Solar Panel String Size Calculator | NEC 690.7 Compliant
Solar Panel String Size Calculator
Calculate the minimum and maximum number of solar panels per string based on your inverter's MPPT voltage window, and local temperature extremes. NEC 690.7 compliant.
Panel specifications
Find these values on your panel's datasheet under STC conditions.
Inverter specifications
Site conditions
Coldest expected temp. Colder = higher Voc. NEC 690.7 uses ASHRAE 2% design temp.
Hottest expected temp. Hotter = lower Vmp. Affects minimum string size.
How solar string sizing works
Solar panels connected in series form a "string." The voltages of all panels in a string add together while the current stays the same. The inverter's MPPT (Maximum Power Point Tracker) has a voltage window it can operate in. Your string voltage must stay within this window under all temperature conditions.
Why temperature matters (NEC 690.7)
Solar panel voltage increases as temperature decreases and decreases as temperature increases. On the coldest day of the year, your string's open circuit voltage (Voc) will be at its highest. This cold-adjusted Voc must not exceed the inverter's maximum DC input voltage, or you risk damaging the inverter. On the hottest day, your string's operating voltage (Vmp) will be at its lowest. This hot-adjusted Vmp must stay above the inverter's minimum MPPT voltage, or the inverter will stop producing power.
The formulas
Cold-adjusted Voc: Voc × [1 + (TempCoeff/100 × (Tlow - 25))]
Hot-adjusted Vmp: Vmp × [1 + (TempCoeff/100 × (Thigh - 25))]
Max panels per string: floor(Inverter Max Voltage / Cold-adjusted Voc)
Min panels per string: ceil(MPPT Min Voltage / Hot-adjusted Vmp)
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