Techie Top Tip No.3.

Techie Top Tip No.3.

Following on from his recent article on cold weather considerations for designing & installing battery systems, Segen’s Technical Support Manager, Steve Donovan offers a few comments on how cold temperatures should be considered when thinking affect PV string design.


From a PV generation perspective, cold temperature is our friend. Module open circuit voltage (Voc), and therefore Vmpp is inversely related to the ambient temperature, i.e., voltage increases as the temperature reduces.

Is the Voc within limits?

To ensure the PV system can cope with temperatures at this time of year, the system designer should make especially sure that the total string Voc is within the limits of the string inverter to which it’s connected.

Snow On Panels

We do hear of cases where inverters are ‘alarming’ at this time of year due to designers not considering the temperatures when designing their string length. If systems are installed in summer without considering winter temperatures, you run the risk of becoming unstuck. You’ll no doubt be familiar with the module datasheets, which can be found in the documents section of the product pages on the Segen portal. The Voc however is typically quoted on datasheets at STC (standard test conditions), which from a temperature perspective is 25℃.

Get Your Calculator Out

To calculate the theoretical absolute worst-case scenario Voc in cold weather, look at the ‘Voc Temperature coefficient’ on the datasheet which is often quoted as a % per ℃. Therefore, you calculate based on the temperature swing away from STC. Typically, we tend to use a low of -10℃, therefore a swing value of 35℃ away from STC of 25℃.

Let’s look at a real-world scenario with a popular module like the JA Solar 60cell 385WVoc at STC is 41.78V. While the Voc Temperature Coefficient is a value of -0.272%/℃. That negative value tells us that Voc increases as the temperature reduces.

Consider a swing of 35℃. Thus 0.272% x 35℃ = 9.52%. Maximum Voc = 41.78 +9.52% = 45.76V

This is your theoretical maximum Voc. If you have a maximum inverter limit of 600V, as a typical example, then you can then work out your maximum string length.

600V / 45.76V = 13.11 Therefore maximum 13 modules per string.

The Tech Team are here for you!

Our growing Tech Support team are always happy to speak to you, but it’s important to realize there is little we can advise in this sort of scenario, other than helping you to manage expectations.

– the Technical Team

Glossary of terms
• Voc – Open Circuit Voltage
• Vmpp – Maximum Power Point Voltage
• STC – Standard Test Conditions

Techie Top Tips so far:
No.1. Winter considerations for battery energy storage systems
No.2. Installation Addendum – Solis EH1P Hybrid Inverter – available on the Segen customer portal only.


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