Solar PV String Sizing Calculator

A solar string is a set of panels wired in series. Their voltages add up, and the inverter has to live with that total at every moment of the array’s life. Get the count too high and a cold winter morning will push the open-circuit voltage past the inverter’s hard limit, killing it on the spot. Get it too low and the inverter spends most of the day below its MPPT working range, leaving free electricity on the roof.

This calculator handles the cold-day side of that problem. Most code books in North America and Europe require the design open-circuit voltage to be evaluated at the record-low ambient temperature, not at standard test conditions.

The formula is:

V_string_max = N × Voc × (1 + β × (T_min − 25))

where Voc is the panel’s open-circuit voltage at 25°C, β is the temperature coefficient of Voc (a negative number, typically around −0.0028 per °C for crystalline silicon), and T_min is the lowest ambient temperature you expect at the site. Multiply by N panels in series and that is your worst-case string voltage.

For the lower end, you want at least one panel of margin above the inverter’s MPPT minimum on the hottest day:

V_string_min ≈ N × Vmpp × (1 + β × (T_max − 25))

Vmpp is the maximum power-point voltage at standard test conditions, around 30 to 40 V for most residential 60-cell modules. Hot panels lose voltage, so the inverter wants the string long enough to stay above the MPPT floor.

A worked check. Take a 365 W panel with Voc = 41.0 V, Vmpp = 33.5 V, β = −0.0028 per °C. Designing for −10°C cold and +60°C cell temperature against an inverter rated 200 to 600 V MPPT, 600 V absolute max:

• Cold-day Voc per panel = 41.0 × (1 + (−0.0028) × (−10 − 25)) = 41.0 × 1.098 = 45.0 V
• Max string length = 600 / 45.0 = 13.3, so 13 panels in series
• Hot-day Vmpp per panel = 33.5 × (1 + (−0.0028) × (60 − 25)) = 33.5 × 0.902 = 30.2 V
• Min string length = 200 / 30.2 = 6.6, so 7 panels in series

Anything from 7 to 13 panels per string is electrically valid. Most installers pick the longest string that fits, because power loss in the DC wiring scales as I² and longer strings carry less current.

A few practical points. Real panel data sheets list Voc and the temperature coefficient explicitly; do not estimate them. Some inverter brands (notably high-frequency string inverters with optimisers) have looser MPPT windows than the raw data suggest, but the absolute Voc maximum is always hard. And in places that get true Arctic cold, design for the actual record low, not the climate average. A 2 V miss on a 600 V inverter is a warranty conversation.