Why is it even necessary to deal with this?
On the DC side, excessively high voltage is one of the most dangerous faults. The maximum DC voltage specified on the inverter's datasheet (e.g., 1000 V) is not a "guideline" but a limit value. If the string exceeds this in cold weather, the inverter may shut down, produce an error, or, worse, be damaged.
The trick is that this often doesn't occur in summer, but in cold, sunny winter weather when the solar panel voltage is at its highest. Therefore, the maximum string voltage should always be checked for cold temperatures.
Voc – what does it simply mean?
Voc (open-circuit voltage) is the highest voltage of the panel when there is no load, meaning no current is flowing. If multiple panels are connected in series, these voltages add up.
Important: The Voc value on the panel's datasheet is given for 25 °C (STC). In winter, it can be much colder than this, and in such cases, the Voc increases.
Why does Voc increase in cold weather?
The solar panel's voltage is temperature-dependent:
- voltage increases in cold weather
- voltage decreases in warm weather
The manufacturer specifies this with a number: Voc temperature coefficient (e.g., −0.28%/°C). The minus sign means it decreases with warming, so it increases with cooling. In practice, when calculating, we work with its magnitude (0.28% per degree).
What question do we need to answer?
Not "what is the average voltage," but "what is the maximum possible voltage in the worst case scenario." In cold weather (e.g., at −10 °C), does the maximum Voc of the series-connected panels fit within the inverter's DC limit?
Required data (only what is truly needed)
- Panel Voc value (from datasheet)
- Voc temperature coefficient (from datasheet, e.g., −0.28%/°C)
- Lowest design temperature (often −10 °C in Hungary)
- Number of panels connected in series in the string
Calculation method
1) Temperature difference:
ΔT = 25 °C − (lowest temperature)
If the lowest temperature is −10 °C, then:
ΔT = 25 − (−10) = 35 °C
2) Voc increase in percentage
Increase (%) = ΔT × (absolute value of Voc coefficient %/°C)
If the coefficient is −0.28%/°C, then the absolute value is 0.28%/°C.
Thus:
Increase (%) = 35 × 0.28% = 9.8%
3) Cold Voc of a single panel
Voc_cold = Voc_datasheet × (1 + increase %)
If Voc_datasheet is 49.5 V, then:
Voc_cold = 49.5 × (1 + 0.098) = 49.5 × 1.098 ≈ 54.3 V
4) Maximum string voltage in cold weather
String_Voc_cold = number of panels in series × Voc_cold
If 15 panels are connected in series:
String_Voc_cold = 15 × 54.3 ≈ 814.5 V
Comparison with the inverter
If the inverter's max DC voltage is 1000 V, then:
814.5 V < 1000 V → OK, safe.
However, if there were 19 panels in series:
String_Voc_cold = 19 × 54.3 ≈ 1031.7 V
1031.7 V > 1000 V → not permissible, too high.
Why is it not good to design to the limit?
Even if it fits on paper, it's not worth pushing the DC limit to its absolute maximum. In reality, it can be colder, there can be datasheet discrepancies, and a safety margin always protects the inverter's lifespan. Installer's logic: one extra panel isn't worth an inverter risk.
Typical errors
- Cold correction is omitted ("it'll be fine like this anyway")
- Vmpp is confused with Voc (they are not the same!)
- Too many panels in series because "it's not high in summer anyway"
- The coefficient is interpreted with the wrong sign (increases with cooling!)
