# With two 50W panels best to wire in series or parallel?

Charge controller (Renogy Wanderer 10A PWM) can handle the combined voltage when wired in series. From the spec sheet, it has a max PV input voltage of 50VDC.

Panels connect to a charge controller which charges a 12V lead acid battery. Panels are 22.7V open circuit voltage each at 2.84A short circuit current.

Which wiring method would be more advantageous? Higher voltage/lower amperage or same voltage higher amperage?

• Can you provide the make/model of the charger? Most likely you'll want these in series (lower current reduces losses) but it can depend on the charger. The rated allowance vs the optimum arrangement can be different.
– LShaver
May 7, 2019 at 17:48
• From spec sheet: May 7, 2019 at 18:07
• Max pv input voltage 50VDC renogy wanderer 10A May 7, 2019 at 18:09
• Panels 22.7v open circuit voltage (each so x2) at 2.84 short circuit amps May 7, 2019 at 18:10

Energy losses in a cable — due to resistance — are proportional to the current squared (I²). Thus if you double the current (by putting your panels in parallel) then you'll quadruple your losses ( (2I)² = 4I² ).

When panels are connected in series, the voltages add up but the current remains constant. Thus to minimise line losses, connect panels in series.

Another advantage of keeping current low is that you can minimise the thickness of the cable between your array and your charger. If the length of that run is long, the savings can really add up.

Your Renogy charger's open circuit voltage limit is 50V, so it should handle the voltage produced by your panels in series without a problem (2x22.7=45.4; 45.4 < 50). If you do end up with the occasional over-voltage condition, it will probably be due to transient cloud-edge effects.

If you want to play it really safe, size your cable for a parallel setup (2x2.84=5.68A) but then deploy it in series to start with. Since line losses are proportional to resistance, and resistance is inversely-proportional to cross-sectional area, a thicker cable will offer less resistance and reduce losses even further. If you do start experiencing over-voltage conditions due to cloud-edge effects, you'll be able to simply switch over to parallel without having to buy a new cable.

UPDATE:

LShaver spotted that the Renogy Wanderer 10A is not in fact a MPPT charge controller, but actually a simpler/cheaper PWM charge controller, and checking the manual confirms that it does not feature voltage conversion.

What that means is that — regardless of array voltage — the maximum current that can exit the controller (to the battery) is equal to the current coming into the controller (from the array). Since the battery is 12V, that means that:

• a series setup, generating 2.84A, is limited to a bulk charge rate of about (12.6*2.84=) 36W
• a parallel setup, generating 5.68A, is limited to a bulk charge rate of about (12.6*5.68=) 72W

The PWM controller is clearly creating a bottleneck here. With (2*50=) 100W of panels, around 64% of the energy from a series configuration would be wasted. Even in a parallel configuration, around 28% of the energy would be wasted. Both of these figures far exceed the line losses discussed earlier.

If you have to stick with the PWM controller, then connecting the panels in parallel is clearly the way to go.

If controller losses of 28% are considered unacceptable, then there's little option but to replace the PWM controller with a MPPT controller. If you do that then connecting the panels in series (as discussed earlier) is the way to go.

• Interesting explanation. On such a small scale (less than 3’ of line) I predicted the losses would be negligible but they are losses nonetheless and I paid dearly for those electrons just to lose them. I shall wire in series. Thanks. May 8, 2019 at 1:14
• @AlexHale I added a strategy to the answer that improves efficiency in series, and lets you easily fall back to parallel if need be.
– Tim
May 8, 2019 at 1:21
• Yes the cables have already been purchased with mc4 connectors. The charge controller only accepts up to 14ga wire due to the cheap screw terminals. I had been concerned about the charge controller seeing 22VDC at 5A vs 44VDC at 2.2A and how its performance on battery charging is concerned. I guess it depends on the circuitry of the CC. Thanks for you input on resistance as it relates to voltage though I totally forgot about that. May 8, 2019 at 2:12
• While I was thinking the same thing as you at first I realize now with these cheaper PWM chargers the panel and battery nominal voltages should match. The charger will dump all current into the battery at first, then start pulse width modulation when the battery is nearly full. If the input voltage is significantly higher than the battery, it will be in PWM mode almost constantly, resulting in switching losses far greater than the resistive losses of the cables.
– LShaver
May 8, 2019 at 2:14
• Wow, good catch. Yes is is cheap \$15 and the connectors they provide for connecting two panels together are incidentally parallel. May 8, 2019 at 2:54