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As we all know, winter days are shorter and the sun rises later and sets earlier. For the folks depending on solar energies for their electrical needs, it’s even more obvious with 50% less hours of sunlight per day in comparison to summer. For that reason, we say that in winter, it charges less…
Why’s that? Like all semi-conductors (electronic components), a low temperature enhances electrical efficiency. That’s why it’s important for a computer to stay well tempered in order to give its full potential. It’s the same thing for a solar panel, the colder it is, the better it performs.
Solar panels perform better in winter; this could make-up for the lack of sunshine. However, to be sure that you get the most out of your solar panel, you need a MPPT solar controller, a more sophisticated controller with better efficiency. With a regular controller (PWM), you won’t notice any difference.
There are many equations to evaluate the efficiency of solar panels in the cold. Normally, you can find on the back of the solar panel a grid that contains the values based on STC norms which are a result of different environmental facts, including temperature at 25°C.
What is needed for the equation? We need to know:
- the STC values
- the temperature coefficient* for the value we are looking for ; like watts, amperage or voltage
- The temperature difference between the temperature desired and the STC value.
*The temperature coefficient grid can generally be found on the spec sheet of the panel.
For example, we want to know how much a 140W solar panel is more efficient at -30°C. The temperature coefficient for the power (W) is -0.45%/°C. The temperature difference -55°C [-30°C (wanted temperature) - 25°C (STC) = -55°C].
Therefore, the equation should be: (140 W x 55°C x 0.45%) = 34.65, 140 W+34.65 = 174.65 Watts.
Since the temperature coefficient is negative and the temperature is going down, we must add the result to the original wattage.
This solar panel is clearly more efficient at -30°C with 174 watts compared to 140 watts at 25°C.
We want to know how much the performance is affected at 40°C. The temperature difference 15°C [40°C (wanted temperature) - 25°C (STC) = 15°C].
Therefore, the equation should be: (140W x 15°C x 0.45%) = 9.45, 140W- 9.45 = 130.55 Watts.
Since the temperature coefficient is negative and the temperature is going up this time, we must subtract the result to the original wattage.
This solar panel is clearly less efficient at 40°C with 130.55 watts compared to 140 watts at 25°C.
This calculation method helps us to:
- Determine the maximum voltage for an array of solar panels in order to be sure it does not exceed the limits of the MPPT controller.
- Determine the maximum amperage of the panels in order to know which fuse and/or breaker to install at the regulator when putting together a solar system.
Even if the solar panels are more efficient in winter and you’re using a MPPT controller, the lack of sun can still be problematic. A good evaluation of your energetic needs, with one of our experts when putting together a solar system, can help you cope better with winter.