String Inverters 

Figure 27. Cells to Modules to Strings 

Now the discussion will move away from solar modules and into inverters. One individual solar circuit is wired in series, with the modules plugging into each other positive to negative, head to tail. The industry labels that circuit a “string” of solar modules. 

Solar arrays can be made of multiple strings of modules, and these circuits would wire into an inverter called a string inverter.

This is very traditional kind of system, with all the solar panels on the system being wired into one single inverter. Sometimes, ahead of the inverter, all the circuits are wired together and reduced down to a single cable. This process is similar to how power lines come into a house and distribute power on a breaker panel in that individual circuits are wired together in parallel and have individual fuses or breakers. This combining of circuits is parallel wiring, but most inverters today do any combining internally, such that the module strings are wired directly up to the inverter itself.

So a string inverter will manage multiple circuits of solar panels. Easy enough. At the commercial level, when the inverters are managing hundreds of circuits, they are called central inverters to denote them as larger than string inverters.

Shade and Safety

Figure 28. Shade Impact

Before we leave string inverters, let’s come back to that term maximum power point tracker. The solar panel produces the most power operating somewhere just under its maximum rated voltage and current. The inverter, being a computer, helps it determine those levels, but the system voltage and amperage is ultimately determined by sunlight and things like shade will directly impact it. 

In the past, 10% shade could result in a 40% production loss. Having multiple power point trackers, such as module-level panel electronics, only 10% of the energy would be lost. This means more of the roof can be used as an array site.

Inverters might have one, two, or even a dozen “power point trackers”, which means that individual circuits can be individually controlled. This means the impact of shade is now confined to only one circuit. Although it is worth noting that a small amount of shade will still reduce the circuit power by 1/3rd to 100 percent, depending on the layout of the shadow.

But string inverters overall, with more powerpoint trackers, have become more shade tolerant, and can have arrays facing different angles, but only if all the panels on the same circuit or power point tracker face the same way and have the same electrical characteristics.

Back in the day, with only one power point tracker per inverter, all of the solar panels had to be uniform in circuit size and face the same direction.

Figure 29. Module Level Panel Electronics

Rooftops, as opposed to ground mounts, are subject to stricter fire code regulations, and string inverters by themselves are no longer good enough to be compliant with fire code. This is because individual solar circuits can reach 600V, or even more on a commercial building. But fire code wants the same circuits to be restricted to less than 90V on command. This essentially means that each solar panel on the roof must have a fire safety controller built into it. 

This is a unique rule specific to the United States, and it drives up installation cost, and increases the need for accessible installation sites. Nonetheless, it is a safer design option and it is code.

Most rooftop installations will then take an additional step and have these module-level panel electronics also be voltage controllers. Optimizing the system voltage allows for longer circuits, and lets individually shaded modules to be bypassed from the rest of the system. This means solar panels that have particle shade from trees or chimneys no longer impact the rest of the array.

Micro-inverters

Figure 30. Early Growth in MLPE 

Because they are code mandated, as well as useful, installing little boxes behind every or every other solar panel on a roof has become a standard practice in the United States. 

There are two categories of these products. Starting small, we have micro-inverters. Every solar panel gets its own inverter and that means AC electricity comes out of the array instead of DC electricity, so general construction feels quite at home here. 

Separately, micro-inverters are great for small projects. If you want to only install a couple solar panels, a micro-inverter circuit can be a great way to go.

Figure 31 Inverter Selection 

The main problem with micro-inverters is that it is more expensive to buy a whole bunch of inverters instead of one big inverter and so the the any kind of module level panel is more expensive than a single string inverter, and the other category of Module Level Panel Electronics has a pricing advantage to micro-inverters, because it is closer to a string inverter.

So micro-inverters are great for beginners, and are used by many mainstream installers, but most solar installers who regularly install on rooftop projects prefer this other second solution.

