String Sizing For Solar Calculator

Work out string sizing for solar instantly with clear inputs, formula shown and shareable results.

About String Sizing For Solar Calculator

String length is limited by open circuit voltage on the coldest morning, because Voc rises as temperature falls — typically 0.25-0.35% per degree below the 25 C rating. The cold Voc multiplied by module count must stay below the inverter's maximum DC voltage, or the inverter is destroyed.

Formulas

Cold Voc

Voc_cold = Voc_STC x (1 + temperature coefficient x (T_min - 25))

String length

Maximum modules = floor(inverter maximum DC voltage / Voc_cold)

Worked example

With the default values below, String Sizing For Solar Calculator returns Maximum Modules Per String of 27. Change any input above and every figure here is recalculated in your browser.

Inputs used

Module Open Circuit Voltageminimum 0.01 V: 49.5 V
Voc Temperature Coefficientaccepts -1–0 %/degree C: -0.27 %/degree C
Minimum Cell Temperatureminimum -60 degrees C: -10 degrees C
Inverter Maximum DC Voltageminimum 0.01 V: 1,500 V

Results

Maximum Modules Per String: 27
Module Voc At Minimum Temperature: 54.178
String Voltage At Minimum Temperature: 1,462.80
Margin To Inverter Limit: 37.20

Step by step

1Values used
2Cold Voc
3String length
4Maximum Modules Per String= 27
5Module Voc At Minimum Temperature= 54.178
6String Voltage At Minimum Temperature= 1,462.80
7Margin To Inverter Limit= 37.20

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Photovoltaic string design must comply with the applicable electrical installation standard and the inverter manufacturer's limits. Exceeding maximum DC voltage is a fire and shock hazard.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Photovoltaic string design must comply with the applicable electrical installation standard and the inverter manufacturer's limits. Exceeding maximum DC voltage is a fire and shock hazard.

Frequently Asked Questions

Which temperature should be used?

The record minimum ambient at the site, not the average winter minimum. Standards typically require the lowest expected cell temperature, which for a clear cold morning approximately equals ambient.

Is there a minimum string length too?

Yes. At maximum cell temperature the string's maximum power point voltage must stay above the inverter's MPPT window minimum, or the inverter cannot track and output collapses.