Before revision: Again, a vote would yield the socially optimal solution: If the policy were implemented, each resident would avoid it's 1/100 x 300 = 3 share of the harm from each of 100 panels yearly, but this 300 benefit would fall below the 400 annual cost of a panel, so each resident would vote against the requirement and collectively the community would achieve the socially optimal outcome.
After revision: Again, a vote would yield the socially optimal solution: If the policy were implemented, each resident would avoid its 1/100 x 300 = 3 share of the harm from each of 100 panels yearly, but this 300 benefit would fall below the 400 annual cost of a panel, so each resident would vote against the requirement and collectively the community would achieve the socially optimal outcome.