# Need Help With 3 Line Drawing And Calcs For City Of Tucson

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• Solar Expert Posts: 46
Re: Need Help With 3 Line Drawing And Calcs For City Of Tucson

Thanks for that Lorlec, my partner will LOVE working that formula.
• Solar Expert Posts: 46
Re: Need Help With 3 Line Drawing And Calcs For City Of Tucson
BB. wrote: »
You can also use the Wiring Calculator in this thread to check your losses at various temperatures (typically, design for less than 3% total loss at worst case temperature).

The other place where you will have losses is the solar panels themselves. There Vmp output voltage falls with increasing temperature (the current rises a little bit with increasing temperature--but you can ignore the effect with little error).

My system (GT 3 kW) seems to vary from a cool spring day (60 degrees or so) to hot spring day (95F)--by about 5-10% output drop at higher temperatures (could also include the typically hazy sky we have in hot weather).

You can look up your panel's Vmp and temperature coefficient and plug the numbers into:

Note: I messed up the equation a bit--see following post for clarification (-Bill B.)

P=V*I=[Vmp @77F + Vtcf(77F-xxF ambient)] * Imp

I think that will work for you--I have to go.

-Bill

Thanks Bill ................
Re: Need Help With 3 Line Drawing And Calcs For City Of Tucson

Larry,

I got a sign wrong. should be a "-" in the middle of the equation:

P=V*I=[Vmp @77F - Vtcf(77F-xxF panel temp)] * Imp

Assuming that Vtcf/Vtoc is a negative number (such as "-0.0704 V/°C")... And that you will have to switch between °C and °F--depending on which your units your specs. are in.

-Bill

PS: Also, note that xxF or xxC is the panel temperature, not the ambient temperature... Use a contact thermometer or IR thermometer to measure panel temperature.
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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