In the previous chapter, we examined whether the system exceeds the inverter's maximum DC voltage even in cold weather. That was a protection-related calculation. Here, the question is not whether it will "break down," but whether it will operate properly.
A solar power system operates stably if the string's operating voltage falls within the inverter's MPPT range. If it does not, then everything else is in vain: the system either does not start on time, operates unreliably, or simply does not deliver the power it could.
What does Vmpp mean in practice?
Vmpp is the voltage at which the panel delivers the most power. It is not the maximum voltage (that is Voc), but the point at which the system actually operates for most of the day.
So the panel:
- does not operate at Voc,
- but around Vmpp.
And when multiple panels are connected in series, these Vmpp voltages add up.
For example, if a panel's Vmpp value is 41 V, and 10 panels are connected in series, then under ideal conditions, the string will operate at approximately 410 V.
But in reality, conditions are not ideal.
Why is temperature important?
Vmpp is temperature-dependent. This is not theory, but a very practical phenomenon.
- In hot weather, the voltage decreases.
- In cold weather, the voltage increases.
In summer, the cell temperature of panels can easily be 60–70 °C, even if the air is "only" 30 °C. This is a significant deviation from the datasheet value of 25 °C.
If this is not calculated, it can easily happen that the system seems good on paper, but in reality, it operates at too low a voltage in hot weather.
Assume that:
Panel Vmpp (according to datasheet, at 25 °C): 41.0 V
Vmpp temperature coefficient: −0.35 % / °C
Number of panels: 10
Inverter MPPT range: 200–850 V
Summer cell temperature: 65 °C
1. Temperature difference
DeltaT = 65 − 25 = 40 °C
2. Magnitude of voltage drop
40 × 0.35 % = 14 %
Thus, the panel's Vmpp value decreases by approximately 14% in hot weather.
3. Vmpp of one panel in hot weather
