Bringing It All Together: Combining Your Power and Voltage Information
This section is your chance to put all the information you’ve acquired (specifically, matching the array’s power to the inverter and defining the string length based on the maximum and minimum number of modules allowed in each string) and put it to use for your client’s site.
Earlier in the sizing process, you determined the number of modules you can fit on the roof and possible inverter sizes (see the earlier “Matching Power Values for an Array and an Inverter” section). Now, with the string lengths defined, you can see whether that array size is a real possibility. For example, in the earlier “First Things First: Evaluating the Budget and the Available Array Area” section, you determined that the roof (or ground, the physical site isn’t crucial, just the space limitations) could hold 24 of the modules and that the inverter you based your calculations on can handle the power from 24 modules. Can you really fit all 24?
The minimum string length was 9 modules (or 10, if you follow my advice), and the maximum was either 11 or 12, depending on how you calculated the temperature adjustment (see the earlier “Working the steps” and “Using NEC info in a pinch” sections).
If you took the time to calculate the adjusted Voc by hand instead of using Table 690.7 from the NEC, then yes, you can fit all 24 modules on the roof by placing two strings of 12 modules in parallel. If, however, you only used Table 690.7 and calculated a maximum of 11 modules in series, then no, you could place only 22 modules on the roof because you’d be limited to strings of 11 modules (because the space is limited to 24 modules or less, two strings of 11 is the closest you can get).
Unless you’re using an inverter that can handle different maximum power point inputs, the strings must be the same length and face the same direction. Otherwise, they’ll have different maximum power points, and the inverter won’t be able to harvest all the potential power.
What if the roof could hold only 18 modules? In that case, the array and inverter relationship in this example will work, but you can probably do better. I suggest finding a different inverter that has a lower DC input voltage than 250 VDC, just to make sure the system doesn’t shut off in high heat conditions or in a number of years as the modules degrade.
One Last Check: The Inverter’s Maximum Current Input
The maximum amount of power input and the DC voltage window are generally the only considerations you need to make when sizing a PV array to the DC side of the inverter. One final check to make, though, is the maximum current input from the array to the inverter.
All inverter manufacturers list the maximum current input allowable on their inverters, but this value isn’t always the easiest specification to find because the manufacturers tend to list it in the backs of their installation manuals. Be sure to find out the maximum current input value so you can make sure the array size you’ve calculated doesn’t exceed it.
To verify that the array you’ve sized doesn’t exceed the inverter’s maximum current input, divide the value reported by the inverter manufacturer by the short circuit current rating of the array at STC.