Paritian

Electricity

PV String Sizing Calculator

Maximum modules per string from the cold open-circuit voltage, with the minimum for the MPPT window.

Results

Modules per string 22
Module open-circuit voltage 45.422 V
String voltage when cold 999.28 V
Modules per string 9
Voltage 0.72 V

What this tool does

The temperature coefficient of open-circuit voltage is negative and printed on every module datasheet, typically around −0.25 to −0.30 % per kelvin. At minus ten degrees a 41.5 V module produces over 45 V, so a string that looks safe at room temperature can exceed the inverter's limit on the first frost. The last two results give the lower bound, so the string still stays inside the tracker's window on a hot afternoon.

Formula

Voc cold = Voc × (1 + β × (Tmin − 25))

Variables

SymbolMeaningUnit
voModule open-circuit voltageV
btVoltage temperature coefficient%/K
tnLowest site temperature°C
vmInverter maximum voltageV
vpModule voltage at maximum powerV
vsInverter minimum MPPT voltageV
NMModules per string—
VCModule open-circuit voltageV
VTString voltage when coldV
NLModules per string—
MGVoltageV

Worked example

  • Module open-circuit voltage41.5 V
  • Voltage temperature coefficient-0.27 %/K
  • Lowest site temperature-10 °C
  • Inverter maximum voltage1000 V
  • Module voltage at maximum power34.5 V
  • Inverter minimum MPPT voltage250 V
  • Modules per string22
  • Module open-circuit voltage45.422 V
  • String voltage when cold999.28 V
  • Modules per string9
  • Voltage0.72 V

Limitations

  • Electrical installations are governed by national wiring rules. Cable sizing also depends on installation method, grouping, ambient temperature and protection devices, which this calculator does not evaluate.
  • For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
  • The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.

Frequently asked questions

Why size the string on the coldest day?

Because a solar module's voltage rises as it gets colder, and the highest voltage it will ever produce is at open circuit on a cold, bright morning before the inverter starts. That is the moment that can destroy an inverter's input stage, and it is the moment nobody is watching. Use the lowest ambient temperature on record for the site, not the average winter temperature.