Maximum open circuit voltage calculator

[upnorthandpersonal]

I'm posting this in a thread here since it might be useful for people. Please keep in mind that the below calculator contains bugs and doesn't do error checking - it's literally been 5 minutes to put together. Ok, so what does it do? It calculates the maximum open circuit voltage you would see on your solar panel string when the temperature drops. You just enter your Voc at 25C, the temperature coefficient (both should be available for panels in their datasheet, the former per panel and should be multiplied with the total panels per string in series, the latter is given as a percentage) and the coldest temperature you expect to experience at your location. The calculator gives, based on these numbers, the maximum Voc you could see on your string. Since this increases when the temperature drops, this info is needed to make sure your charge controller can manage. Also, you might want to build in some safety margin still.

Link to the calculator.

I'll work on improving if there is interest. It only takes in °C for now for example, and as I said, no error checking/input validation is done...

Other links of interest:

• What does it mean to have solar panels in parallel and series?
• Figuring out how many panels in series and parallel based on your MPPT

 

[FilterGuy]

Thank you @upnorthandpersonal !! I think this is very useful.

It is easy to overlook the cold temperature voltage increase on solar panels and I suspect a lot of people have potential problems with their install.

 

[FilterGuy]

For people that want to know the math behind the calculator:

Starting values.

• Total string voltage (Rated Voc times number of panels in series)
• The worst case cold temperature in c.
• The panels temperature coefficient in %/C
• Temperature the panel is rated at. (As far as I can tell this is *always* 25C)

Calculations.

1. Calculate the worst case temp differential: 25 - (worst case cold temp) = Worst Case Differential
2. Calculate the Voltage increase percentage: Temp Coefficient * Worst Case Differential = % increase of voltage at worst case cold temp.
3. Calculate the voltage increase: % increase * Total String Voltage = Voltage increase at worst case cold temp.
4. Calculate the Max voltage at worst case cold temp: = Total String Voltage + Voltage increase

Real World Example:

• total string voltage is 136.2.V
• worst case temp is -17.8C
• temp coefficient is .3%/C

1. worst case temp differential = 25 - (-17.8)= 42.8C
2. Voltage increase % = .3%/C * 42.8C = 12.84%
3. voltage increase = .1284*136.2 V = 17.48V
4. Max voltage = 136.2+17.48 = 153.68V

 

[kenkoh]

I'm posting this in a thread here since it might be useful for people. Please keep in mind that the below calculator contains bugs and doesn't do error checking - it's literally been 5 minutes to put together. Ok, so what does it do? It calculates the maximum open circuit voltage you would see on your solar panel string when the temperature drops. You just enter your Voc at 25C, the temperature coefficient (both should be available for panels in their datasheet, the former per panel and should be multiplied with the total panels per string in series, the latter is given as a percentage) and the coldest temperature you expect to experience at your location. The calculator gives, based on these numbers, the maximum Voc you could see on your string. Since this increases when the temperature drops, this info is needed to make sure your charge controller can manage. Also, you might want to build in some safety margin still.

Link to the calculator.

I'll work on improving if there is interest. It only takes in °C for now for example, and as I said, no error checking/input validation is done...

Before this is done, can a real simple rule of thumb guide be posted? I have posted my setups dozens of times and until recently, a random YouTuber warned me about the coefficient witch smoking my system with the temps I have in PA.

 

[upnorthandpersonal]

Rule of thumb would be 20% or thereabout safety margin. Make it 25% to be sure.

 

[kenkoh]

Rule of thumb would be 20% or thereabout safety margin. Make it 25% to be sure.

Thanks. That works for my brain!

 

[kenkoh]

Rule of thumb would be 20% or thereabout safety margin. Make it 25% to be sure.

Please check my setup & math.

5x 150w (22.7 voc)
22.7 x 5 = 113.5 + 25% (28.3) overhead = 141.8 so an Outback 80a 150v is good?

 

[upnorthandpersonal]

Yes, that should be good to go! The Outback has a 145V start-up and operating maximum if I remember correctly, so you should still be well below that in your worst case scenario.

 

[kenkoh]

Yes, that should be good to go!

Thanks so much. With all the brains & experience here you'd think this would have been brought up, maybe easy to miss? I thank Will for this forum.

 

[upnorthandpersonal]

Thanks so much. With all the brains & experience here you'd think this would have been brought up, maybe easy to miss.

I guess because a lot of people are in warm climates where 'freezing' has a completely different meaning. Looking at you @Will Prowse

 

[kenkoh]

I guess because a lot of people are in warm climates where 'freezing' has a completely different meaning. Looking at you @Will Prowse

Oh yeah Las Vegas I'll be using 200w and get the same power.

 

[FilterGuy]

Thanks so much. With all the brains & experience here you'd think this would have been brought up, maybe easy to miss? I thank Will for this forum.

See this post.
https://diysolarforum.com/threads/how-can-i-make-this-600w-safer.4389/page-2#post-43272

Sorry, I should have been more direct.

 

[tictag]

There is usually a temperature coefficient for power output too. Would be a nice feature addition...

 

[upnorthandpersonal]

There is usually a temperature coefficient for power output too. Would be a nice feature addition...

I'll check it out!

 

[upnorthandpersonal]

There is usually a temperature coefficient for power output too. Would be a nice feature addition...

Added. I'm not to be held responsible for any bugs - quickly added this during my train journey

 

[Dzl]

Very cool! thanks for creating this.

 

[Dzl]

The panels I'm looking into don't give a temperature coefficient as a %, they list mV/mA per degree celsius

I know this value (the mV/mA per degree) would be the numerater used to determine temp coefficient % but would the denominator be Voc/Isc or Vmpp/Impp

 

[upnorthandpersonal]

Voc and Isc - always open circuit/short circuit values for these.

 

[LearningIt]

Rule of thumb would be 20% or thereabout safety margin. Make it 25% to be sure.

Do you mean a 20-25% safety margin based on the specified Voc at 25C or do you mean a 20-25% safety margin based on the calculated Voc (or F factor adjusted Voc) at the location's low temperature?

For example, when I consider a panel with a specified Voc of 44.5 V and the adjusted Voc based on a coefficient of -0.156 V/K or an F Factor of 1.12 for my location, I get a Voc of 49.96 or 49.8 V respectively, or 50.7 for a F factor of 1.14 (once a century freeze temperatures). With two series panels, I am still +/- 1 Volt of the maximum 100 V that many controllers like the epever Tracer 4210An specify.

Is that alarming enough to go with a controller with a higher 150 V input rating like the epever 4215?

 

[upnorthandpersonal]

Do you mean a 20-25% safety margin based on the specified Voc at 25C

Yes. Just add a 20% margin to the specified Voc in the panel datasheet (at 25C) and make sure your controller can handle that. Anything more is overkill. 15% could even be good enough, but if you want to be correct, calculate for worst case scenario for your region.

For your particular case, I would say you're fine if it's really only a once in a century thing. Do make sure however that your MPPT can operate at this voltage - some controller can accept a high voltage, but this is outside the MPPT operating range and it won't supply power to the battery.

If it were me though... I would get the 150V controller.