PV Source
String Sizing (Voc Temp Correction)
Open-circuit voltage rises as cells get cold. A string that measures 480 V on a mild afternoon can push past an inverter's 600 V ceiling on the first clear morning below freezing, and that destroys the inverter. This calculator corrects Voc to your site's record low, then checks the hot-day Vmp against the MPPT floor so the string still tracks in August.
What it asks for
- Module Voc (STC) (V) — From the module nameplate, not the spec sheet marketing number.
- Module Vmp (STC) (V)
- Voc temp coefficient (%/°C) — Enter as a negative number, e.g. −0.27.
- Record low ambient (°C) — The mean annual minimum for the site. This governs max string length.
- Design high ambient (°C) — The hottest day you design for, in the shade.
- Cell temp rise over ambient
- Inverter max DC input (V) — Residential US: usually 600 V. Commercial: 1000/1500 V.
- MPPT minimum voltage (V) — The lowest voltage the MPPT will still track.
- Vmp / Pmax temp coefficient (%/°C) — Steeper than the Voc coefficient. If the sheet only lists Pmax, use that.
Per NEC 690.7 you may use the module datasheet coefficient, the ASHRAE extreme minimum, or Table 690.7(A). This calculator uses the coefficient method.