PV string Voc temperature correction — the maximum number of panels in series at low temperature

The guide on solar photovoltaic systems (§712) mentions the DC disconnector and the inverter as core components of a PV installation. This article goes deeper into a calculation step that precedes the design of every PV string: how many panels may be connected in series without exceeding the inverter's maximum DC input voltage — and why that calculation must not be done with the nameplate Voc.

STC Voc is not the design-relevant value

The open-circuit voltage (Voc) on a PV module's nameplate is measured under Standard Test Conditions (STC): a module temperature of 25 °C. In practice the actual module temperature is rarely 25 °C — on a bright summer day the module temperature can rise well above ambient temperature, while on a cold, clear winter morning the module temperature stays close to the low ambient temperature, combined with full irradiance as soon as the sun rises.

Why low temperature gives a higher voltage

A PV module has a negative temperature coefficient for Voc: the voltage rises as module temperature falls. For crystalline silicon modules this coefficient (β) is typically on the order of -0.25 to -0.4 %/°C, and is stated on the module datasheet. The temperature-corrected Voc is approximated by:

Voc(T) ≈ Voc,STC × [1 + β × (T − 25 °C)]

For example, at a minimum design temperature of -10 °C with β = -0.3 %/°C, this means a voltage increase of 0.3 % × 35 °C = 10.5 % above the STC Voc. For a string of twenty panels each with an STC Voc of 45 V, the string voltage then rises from 900 V to just over 994 V.

The requirement from IEC 60364-7-712

IEC 60364-7-712 (on which NEN 1010 §712 is based) requires that the maximum DC system voltage of a PV installation be determined at the lowest expected ambient temperature at the installation site, not at the STC temperature. The maximum permissible number of panels in series follows from:

Maximum panels in series = Umax,inverter / Voc(T_min)

where Umax,inverter is the maximum DC input voltage of the inverter as specified by the manufacturer, and Voc(T_min) is the temperature-corrected open-circuit voltage of one panel at the lowest expected ambient temperature.

Note: the exact lowest expected ambient temperature is site-dependent; for the Netherlands a minimum design temperature between -10 °C and -15 °C is often used in practice, but the actual value and the precise temperature coefficient of the chosen panel are decisive — both are found on the panel manufacturer's datasheet and local climate data, respectively.

Why this can lead to an unnoticed overshoot

A string that stays comfortably under the maximum inverter voltage in an STC-based calculation can, on a cold, clear morning — precisely the moment when panels deliver their highest output due to the combination of low cell temperature and full irradiance — still exceed the inverter's maximum DC input voltage. This risk does not manifest structurally but only under that specific weather condition, so an undersized string can easily go unnoticed at the first (warmer) commissioning, until a cold morning damages the inverter or forces it to shut down.

Also relevant on the other side: Vmp at high temperature

Besides the maximum Voc at low temperature, there is also a lower bound: at high module temperature (summer day, the module can become considerably warmer than ambient air), the string's MPP voltage (Vmp) drops. A string that is sized too short can therefore fall below the inverter's MPPT range in warm weather, causing power loss. The maximum number of panels is thus determined by the cold Voc, the minimum number of panels by the warm Vmp — both limits follow from the same principle (temperature dependence of module voltage), but in opposite directions.

Practical relevance

When designing a PV string — especially for larger rooftop systems with strings that sit close to the maximum inverter voltage — it is important to always perform the temperature correction using the actual temperature coefficient of the chosen panel and the lowest expected temperature at the site, rather than the simpler but unsafe STC-Voc calculation. For existing installations that sit close to the limit, it is worth checking whether the original sizing was actually done with temperature correction.

Common mistakes

Calculating the maximum string length using the nameplate STC Voc, without applying temperature correction for the lowest expected ambient temperature.
Assuming a generic temperature coefficient instead of using the specific β value of the chosen panel type — this varies by panel technology and manufacturer.
Only looking at the summer situation when determining the maximum string voltage, while a cold, clear winter morning is actually the voltage-critical condition.
Overlooking the minimum string length (warm Vmp, MPPT lower bound) while only paying attention to the maximum string length (cold Voc) — both limits are needed for a correctly sized string.