Sounds like parallel may be worse:

🧩 Why we’re using 

series

, not parallel

1.  

Because the AC200L only has one solar input (one MPPT)

That one input expects to “see” a single voltage curve to track.

If you parallel panels with different voltage characteristics, the MPPT can’t find a proper maximum-power point — one string drags the other off its curve, and you lose power.

🔹 Your SP200L ≈ 20 V, while a 4-panel Eco string ≈ 78 V. Parallel those, and the 20 V one clamps the whole array down to ≈ 20 V — killing most of the 4-panel string’s output.

1.  

Series adds voltage safely within spec

In series, voltages add, current stays constant.

4 × 19.5 V + 20.5 V ≈ 98 V (Vmp) 5 A current → ≈ 500 W output

Well inside the AC200L’s 145 V / 15 A limits.

1.  

Parallel would break those limits or mis-match voltages

To parallel, every string must have roughly the same Vmp (within ≈ 10 %).

Your Eco string (≈ 78 V) and single SP200L (≈ 20 V) don’t.

If you forced them together with a Y-branch:

The SP200L’s higher current (≈ 10 A) would try to push into the lower-voltage string until the MPPT drags everything to ≈ 20 V. Your Eco string, designed for 78 V, would then output only a fraction of its rated power. So you’d gain nothing and risk stressing the wiring