Best Watt per panel

[equinox]

what is the best Watt per panel as there are various offer in the market varying from 10 to 100 watts per panel..
I have built an 80 watt panel and found out that it produce less than required current (V) and therefore it is not recognized by the controller (30A PWM).
So what do I need to add? serial another 80 watt panel or just add 20Watt to produce 12Volts current?
Is it possible to mix panels of various watts?

[sensij]

You are mixing up voltage and current, making it hard to understand your problem.

Sunpower has a 475 W panel coming, but that won't work with a 30 A PWM controller either.

What cells did you use to build your panel?

[equinox]

20 cells, 4.6 Watts per cell
my problem is the my controller is not showing any panel.. insufficient voltage I assume

[sensij]

6" x 6" cells? Yeah... those might be 0.63 Voc per cell, 0.5 Vmp, so you are only at 10 V. You would need ~36 of those cells in series to charge a 12 V battery.

[equinox]

Yes. so what are you suggesting.. serial another 80 W panel or 20W is sufficient?
What would be the drawback of mixing panels of various watts.

[zed]

Yes. so what are you suggesting.. serial another 80 W panel or 20W is sufficient?
What would be the drawback of mixing panels of various watts.

Its like a chain....the weakest link dictates maximum power by virtue of the fact that one of them may have less current capacity.

Here's an analogy.
Imagine stringing together three garden hoses together...but the middle hose is thinner than the other two.
Maximum flow would always be limited by the thin hose....no matter how thick the other hoses are.

Hope that makes sense?

[inetdog]

Yes. so what are you suggesting.. serial another 80 W panel or 20W is sufficient?
What would be the drawback of mixing panels of various watts.

When you put two panels in series their current ratings need to be similar.
When you put two panels in parallel their voltage ratings need to be similar.
Power = Volts times Amps, so just knowing the power of the panel does not give you enough information.

In your case if you use the exact same type of cell to make another panel, it would need to be at least 10 cells to bring the voltage up to 15V when you put the two panels in series. They will then both produce the same amount of current, since they use the same size cells.
Even better would be 16 more cells, and just making another identical 20 cell panel would not be much of a waste.

[equinox]

Thank you guys...
So my conclusion is that one should use at least 100W panel to be able to charge a 12V battery.. am I correct?
@Sensij... just to my curiosity.. why is your 475W panel not working with a 30A PWM?

[inetdog]

Thank you guys...
So my conclusion is that one should use at least 100W panel to be able to charge a 12V battery.. am I correct?
@Sensij... just to my curiosity.. why is your 475W panel not working with a 30A PWM?

No. It all depends on the size (in AH) of the battery.
If you have a 50AH 12V FLA battery, then a 50A panel is fine.
For a 200AH 12V battery a 100W panel is too small.

[sensij]

Thank you guys...
So my conclusion is that one should use at least 100W panel to be able to charge a 12V battery.. am I correct?
@Sensij... just to my curiosity.. why is your 475W panel not working with a 30A PWM?

I think you are assuming all panels are constructed from cells that are identical to yours. For cells that produce 4.6 W at 0.5 Vmp, then you should be really looking at building panels of at least 150 W to charge a 12 V battery. Many panels available commercially are able to charge an appropriately sized 12 V battery with 100 W or less, because even though the cells in those panels might still be 0.5 Vmp (so 36 of them are required), they produce less wattage per cell.

The 475 W panel in my example has a much higher Vmp, 70 V or more, which is not appropriate for a 12 V battery (with a PWM charge controller).

[zed]

Thank you guys...
So my conclusion is that one should use at least 100W panel to be able to charge a 12V battery.. am I correct?

Nope...you can charge a 12v battery with a one watt panel...it!s not the watts its the volts that are your problem.

Most charge controllers need at least 3 volts more going in than coming out...so your problem is you need more cells to get a bigger voltage into the charger.

[equinox]

sensij : Got it... so what is the range of allowable/identifiable voltage to go through a 30A PWM controller?
zed : "Most charge controllers need at least 3 volts more going in than coming out...so your problem is you need more cells to get a bigger voltage into the charger." ... I got 10V and the solar panel icon does not lit.

[sensij]

If you were to buy a panel to charge a 12 V battery, it would have a Voc of 21-22 V, and a Vmp of 17-18 V. That would be a good target. If you go higher, the panel will get less efficient, so you won't get as much power out of it. If you go much lower, on hot days, you'll risk dropping the panel voltage below the charge voltage and you'll see the same thing you're seeing now.

[equinox]

Thank you Sensij, I believe newbie with limited budget like me should be aware of this and not building panels with less or too much voltage to produce.
Is there cells that produce more than 0.5 Vmp? if the answer is no, then 12V / 0.5 = 24 cells which is ~120 Watt panel is the minimum requirement... adding another 80 Watts to my existing 80 Watts would be a waste, but do I have an option??

[zed]

sensij : Got it... so what is the range of allowable/identifiable voltage to go through a 30A PWM controller?
zed : "Most charge controllers need at least 3 volts more going in than coming out...so your problem is you need more cells to get a bigger voltage into the charger." ... I got 10V and the solar panel icon does not lit.

Listen carefully....you cannot charge a 12v battery with a 10v solar panel.

You need at least a 15v solar panel.