Solar String Size Calculator

Enter panel electrical specs and string layout to check cold-temperature Voc against the inverter maximum — the check that protects your equipment.

Last updated: 2026-09-15

How the calculation works

Voc rises as panels cool — about 0.29% per °C below the 25 °C rating point for crystalline silicon.
The string's cold Voc (panels × corrected Voc) must stay under the inverter's absolute maximum DC input, or the inverter can be destroyed on the first cold morning.
String Vmp must fall inside the inverter's MPPT operating window; array Isc (×1.25) sizes the DC disconnect and wiring.

Formula

Voc_cold = Voc_STC × (1 + β × (25 − T_min))
String Voc_cold = Voc_cold × panels
β ≈ −0.29%/°C (c-Si)

Formula variables
Symbol	Meaning	Unit
`Voc`	Open-circuit voltage	V
`β`	Temperature coefficient of Voc	%/°C
`Vmp`	Voltage at max power	V

Worked example

A 37.6 Voc panel at −10 °C: Voc_cold = 37.6 × (1 + 0.0029 × 35) = 41.4 V. Ten panels: 414 V — fine for a 500 V inverter. Eleven: 455 V, still OK. Fourteen: 580 V — over the limit, and 14-panel strings destroy 500 V inverters in winter.

Interpreting the result

String sizing has two hard constraints: cold Voc under the inverter max (a survival limit) and Vmp inside the MPPT window (a harvest limit). Cold Voc is the dangerous one because it happens at dawn on the coldest day with no load current to pull the voltage down. Always use the site's record low, not the average winter temperature.

Assumptions

Crystalline silicon temperature coefficient −0.29%/°C — verify against the panel datasheet (bifacial and thin-film differ).
All panels in a string are identical and identically oriented.

Limitations

Partial shading breaks the uniform-string assumption and can push one panel past its voltage limit.
Does not size string fusing (needed above 2 strings) or DC cable ampacity.
Grid-tie string inverters only — battery chargers need the charge controller calculator.

Frequently asked questions

Why does cold weather increase panel voltage?

Silicon's bandgap widens as temperature falls, raising the cell's open-circuit voltage. Voc at −20 °C can be 13% above its STC rating — enough to exceed inverter limits sized at STC.

What happens if string Voc exceeds the inverter max?

The input capacitors and switches see overvoltage beyond their rating — typically instantaneous, catastrophic failure, usually not covered by warranty.