You are ignoring investing the savings which if the stock market returns 8% vs the panels at 13%, you would get a 50% higher return with the panels + investing the savings in the stock market vs just the stock market after 30 years. Albeit, it does take 13 years to come out ahead.

  >>> x = 0
  >>> for y in range(0,30):
  ...   x *= 1.08
  ...   x += 2125
  ...   print(y, x)
  ...
  0 2125.0
  1 4420.0
  2 6898.6
  3 9575.488000000001
  4 12466.527040000003
  5 15588.849203200003
  6 18960.957139456004
  7 22602.833710612485
  8 26536.060407461486
  9 30783.945240058405
  10 35371.66085926308
  11 40326.39372800413
  12 45677.50522624446
  13 51456.705644344016
  14 57698.24209589154
  15 64439.10146356287
  16 71719.2295806479
  17 79581.76794709972
  18 88073.30938286771
  19 97244.17413349713
  20 107148.70806417691
  21 117845.60470931107
  22 129398.25308605596
  23 141875.11333294044
  24 155350.1223995757
  25 169903.13219154175
  26 185620.3827668651
  27 202595.01338821434
  28 220927.6144592715
  29 240726.82361601325
  >>> x = 16500
  >>> for y in range(0,30):
  ...   x *= 1.08
  ...   print(y, x)
  ...
  0 17820.0
  1 19245.600000000002
  2 20785.248000000003
  3 22448.067840000003
  4 24243.913267200005
  5 26183.426328576006
  6 28278.100434862088
  7 30540.348469651057
  8 32983.57634722314
  9 35622.26245500099
  10 38472.04345140108
  11 41549.806927513164
  12 44873.79148171422
  13 48463.69480025137
  14 52340.79038427148
  15 56528.0536150132
  16 61050.29790421426
  17 65934.32173655141
  18 71209.06747547553
  19 76905.79287351358
  20 83058.25630339466
  21 89702.91680766625
  22 96879.15015227956
  23 104629.48216446194
  24 112999.8407376189
  25 122039.82799662842
  26 131803.0142363587
  27 142347.25537526738
  28 153735.0358052888
  29 166033.8386697119