Solar string and charge controller sizing
Panel voltage rises when it is cold. A string that is fine on a mild day can exceed a controller's maximum input voltage on a frosty morning and damage it. Enter the numbers from your panel's and controller's spec sheets; we correct Voc for your record low and tell you how many panels fit.
How this is worked out
- NEC 690.7(A) (2023 NEC): maximum PV system voltage is the rated Voc corrected for the lowest expected ambient temperature, using the maker's temperature coefficient. Cold Voc = Voc x (1 + coefficient x (low - 25 °C)).
- Series count rounds down so cold voltage never exceeds the controller's maximum input voltage. Use a record low, not an average; leave margin if unsure.
- NEC 690.8(A)(1): maximum circuit current is 125% of Isc. If the controller's current limit is an operating rating we use that figure; if it is a short-circuit rating we compare plain Isc, as the maker states it.
- Not checked here: the controller's minimum start voltage, array power limit, wire and breaker sizing (690.8(B)), rapid shutdown (690.12) and grounding. Power stations often state a PV input voltage and current window; use those as the controller limits.
- Coefficient and limits are user-entered from spec sheets. Typical Voc coefficients are around -0.25 to -0.30 %/°C, but use your panel's own value.