Solar String Sizing Calculator
String length is set by temperature, not by the module's rated voltage. Open-circuit voltage rises as it gets colder, so the binding constraint is the coldest morning of the year: if cold-weather Voc exceeds the inverter's maximum DC input, the string damages the inverter. At the other end, maximum-power voltage falls as it gets hotter, so a string that is too short drops below the inverter's MPPT window on the hottest afternoon and stops tracking. The usable range sits between those two.
Run the numbers
The working calculator is free and needs no signup.
The formula
Vmp(hot) = Vmp × (1 + (Tc·Vmp / 100) × (Tmax − 25))
| Voc | Module open-circuit voltage at standard test conditions |
| Vmp | Module maximum-power voltage at STC |
| Tc·Voc | Temperature coefficient of Voc, percent per °C, always negative |
| Tmin | Record low ambient at the site |
| Tmax | Design cell temperature, which runs well above ambient in full sun |
Worked example
A module with Voc 49.5 V and a coefficient of −0.27%/°C, at a record low of −15°C:
Voc(cold) = 49.5 × (1 + (−0.27/100) × (−15 − 25)) = 49.5 × 1.108 = 54.8 V
Against a 1500 V inverter limit: 1500 / 54.8 = 27.3, so 27 modules maximum.
Sizing on the 49.5 V nameplate instead would have suggested 30 — and exceeded the inverter limit on the first cold morning.
Which standard governs this
NEC 690.7 governs maximum PV system voltage and permits the temperature-coefficient method. Module coefficients come from the manufacturer's datasheet at STC per IEC 61215.
What this calculation does not account for
The result is only as good as the design temperatures. Record low is not average winter low, and cell temperature in full sun runs 20-30°C above ambient. Uses module datasheet values, which carry their own tolerance.
Common mistakes
Using average rather than record low temperature, which produces a string that is legal on paper and over-voltage in practice. Ignoring the hot-weather minimum entirely and building strings that fall out of the MPPT window in summer. Applying one site's temperatures to another project.