A string that looks right at 25 °C can kill an inverter in January, or stop tracking in July. This checks both directions: cold-morning open-circuit voltage against the inverter's DC ceiling, and hot-day maximum-power voltage against the MPPT floor. Free, instant, nothing uploaded.
Module datasheet (STC)
Site temperature
Inverter limits
Array
Limit checks
| Check | Value | Limit | Result |
|---|
Which string lengths actually work
| Modules | Cold Voc (V) | vs ceiling | Hot Vmp (V) | vs MPPT min | Verdict |
|---|
Detail
A 540 W module, Voc 49.5 V, temperature coefficient −0.28 %/°C. The site sees −10 °C. Inverter: 1000 V maximum DC input, MPPT window 200–850 V, 3 % design margin.
Correct the module voltage to the cold cell temperature:
Voc(−10 °C) = 49.5 × [1 + (−0.0028) × (−10 − 25)] = 49.5 × 1.098 = 54.35 V
Twenty modules in series give 1087 V against a 970 V ceiling (1000 V less the 3 % margin). That is the over-voltage case. The ceiling allows ⌊970 / 54.35⌋ = 17 modules, which lands at 924 V.
Now the hot end. Roof-mounted cells reach 40 + 25 = 65 °C:
Vmp(65 °C) = 41.6 × [1 + (−0.0035) × (65 − 25)] = 41.6 × 0.86 = 35.78 V
Seventeen modules give 608 V at the hot extreme — comfortably inside the MPPT window. The lower bound from this constraint alone would be ⌈200 / 35.78⌉ = 6 modules, so the design is governed by the cold case, as it usually is.
Every figure is a linear correction from the 25 °C datasheet value:
V(T) = VSTC × [1 + (γ / 100) × (Tcell − 25)]
The result is far more sensitive to the temperature coefficient and the record low than to anything else. Two practical cautions: take the coefficient from the actual module datasheet — some high-efficiency cells run closer to −0.25 %/°C, thin-film is very different — and take the record low from a weather basis with a return period, not from a single cold night someone remembers.
Rooftop arrays deserve a larger hot-case allowance than ground mounts: restricted airflow pushes cell temperature up and Vmp down, and that shortens the string's usable hours on exactly the days when production matters most.
Voltage rises as cells cool. A string sized from the 25 °C datasheet value can exceed the inverter's DC ceiling on the coldest morning of the year, which is a genuine over-voltage condition on the DC bus and a common cause of winter inverter failure.
Vmp falls as cells heat up. On a hot day a short string can drop below the MPPT lower limit and the inverter stops tracking, losing output during peak sun.
Some jurisdictions require you to compare against the stated maximum with no reduction. Set the margin to 0 in that case — the tool then compares against the manufacturer's figure directly.
If the hot-case floor exceeds the cold-case ceiling, no series length satisfies both. That happens with very high module Voc, very cold sites, or a narrow MPPT window. The usual fixes are a different inverter with a wider window or a higher DC ceiling, or fewer modules per string with more parallel strings.