# Mixed voltage panels

Registered Users Posts: 27 ✭✭
edited September 2019 #1
I may be totally confused but here goes
I have 2 190 panels 7.12 imp 26.6 vmp in series 53.2 volt total. And 3 100 watt 5.29 imp 18.9 vmp 56.7 volt total.
First question
Could I series the 3 100s and series  the 2 190 watt panels.
then parellel them with only a ~7% volt difference?
If so what kind of losses should I expect? Would it be the lower of the 2 voltages × the 2 amperages combined 53.2 x12.41 amps for 660 watts?
Question 2
When sizing a cc volt limit which rate do you use? voc from  noct or stc?

• Registered Users Posts: 4,496 ✭✭✭✭✭
IMHO, 7% is close enough.  Even in an array of identical panels, there will be variances from manufacturing, airflow, reflected light, wiring, etc.

For max voltage into the controller, use string Voc adjusted up for the record cold temperature at your location.  Controller makers (eg Midnitesolar) sometimes have online calculators to help with this.
Off-grid.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
• Registered Users Posts: 27 ✭✭
Excuse my lack of understanding
So use standard test conditions voc?
For string calculations.
Each has its own reason for being used... For predicting average output:

Vmp*Imp*0.77=Power (Watts) average best case (cool weather/direct noontime sun with panels "square to sun").

Vmp is variable... Vmp-stc is with cells at ~25C. But, in full sun, the Vmp falls as temperature increases (cells get hot and hot weather/no wind, cells get even hotter). In subfreezing weather, Vmp rises (and power increases). The 0.77 (or 0.75) derating gives you "practical" day to day average peak power and relatively close prediction of solar harvest.

Imp is also temperature variable (Imp rises, slightly, as cell temperature increases), but is 1/5 or 1/10 that of Vmp--So temperature compensation is usually ignored (close enough for solar work).

Voc (voltage open circuit)--Also temperature variable. We use this value based on the coldest temperature in your area (Voc-cold--Increases as temperatures drops). Used to ensure that the Vmp-array does not exceed Vpanel-max-input for solar charge controllers (i.e., for very cold conditions, Vmp-array-stc at ~100 VDC will be less than ~150 VDC max for below 0F temperatures).

Isc is the maximum short circuit current for Isc-std. Used to size wiring/fuses/breakers. This is the maximum current the solar panel/array output.

Voc is zero current flow. Isc is zero output voltage (current flow into a dead short). Remember that Power=Voltage*Current -- So a Zero in either voltage or current variable, you will have zero effective power.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset