider the type of array you’re working with so you know which features you’ll need. This way, you can tailor your inverter to your panel configuration to maximize power output.
The inverter has one job: to convert DC power from the solar panels into AC power for the utility grid. All available technologies offer a high conversion rate, so there’s little difference there. However, there will be significant differences based on your specific solar needs:
a) How panels in the same string (or branch) perform when a few panels are shaded or have a different orientation, and
b) Whether or not panel-level remote monitoring is required for your system
Solid Non-Shaded Solar Array
In a perfect or near-perfect solar setup, all panels are in one solid square array and facing the same direction. If this describes your solar setup, the solution is simple and cost-effective: a central inverter.
With a central inverter, all the solar panels are grouped into strings. Each string is brought to the inverter separately. Many newer central inverters also include multiple MPPT channels (usually one per string). For example, let’s say you have a ground-mount system with a total of 30 panels. You could mount a central inverter in the back of the structure, group the panels into two strings of 15 panels each, and connect each string to an independent MPPT channel.
In this case, a central inverter offers an elegant and cost-effective solution. The number of parts in the system is also significantly reduced, which keeps setup and maintenance simple.
Another advantage of central inverters is that the power electronics are not mounted on the roof behind the panels where temperatures tend to be high. Central inverters are often placed in cooler spaces where their performance and longevity increases.
The major limitation of using central inverters is the inability to monitor the performance of each module, which is only possible with microinverters and optimizers. With a central inverter, you can only monitor the performance of the system as a whole. However, for an open, solid array, system-wide monitoring is all you need. Panel-level monitoring is usually only useful when shading is present in the solar array.
Pros: Simple installation, reduced number of connections, all electronics in a cool, ventilated space as opposed to behind the panels. Cost-effective solution.
Cons: No panel-level monitoring
Our Central Inverter Recommendation: SMA US-40 Sunny Boy
Some Shading and Different Orientation
In the real world, most solar arrays are not perfect. In most roof-mount residential installations, it’s rare to have the roof space for a nice square PV array. It’s more common to have the solar panels distributed on multiple roof faces, with some of the panels shaded for a portion of the day.
In this case, it’s beneficial to control the output of each individual solar panel. If a few panels perform poorly during a portion of the day, other panels in the same string can then adjust their output to compensate for those losses.
The best way to do this is with power optimizers (one optimizer per PV panel). Optimizers constantly monitor and regulate the output of the panels in the string, sending optimum input to the inverter. This pushes the system to the highest possible efficiency in any given condition. Each optimizer in the system communicates with the inverter, offering panel-level monitoring where internet connectivity is available.
Another benefit of power optimizers is the built-in rapid shut down feature (which is required now by most jurisdictions). If power is switched down or the AC voltage is lost, the optimizers will bring the output voltage of each individual panel below 1V, complying with the rapid shut down requirement of 30V maximum for the system.
Pros: Panel-level monitoring, built-in rapid shutdown, shading mitigation, flexible string sizing
Cons: More hardware and connections (not much of a downside)
Our Optimizer Recommendation: SolarEdge optimizers plus inverter
Severe Shading or Challenging Layout
Just like central inverters, the use of power optimizers requires that each string feeding the inverter has a minimum number of modules.
However, there are instances when your roof layout demands additional flexibility. For example, let’s say that we have a string of 9 modules with power optimizers on a roof face right next to the neighbor’s home. At 3pm the neighbor’s roofline casts a shadow on 2 of the panels. Given that we need a minimum of 8 optimizers operating in a string, at 3pm that string will simply not produce power.
The same group of modules in the example above will benefit from using microinverters. Microinverters are mounted behind the panels and convert the solar DC voltage to AC voltage at 240V, compatible and ready to be transferred to the utility grid.
All microinverters produce energy at the same voltage (240VAC), and each one can be considered an independent unit. With this architecture, it’s irrelevant for a microinverter whether the adjacent unit is fully shaded or installed with a different orientation.
Pros: Maximum flexibility, panel-level monitoring
Cons: More parts and hardware, slightly higher system cost, power electronics installed on roof
Our Microinverter Recommendation: Enphase microinverters
Small Arrays and Future Expansion
Sometimes you only have enough space for a small solar system. In this case, efficiency and maximum output are even more important.
String inverters and power optimizers have a minimum system size requirement, usually around 3,000W of PV power. For a system smaller than 3000W, the best solution (and practically the only one) is to use microinverters. With microinverters you can have a solar system as small as 1-2 panels.
What if you’re building in stages, or adding to an existing system? Microinverters are still usually the best bet because you’ll get maximum flexibility and panel-level monitoring.
Small System Verdict:
Pros: System of any size, modular additions
Cons: Higher system cost, power electronics on roof
Our recommendation for smaller systems: APsystems dual microinverters
Shop for SMA and SolarEge inverters as well as Enphase and APsystems microinverters.