BritCanuck
Senior Member
Yes all parallel panels will contribute power but sub optimally. The shaded panels will drag down the optimal power.
Here’s how Grok explains it :-
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### Understanding Mismatch in Parallel Solar Panels with a Single MPPT
You're spot on that in a parallel connection—like your 190W rooftop and 200W mobile panels wired together into the Victron 100/30—the MPPT controller does see the panels as a single combined array. Their currents add up at the shared voltage, so under ideal conditions (both in full sun, same angle), you'd get roughly 390W total input, with the controller outputting ~25-30A to your batteries depending on voltage and efficiency.
But the "drag-down" effect from a lower-output panel (e.g., the rooftop one shaded by a tree or the mobile one flat on the ground) isn't about the MPPT ignoring the weaker panel—it's about how solar panels generate power via their I-V (current-voltage) curves. The MPPT can't perfectly optimize both panels at once because they're forced to operate at the same voltage. Let me break it down step by step.
#### 1. How a Single Panel's Power Works (Quick Refresher)
- Each panel has an "ideal" operating point called the Maximum Power Point (MPP), defined by its Vmp (voltage at max power, ~17-20V for 12V panels) and Imp (current at max power, ~10-12A for 190-200W panels).
- Power (W) = Voltage (V) × Current (A). The MPPT sweeps the voltage to find this sweet spot, extracting max watts.
- If conditions change (shade, heat, angle), the curve shifts: the panel might produce less current at its original Vmp, or its MPP voltage changes slightly.
#### 2. Parallel Connection: Shared Voltage, Added Currents
- In parallel, both panels connect with positives together and negatives together, so they must operate at the exact same voltage set by the MPPT.
- The total current is just the sum: I_total = I_rooftop + I_mobile.
- The MPPT treats this as one big panel (~390W array) and adjusts one voltage to maximize total power (V × I_total).
- The catch: It optimizes for the array's combined MPP, not each panel's individual one. If the panels' curves don't align perfectly, the chosen voltage might be a compromise.
#### 3. How the Weaker Panel "Drags Down" the Stronger One
- Imagine the rooftop panel is in full sun (optimal: 18V at 10.5A = 189W), but the mobile is partially shaded/wrong angle (now only 15V at 8A = 120W effective).
- Their I-V curves differ: The strong panel wants ~18V for max output; the weak one might "prefer" a lower voltage (e.g., 16V) where it still pulls decent current, but at 18V, its current drops further (say, to 7A).
- The MPPT scans and picks a voltage (e.g., 17V) that maximizes total power: Strong panel at 17V might give 10A (170W, a 10% loss from ideal), weak at 17V gives 7.5A (127W, close to its max). Total: ~297W.
- Without drag: If separate, strong panel hits 189W, weak hits 120W = 309W total (4% more).
- The weak panel forces the MPPT to "settle" at a voltage slightly off the strong panel's peak, clipping some potential watts from the good panel. In bad mismatches (e.g., 50% shade on one), losses can hit 20-30% overall because the weak panel acts like a "load" at higher voltages, reducing its current disproportionately.
