A module datasheet gives you four voltage/current figures at Standard Test Conditions (STC: 1000 W/m², 25°C cell temperature, AM1.5 spectrum) — Voc, Vmp, Isc and Imp. None of them describe what the string actually experiences on site, because the site is essentially never at exactly 25°C. Getting string sizing right means correcting for that gap before the numbers go anywhere near a design.
The four numbers, and what they actually mean
Voc (open-circuit voltage) is the voltage across the module with no load connected — the highest voltage the module ever produces. Vmp (voltage at maximum power) is the voltage the module sits at when it's actually delivering its rated power into a load, always somewhat lower than Voc. Isc (short-circuit current) is the maximum current the module can produce, and Imp (current at maximum power) is the operating current — the one the module actually delivers.
Voc matters for a safety limit: it's the worst-case voltage the string can reach, checked against the inverter's maximum DC input voltage. Vmp matters for the MPPT tracking window: it's where the string actually operates most of the time, checked against the inverter's MPPT voltage range.
Why current doesn't multiply when you connect modules in series
Wiring modules in series means connecting the positive terminal of one to the negative terminal of the next, forming a single current path. In a series circuit, current is the same at every point in the loop — connecting five modules in series doesn't give you five times the current, it gives you five times the voltage at the same current. This trips people up because parallel connections behave the opposite way: parallel strings add current at the same voltage. Confusing the two leads to a wildly wrong current or voltage estimate depending on which direction the mistake goes.
Temperature correction: why cold weather is the Voc problem
Crystalline-silicon module voltage rises as cell temperature drops below 25°C, and falls as it rises above 25°C — the opposite of the intuitive assumption that heat always makes things “work harder.” The relationship is approximately linear, described by the manufacturer's voltage temperature coefficient (in %/°C, normally negative):
V(T) = V(STC) × (1 + (coeff / 100) × (T − 25))
The critical case for Voc is the coldest expected cell temperature at the site — often well below ambient air temperature on a clear, still, sunny winter morning, since irradiance heats the cell above ambient even when the air is cold, but the largest Voc excursions typically occur at low irradiance/low ambient combinations right at sunrise. A string sized only against the STC Voc figure can exceed the inverter's maximum DC voltage on exactly that kind of morning, months after the design was “checked.”
Why hot weather is the Vmp / MPPT problem
The same correction applies to Vmp, but the design concern is different: at high cell temperature, Vmp falls, and if it falls below the inverter's MPPT minimum voltage, the inverter can't track the array's maximum power point efficiently — the string is still electrically safe, but it's not being harvested properly. The critical case here is the highest expected cell temperature, which on a hot, high-irradiance afternoon can run 20–30°C above ambient air temperature depending on mounting and ventilation.
Cell temperature is not ambient air temperature. Use the site's actual design minimum and maximum cell temperatures — derived from local climate data and the specific mounting/ventilation configuration — not a generic assumption. This guide and its calculator do not supply a default; that number is project-specific.
Common mistakes
- Using STC Voc directly against the inverter's maximum DC voltage without a cold-temperature correction.
- Checking Voc but forgetting the separate Vmp/MPPT-window check for hot conditions.
- Multiplying current instead of voltage when adding modules to a string (a series/parallel mix-up).
- Using ambient air temperature instead of expected cell temperature for the correction.
- Assuming a positive temperature coefficient behaves like a negative one — some thin-film technologies do differ, and the sign matters.
Preliminary worked example
A module has Voc = 49.5 V at STC with a voltage temperature coefficient of −0.27%/°C. At an expected minimum design cell temperature of −10°C:
V(−10°C) = 49.5 × (1 + (−0.27/100) × (−10 − 25)) = 49.5 × (1 + 0.0945) ≈ 54.18 V
For a 28-module string: 54.18 × 28 ≈ 1,517 V — this figure, not the STC 49.5 V × 28 = 1,386 V figure, is what must be checked against the inverter's maximum DC input voltage. The gap between the two (≈131 V here) is exactly the kind of margin that gets missed when temperature correction is skipped.
Once the string configuration is fixed, the next design step is usually sizing the DC cable that carries the string's current back to the combiner or inverter — see the companion guide on DC voltage drop.