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[Cross-posted on Victron's community https://community.victronenergy.com/questions/85929/difference-in-solar-wattage-between-24v-and-48v-sy.html ]

Background: I'm on a boat with a 48V system and a 24V system. I have the means of charging the 24V system from the 48V system (not covered in this post). I have one set of solar panels which are connected to two MPPTS (I only activate one MPPT at a time), one is on the 48V system, one is on the 24V system. *

What I noticed was odd. When I switched from the 48V to the 24V, I noticed a doubling of wattage. Here are some graphs:

enter image description here

ID 256 is the 48V MPPT, ID 258 is the 24V MPPT. During this time, the MPPTs were in bulk. The lithium batteries are configured to push up to 56.8V/28.4V.**

Here's the voltage on the solar side:

enter image description here

I'm not sure why the voltage is held ~constant on the 48V (but solar) side. However at just above 60V it's still above the 56.8V needed to push to the battery, so I don't expect this is why the wattage is half. But I'm not sure. I do suspect the solar panels could be arranged differently, better (and probably arranged differently to take advantage of whether I'm pushing to 48V or 24V), but I suspect that might not be related to why the wattage is half.

Help?

[* Further background if you're curious: I'm on a boat with 48V propulsion. When I'm underway, I want the solar feeding into the 48V directly. When I'm not, I want it feeding into the 24V house battery bank instead of feeding it first into the 48V then charging the 24V off the 48V. Additionally, this lets me charge the battery over more time during the day.]

[** The 48V battery is a Valence which could be configured to charge up to 58.4V, but since we're not even to 56.8V, I suspect that isn't the issue here. The 24V is a Victron Li and 28.4V is the correct number here.]

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  • Can you provide details on the panels?
    – LShaver
    Apr 21 at 13:22
  • 6 Solara panels (solara.eu). I believe 4 are 140W, 2 are 120W (I know, it's best if they are homogeneous). I also believe (the boat builder assembled them, the wires are well-hidden, and I haven't verified) they are (140+140+120) together, with another set in parallel.
    – Tyler A.
    Apr 21 at 14:50
  • Hard to tell without part numbers, but I looked at a few 120 and 140 W panels and they were all 24 V. Assuming they're all in parallel you'd need your MPPT set to 24 V. To give a proper diagnosis we'd need a full wiring diagram with part numbers.
    – LShaver
    Apr 21 at 15:03
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This was answered on the Victron site https://community.victronenergy.com/questions/85929/difference-in-solar-wattage-between-24v-and-48v-sy.html

The answer from wkirby was that the 24V MPPT was able to pick a lower voltage (~46V) than was the 48V MPPT (60V), and thus the 24V was able to better convert solar energy.

The solution is to rearrange the panels to provide a higher voltage to the 48V MPPT.

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