Every grid-connected solar PV system connects modules in series to form strings. The string voltage is the sum of the individual module voltages — but module voltage isn't fixed. It changes with temperature, and that's what makes string sizing a real engineering calculation rather than simple arithmetic.
On a cold Queenstown morning at −8°C, a crystalline silicon module can produce 10% more voltage than its STC rating. On a hot Napier afternoon at 50°C+ cell temperature, it can produce 10% less. Get the string size wrong and you either exceed the inverter's maximum input voltage (potentially destroying it), or drop below the MPPT tracking window and lose generation for hours.
AS/NZS 5033:2021 is the installation and safety standard for PV arrays. It covers string sizing, overcurrent protection, cable sizing, earthing, and labelling — and, critically, the maximum voltage limits (Clause 3.1):
The limit applies to the maximum open-circuit voltage at the lowest expected temperature — not at STC. This is where people get caught: 26 × 37.2V = 967V at STC looks fine, but at 0°C (Table 4.1 factor 1.10) that same string produces 1064V and you've exceeded the 1000V limit.
AS/NZS 5033 provides two ways to correct Voc for temperature:
Voc(cold) = Voc(STC) × [1 + (αVoc ÷ 100) × (Tmin − 25)]
Where αVoc is the temperature coefficient from the module datasheet (e.g. −0.27%/°C). This is the more accurate method when the datasheet value is available.
Table 4.1 in AS/NZS 5033 provides a simple multiplication factor based on the minimum expected ambient temperature. This is a quick conservative check that doesn't require the exact temperature coefficient.
This calculator uses the higher of the two values (more conservative) as the corrected Voc for string sizing.
The minimum number of modules is driven by the inverter's MPPT minimum voltage. On the hottest day, when cell temperatures hit 50°C+, the module voltage drops. If the string voltage falls below the MPPT minimum, the inverter either shuts down or operates at reduced efficiency.
Vmp(hot) = Vmp(STC) × [1 + (γVmp ÷ 100) × (Tcell_hot − 25)]
Tcell_hot = Tambient_max + 25°C (typical roof mount)
Min modules = ⌈ MPPT_Vmin ÷ Vmp(hot) ⌉
The +25°C cell temperature rise is a standard assumption for typical rooftop-mounted panels. Ground-mounted or well-ventilated arrays may run cooler; flush-mounted panels on dark roofs in sheltered locations can be +35°C or more.
AS/NZS 5033:2021 requires that the maximum string current used for equipment sizing is:
Istring_max = 1.25 × Isc(STC)
The 1.25 factor accounts for enhanced irradiance conditions — cloud edge effect, snow reflection, or particularly clear atmospheric conditions can push short-circuit current above the STC rating. This value must not exceed the inverter's maximum DC input current per MPPT tracker.
New Zealand's maritime climate means temperature extremes are relatively mild compared to inland Australia or northern Europe. However, there are still meaningful regional differences:
Always use the site-specific minimum temperature, not the regional average. String sizing must account for the worst case — the coldest morning the system will ever experience.
1000V DC per AS/NZS 5033:2021 Clause 3.1. This applies to the maximum open-circuit voltage of the array at the lowest expected site temperature — not at STC. Other (non-domestic) installations can operate up to 1500V DC.
Use the minimum expected ambient temperature at the site for maximum Voc (overvoltage check). Use the maximum expected ambient temperature plus 25°C for cell temperature when calculating minimum Vmp (MPPT dropout check). The location presets in this calculator provide typical values for NZ cities.
Table 4.1 provides a simple multiplication factor to correct the STC Voc for minimum ambient temperature. It's a quick method that doesn't require the module's exact temperature coefficient. For example, at 0°C minimum ambient, the factor is 1.10 — meaning you multiply the STC Voc by 1.10 to get the corrected value.
AS/NZS 5033:2021 requires a 1.25 overcurrent factor on the short-circuit current for equipment sizing. This accounts for enhanced irradiance conditions that can occur during cloud edge effect or snow reflection, where the actual irradiance exceeds the 1000 W/m² STC reference.
NZ solar PV string size calculator. AS/NZS 5033:2021 Table 4.1 cold-Voc factors. Per-module Voc/Isc derating.