The upper bound on environmental harm is the damage that would be done by the most harmful energy production method used: any renewable energy spent on mining could be used instead to replace that harmful production.

(It's only an upper bound because energy isn't perfectly fungible.)

Renewable energy is also better than fossil energy in terms of environmental damage, but it's not free from damage. A dam is a massive localised environmental impact, and a huge amount of concrete. A solar farm requires (holes-in-ground) mining and significant industrial processing to build. The lower bound on environmental harm is the cost of building renewable energy sources that are used for something so unproductive.