How do I select the right equipment for my solar system?
Many customers who have decided to go solar face their first hurdle in selecting the right equipment for their PV system. As with any selection process involving multiple options, you want to be well informed about what each option has to offer. Solar equipment is no exception, but it is a lot easier than you might think. We put together the following guide to help with your selection process of residential solar panels and grid tie inverter.
Selecting the Grid Tie Inverter
Selecting the right grid tie inverter is perhaps the most critical component to ensure the successful performance of your solar system. The job of the inverter is to convert DC power from solar panels into AC power that is compatible with the utility grid. There are a few local options for each scenario, but technology offers a different approach to an efficient power conversion. All available technologies offer a high conversion efficiency, so it is difficult to state with authority that one particular technology is better than another. The two main characteristics that help us differentiate between the inverters in a solar system are:
a) How the panels in the same string (or branch) perform when a few of them are shaded or have a different orientation
b) Panel-level remote monitoring
Grid Tie Inverter for a Clean-cut Solar Array
When you achieve a solar system that is as close to perfect as possible – i.e. one solid square array in which all the panels are facing the same direction – a central inverter would be the most cost-effective option. In this arrangement, the solar panels are grouped in strings and each string is connected to the inverter. Recent central inverters have multiple MPPT channels, usually one per string. To give an example, if you had a ground mount system with a total of 30 panels, you could mount a central inverter at the back of the structure, group the panels in two strings of 15 panels each, and connect each string to an independent MPPT channel.
A central grid-tie inverter provides an efficient and budget-friendly energy solution. The number of parts in the system is significantly reduced. One advantage of central inverters is that the power electronics are not mounted on the roof behind the panels, a location in which higher temperatures tend to threaten their performance and durability.
One limitation of using central inverters, as opposed to, say, microinverters and optimizers, is the inability to monitor the performance of each individual module. With a central inverter you can only monitor the performance of the system as a whole. Panel-level monitoring is most useful when shading is present in the solar array, but it should not be an important consideration for an open solid array like the one in the example above.
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 most roof mount residential installations, it is rare to have sufficient space for a proper square PV array. More commonly, solar panels are distributed in multiple roof faces, with some of them inevitably in shade for a portion of the day. In this scenario, it becomes beneficial to control the output of each individual solar panel; this way, if a few panels are performing poorly during a period of time, other panels in the same string will adjust their output to compensate for the losses.
A great way to do this is with power optimizers (one optimizer per PV panel). They constantly monitor and regulate the output of the panels in the string, sending optimum input to the inverter and facilitating the highest efficiency operation in any given conditions. All optimizers in the system communicate with the inverter, offering panel-level monitoring when internet connectivity is available.
Another benefit of using power optimizers is the built-in rapid shutdown feature, required by most jurisdictions. If the power is turned down or the AC voltage is lost, the optimizers will bring the output voltage of each individual panel below 1V, complying with the maximum rapid shutdown requirement of 30V.
Pros: panel-level monitoring; built-in rapid shutdown; shading mitigation; flexible string sizing
Cons: more hardware and connections (not a big concern, however)
Our optimizer recommendation: SolarEdge optimizers plus inverter
Severe Shading or Challenging Layout
Just like central grid tie inverters, the use of power optimizers requires each string feeding the inverter to have a minimum number of modules. There are instances when a particular 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 a neighbor’s home; at 3pm the neighbor’s roof line 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 not produce power.
The same group of modules in the example above will benefit from using microinverters. These are mounted behind the solar panels and convert the DC voltage of the panels to AC voltage at 240V compatible, making them ready to be transferred to the utility grid. All microinverters produce energy at the same voltage (240VAC) and each one can be considered as an independent unit. Within this architecture, it is irrelevant whether the adjacent unit is fully shaded or installed in a different orientation.
Pros: maximum flexibility; panel-level monitoring
Cons: more parts and hardware; slightly higher system cost; power electronics installed on roof
Our micorinverter recommendation: Enphase microinverters
Small Arrays and Future Expansion
Sometimes there is only enough space for a small solar system. 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 and possibly only alternative is to use microinverters. These allow you to have a solar system composed of only 1 or 2 panels.
With all of the advantages mentioned above, when building a solar system in stages or adding on to an existing system, microinverters often offer the most feasible solution.
Pros: System of any size; modular additions
Cons: Higher system cost; power electronics on roof
Our micorinverter recommendation: Enphase microinverters