CalculatorDesign

String sizing calculator

How many panels fit in one string of a string or hybrid inverter in Southern California weather. Systems with power optimizers (SolarEdge, Tigo) follow the optimizer rules instead — use the manufacturer's designer.

PanelInverterCityColdest air, °C

—

Voc when cold

per panel

—

Vmp when hot

per panel

—

Max per string

—

Min per string

—

Voltage check only. Final design must follow the inverter manual and NEC 690.7, which uses the ASHRAE extreme minimum temperature — usually colder than the typical-year value.

How it is calculated

Voc when coldPanel Voc × (1 + β(Voc) × (coldest air − 25 °C)).
Vmp when hotPanel Vmp × (1 + β(Vmp) × (hottest cell − 25 °C)).
String lengthMax = inverter max DC voltage ÷ Voc when cold; min = MPPT low ÷ Vmp when hot.

Panels — CEC list (most installed in SoCal permits) · inverters — datasheets or CEC test window · weather — NLR (formerly NREL) PVWatts v8 typical-year weather · roof mount, 20° tilt, facing south, 14% system losses