The formula
The coefficient is negative, and Tmin is below 25 °C, so the result is higher than the label Voc.
Worked example
Three 100 W panels with Voc 22.5 V and a coefficient of −0.29 %/°C, at −10 °C: 22.5 × (1 + −0.0029 × −35) = 24.8 V per panel, or 74.4 V for the string. That's safely inside a 100 V controller. Five of the same panels would reach 124 V and could destroy it.
The low-voltage side
An MPPT controller usually needs the panel voltage to be at least 5 V above the battery voltage to start charging, and panels lose voltage when hot. Victron, for example, quotes battery voltage + 5 V to start. Keep the string's Vmp comfortably above that in summer heat.
Tips
- Use the coldest temperature you could realistically see at dawn, not the average winter temperature.
- Series wiring lowers current and cable size, but one shaded panel drags down the whole string. See series vs parallel.
Frequently asked questions
How many solar panels in series on a 100V MPPT?
With typical 12 V-style panels (Voc around 22 V), three in series is safe in most UK and US climates. With 60-cell panels (Voc around 40 V) only two.
What happens if Voc exceeds the controller rating?
The controller can be permanently damaged, and it's not usually covered by warranty. Always check with the coldest expected temperature.