DC Optimizers

The compromise solution between having every solar panel get its own inverter verses having one inverter for the entire system, is to take half of the stuff inside the inverter and put it up on the roof, for module-level voltage control, and then keep some of the stuff down in the inverter on the ground below. There is a slight cost advantage in doing so. Instead of DC to AC up on the rooftop, now the boxes just control the DC voltage and sent a steady voltage down to the inverter below. 

DC optimizers may be the most popular rooftop architecture in the United States, but connecting all the wires correctly is the realm of a professional installer. While not terribly difficult, it is not as easy as wiring to a string inverter by itself. Micro-inverters, however are the easiest of the systems to install.

Just because a solar array is shade tolerant doesn’t mean that it will produce electricity if completely shaded. So ultimately, the total amount of sunlight available may depend on whether the site owner is able to cut down some trees. If shade cannot be avoided, such as a tall chimney, module level panel electronics are a great solution. 

There might be a portion of roof that is sunny for eleven and a half months out of the year but for two weeks in December its shaded by the tree line and it might be economic to install an array there but even so, clients want to see their systems functioning all year round. So do not be reckless with shade analysis just because MLPE is specified.

With MLPE monitoring, every single solar panel can be checked in on, sometimes to the chagrin of the installer. But they fundamentally improve project safety and sometimes improve system production. 

They allow for a safety switch, called a “rapid shutdown device” to be pressed on command and for the electricity from the circuits to drain to ground. This increases firefighter safety, such as if the solar array is burning the house down.

These rapid shutdown requirements have been on the market for some time now, and market leaders SolarEdge and Enphase have grown from startup to industry giants in a short period of time. 

Inverter Specifications

Figure 32. Inverter Specifications

While on the topic, let’s look at a micro inverter specification sheet. This micro inverter is compatible with 60 cell modules

and that micro inverter is compatible with 72 cell modules at a higher voltage. 

This output data is handy. It says “maximum units per 20 amp branch circuit” which is that these micro inverter units are wired into 240 volt double pole circuit breakers off the electric service panel. Sixteen micro inverters can be installed per branch circuit, so certainly when a solar array is 16 solar panels or less, a micro-inverter circuit is a good choice.  The environmental rating is NEMA 6 which is industry-leading. 

A string inverter specification sheet doesn’t tell how many solar panels fit on a circuit. To design with string inverters, a little more knowledge is required. The design work on a micro-inverter circuit is easier.

String Sizing

In string inverter design, the designer must ensure the circuit voltages do not exceed 600V. In other words, the designer must take the module short circuit voltage and make sure that number, multiplied by the number of panels on a circuit, does not exceed 600V at standard test condition, as well as at full power at the coldest design temperature. The actual calculation isn’t too complicated and there is a conversion table in the solar section of NEC for silicon modules.

But inverter manufacturers want to make it as easy as possible for you to design a system using their products and so they publish design software right on their websites for you to use. Many students wonder what the next step is after taking a solar training class, and my common answer is to start playing around with this free manufacturer solar design software. I do teach a 4 hour design course, where we have more time to spend on the capabilities of computer-assisted design in guiding decisions like string length.

Figure 33. Microinverter Specifications

But before moving on, let’s acknowledge that the same temperature considerations are given to micro-inverters, except because it is only one panel per inverter, this is a fairly easy item to figure out. Micro-inverter voltage is kept in range by identifying whether the micro-inverter is for a 60 cell or 72 cell solar panel, as referenced on its specification sheet. Then count the cells on the solar panel, and if it is what you are supposed to have, you are good to go.

Here is an example of a micro-inverter sizing tool, where the module

Specification sheets can be inputted and it’ll tell you whether or not that solar module is compatible with the micro-inverter. 

At which point it is relatively easy to run a 20 amp branch circuit from the attic to the electric service panel. So if a pallet of solar panels has 26 solar panels, the micro-inverter circuit would need two 20A branch circuits, with up to 16 panels each.

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