Serial Good Luck

[Gone Gypsy]

We planned to install 2 Renogy 250 watt solar panels on our davit. Checked references and found variation in recommended cable size for the amps and distance so I was conservative and selected 10 gauge marine tin coated copper.

Then terrible good luck. Met the UPS driver delivering the 2 panels. One box was damaged and we could see shattered cells on the panel. He took that panel and left the good one. Amazon is now delivering 2 new panels and said to not return the good one left here.

Now all the purchased ancillaries intended for 500 watts (27 amps) are too small for 750 watts (40 amps) if wired parallel. Planned to wire parallel because so many cruisers claim better charging in parallel but also planned to test serial as the wire, controller and disconnects were all ok either way with just 2 panels. I have a new Victron 100/50 controller.

Now I either jump to 8 gauge wire and return the disconnects, fuses, glands etc or just wire in series with the existing stuff and no chance to test parallel. The davit panel location has little chance of shading. The panels have diodes and Victron says serial should be better even though most cruisers say parallel works best. I think this is heavily influenced by folks installing panels in shade prone locations like on Bimini under the boom.

What should I do?
Return everything and buy bigger for parallel?
Keep everything and go series? No chance of parallel without replacing wires, disconnects, fuses, etc.

I have popcorn and look forward to your comments.

[Jammer]

Install as originally planned and give the third panel away to someone else.


Or, install it in future with its own controller, wiring, fuses, connectors, etc.


The best answer is neither serial nor parallel but rather a separate controller for each panel. Serial and parallel are merely approaches taken to reduce cost and complexity.

[Statistical]

If there really is little chance of shading and you have room for the three panels then I would go series. Even with modest partial shading series isn't that much worse depending on the exact conditions usually in the range of 15% to 30% less than parallel but you are installing 50% more panels than expected.

Worst case scenario if you later do find out it has more shading than you expect you could rewire it or go with an additional 1 (or even 2) controllers.

The ideal scenario would be 1 controller per panel. Seriously I wish someone made a DC per panel optimizer for marine applications.

[Pete7]

Renogy UK 250w panels say maximum fuse rating is 30A, my MC4 connectors are also limited to 30A, so in parallel you will exceed this unless you have some very high spec MC4s.

However, in the real world its not often panels produce the claimed output and 75% more likely even in sunny conditions.

If you now have 3 panels and the space to fit them, then do so. Don't remember anyone ever saying I have too much solar and need to reduce it Either return the 100/50 and buy 3 x 100/20 instead, or wire 2 panels into the 150 and the third into a new 100/20. The small Victron MPPTs are now cheap as chips and the 100/20 gives you some head room.

The two or three MPPT solution gives you some redundancy too if one panel or MPPT fails.

We have 2 x 295w panels into a 100/50 which is fine. However, I also have a pair of 100/20s on board bought in the depths of winter when suppliers were doing clearance sales. Other than running the extra wires I may be tempted later in the year to switch.

[noelex 77]

The Imp of Renogy 250 w panels is 8.32 A. Therefore the maximum current between the panels and the controller with 3 panels and parallel connection is under 25 A. If you have sized wire assuming a current of 27 A (hopefully considering both voltage drop and current capacity) this will be perfect. There is no need to change this cable.


The cable from the controller to the battery will carry a higher current (A maximum of around 55 A or very occasionally 60+ A). The controllers should be mounted close to the batteries for best performance so this cable should only be short. It will therefore not be expensive to replace if needed (voltage drop will be less over a short distance).

With three panels in parallel you will need to have circuit protection (fuses or CBs) on the output of each solar panel as well as a circuit protection between the output of the the controller and the battery (mounted close to the battery).

However, as others have said, seperate controllers have advantages.

[Gone Gypsy]

Thanks everyone so far. Plan to sit in boat with beer this weekend and ‘visualize’ while I wait for the new panels to arrive. Thanks for the input.