This is an excellent question and it has an important answer - it depends!
There are three types of solar architectures out there and each has its own way of system design. I will not get into the pros and cons of each architecture in this post but here is the high-level overview.
String inverters: get their name from the stringing together of solar modules before connecting to the inverter. These are traditional conversion devices that have been around since the early 1970s.
Microinverters: Small inverters that are mounted underneath the module and convert the sunlight to AC power directly on the roof. Micros are a relatively new technology that became popular around 2010.
Optimizers: Small devices connected to the modules on the roof but perform a DC-to-DC conversion. Some optimizer companies offer a stand-alone product that will work with traditional string inverters, while others offer their own version of a string inverter that works with their optimizer. Appeared around the same time as microinverters.
Microinverters do not require string sizing so they are not applicable to this conversation.
However, you must still perform some minor calculations to determine if the solar module is compatible. Failure to do so could damage the microinverter and impact the warranty. Enphase holds the largest market share for this technology.
SolarEdge optimizers reduce the module voltage so that the entire string maintains a constant voltage of
around 350 to 400VDC. Consequently, SolarEdge allows many more optimizers per string and their string sizing tool dictates not only how many optimizers can be daisy-chained together, but also how much power you can connect to a string.
This is proprietary to SolarEdge. Perhaps there are other optimizers out there that do this, but they are not well known in the US. In fact, you cannot use the SolarEdge optimizer with any other inverters except theirs. The Tigo optimizer works much differently.
Tigo's optimizers work to find the maximum power point but do not maintain a constant voltage. Consequently, string sizing using their solution is the same as if you were not using the optimizers at all. Exactly the way the industry has been doing it since the 70s!
The Tigo optimizer can be used with any PV inverter on the market and their process also boasts a boost in efficiency over other optimizer technologies.
Ok, enough of that. How the heck do we find out how many we need if we are not using microinverters, SolarEdge, or no module level device at all? Since most rooftop installations in the United States must comply with National Electric Code 690.12 Rapid Shutdown, we will eliminate the last option.
There are some very powerful tools out there that not only include string sizing but all kinds of cost analysis and data people like to see when it comes to PV prediction vs. Money. I will not cover those in this blog.
Some inverter manufacturers have their own online string sizing tools that range from downloadable spreadsheets to some that are so comprehensive that you only need to know the make and model of the PV module used in the design and the tool will spit out a recommended design in seconds- using their inverter, of course. Here is a short list of available PV inverter manufacturer string sizing tools:
Tigo Energy also has a string and battery sizing tool in Excel format. Send me an email and I will happily send it to you!
These were the first to appear when Googled "PV inverter string sizing tool". Keep scrolling until you find your inverter manufacturer. If you don't find one for your inverter, the odds are pretty good that you can use someone else's tool and carefully compare their inverter specs against yours. I do it all the time and never had an issue.
There are some third-party online (and free) string sizing tools, but they tend to come and go since it becomes an administrative burden for some poor soul who has to maintain the databases every time a new inverter or module hits the streets. I will not go into the math behind string sizing because it is fucking boring. That's why there are online tools.
Berkely Labs Solar Photovoltaic String Length Calculator is a pretty good online string sizer that you can definitely tell was designed by engineers. It takes local weather data into consideration, has a long list of module manufacturers, and the form is extremely easy to use. It even provides auto calculations for the nerdy inputs you may not understand and uses them as default.
My only complaint is that it only shows the maximum number of modules and no minimum. This is usually not a big deal in the United States since most homeowners fill the roof up with as much as they can afford and/or as much as the roof will hold. However, some people may only want a 4 or 5-module system and will need to check if that many modules will provide a decent amount of production and hit the inverter voltage milestones.
Photonik's String Voltage Calculator is even easier to use than Berkley's since it removes a lot of the extraneous engineering crap. However, it does not provide automatic temperature data, although it does provide a link to an external source to obtain that info. It has a long list of module make and models but no specific inverter data. You will still need to compare the results against the inverter manufacturer's spec sheet. But, this site does an excellent job of showing you how to find that info and it makes it look less intimidating than some of the other sites.
Thank you for reading! Let us know what kind of sizing tools you use. I am sure there are tons more out there.
If you want to learn more about solar and storage, I recommend reading my book, The Battery Powered Home. It is an Amazon #1 Best Seller and contains a lot of easily digestible information for solar professionals and homeowners alike!