5 panels in parallel or 4 in series-parallel?

[coaster]

Hi, I've had 2 x 75 watt 12v panels running through 70' of 12 AWG cable to a PWM controller at a remote cabin for about 10 years that meets most of my needs except during a stretch of overcast. A neighbour is upgrading his solar and has given me 3 more identical panels that I'd like to add. Due to the odd number (5), I can only do 5 in parallel at 12v, or drop one and go with 4 in series-parallel at 24v. The controller can handle the voltages and current either way, and I'd prefer not to replace the cable. My calculations on the milliohm loss for the cable show that I would lose 13% at max current with 5 (12v), vs 6% loss with 4 (24v), but also losing 20% output (4 vs 5 panels). It seems like the setup with 5 is the way to go. Before I go too far along I'd welcome a double check.

[Sunking]

With PWM you have no choice but only parallel. At 70 feet means some really expensive large wire.

Here is what happens and the most important thing you need to know. With a PWM controller Input Current = Output Current. They suck.

So lets say you have 2-75 watt battery panels. Panel Spec is Vmp = 18 volts, and IMP = 4.17 amps. Connect them in Parallel, currents adds and voltage remains 18 volts. That gives you 9.34 amps input thus 9.34 amps output for a total power of 12 volt x 9.34 amps = 112 watts output out of 150 watts. That is as good as it gets with PWM. Take the same two panels and with them in series. Voltage adds to 36 volts and current remains 4.17 amps. On the output of the controller you have 12 volts x 4.17 amps = 50 watts out of 150 watts. You would have the same thing as a single panel.

MPPT is a different animal where Output Current = Panel Wattage / Battery Voltage. 150 watt / 12 volts = 12.5 amps

So with PWM you are screwed. With the right MPPT controller you could wire all 5 panels in series which would give you 90 volts @ 4.17 amps, and allow you to use small inexpensive wire between the panels and controller with almost no losses, (less than 2%) and output 32 amps. With your PWM and panels you will be lucky to ever see 19 amps.

[coaster]

Thank you, Sunking, that's a clear explanation. I hadn't realized PWM limits based on current rather than power, and an all series input is an interesting idea. Testing on a clear day with all parallel gives me 15 A of a possible 22A usung PWM (4.4 A Impp x 5 panels), probably due to wire loss and the age of the panels. That more than meets my needs, but higher efficiency would be great.

[inetdog]

Thank you, Sunking, that's a clear explanation. I hadn't realized PWM limits based on current rather than power, and an all series input is an interesting idea. Testing on a clear day with all parallel gives me 15 A of a possible 22A usung PWM (4.4 A Impp x 5 panels), probably due to wire loss and the age of the panels. That more than meets my needs, but higher efficiency would be great.

If you are looking at the STC output specifications for the panels, then you should not expect full power output except on a very cold day, since STC assumes unreasonably low panel temperatures. (But even though the power will drop because the voltage drops, the available current should actually increase slightly with temperature, I believe.
Also remember that the PWM (or MPPT) controller will only take maximum current from the panels if the battery bank is discharged enough to require that full current.
You can get an independent idea of relative output, including both light reduction and wire resistance effects, by measuring the short circuit current instead.