The number of panels a string inverter can handle depends on several factors, including the output current and voltage of the panel you are using, the number of strings allowed by the inverter, the maximum and minimum voltage allowed per string for the inverter, and the minimum ambient temperature for your location.
The minimum ambient temperature is important because this will affect the output voltage of the panels and therefore will put a limit on the number of panels you can have on the same string. Not having enough panels in a string will result in the inverter not turning on and having too many panels will result in a higher input voltage that the inverter can handle, and this can damage the inverter.
It is also important to note that all the panels connected in the same string will need to be facing in the same direction and have a similar tilt, and so this may limit the number of panels you can connect to each string due to roof space limitations and orientation of the roof mounting planes.
The math behind determining the string size for a certain panel and inverter combination can be confusing, and we recommend you use one of the many string-sizing tools that are offered. Renvu has a very easy to use string sizing tool available – if the panel or inverter you are using is not available in the dropdown menu, you will just need to have the solar panel and inverter data sheets available to pull the information you need to input into the calculator, and you will also need to know the minimum and maximum ambient temperature for your area.
Once you determine the number of panels per string the inverter can handle, you can check the PVwatts calculator to find out what the optimal DC to AC size ratio is for your area. This calculator will give you a very good estimate of your system production and at the end when you get your results, you can scroll down the page and read the DC to AC ratio. The DC size will be determined by multiplying the solar panel’s nominal power rating by the number of panels, and the AC size can be read off the inverter rating, that is, for example a Sunny Boy 5.0-US inverter is rated for 5kW AC. To get this ratio simply divide the DC size by the AC size.
For a quick example on how to use the PV Watts calculator, you can check out this video created by Ian Farneth.