TradesHow this works
A solar string has to stay inside the inverter's voltage window in the two worst moments of the year. On the coldest morning the module voltage climbs, so the string can have only so many modules before it tops the inverter maximum, which is a hard safety limit under NEC 690.7. On the hottest afternoon the voltage sags, so the string needs a minimum number of modules to keep tracking power. This tool corrects the module voltage for both temperatures and gives you the valid range.
Longer strings save wire and inverters but leave less cold-weather margin; shorter strings are safer but need more home runs. Use the module datasheet numbers and the site design temperatures, and let the stamped design and the inspector have the final word.
Common questions
Why does cold weather set the maximum string length?
A solar module makes more voltage the colder it gets. On the coldest morning of the year an idle string sits at its open-circuit voltage, corrected upward for that low temperature. If that cold Voc times the number of modules exceeds the inverter maximum input voltage, the inverter can be damaged and the install fails inspection. That is why the record low sets the maximum count.
What sets the minimum string length?
Heat. On a hot roof the module operating voltage sags, and if the whole string falls below the inverter minimum MPPT voltage it stops tracking peak power and production drops. So the hottest expected cell temperature sets the fewest modules you can use.
Which temperatures should I use?
Use the ASHRAE extreme annual minimum design dry-bulb temperature for the site as the record low, and add the solar heating rise to the record high ambient for the cell temperature. NEC 690.7 lets you use the module datasheet coefficient, which this tool does.
Sources: NEC 690.7 maximum PV system voltage from temperature-corrected Voc; module temperature coefficient method; inverter min/max input and MPPT window from the datasheet (NEC). Engine version 1.0.0. Risk tier 2.