I have no experience with them, but from their website:

"Our web hosting company is 100% powered by green power

Acorn Host purchases Green Certificates monthly for the amount of energy that is used to run the servers, office equipment, lights, etc. Renewable Energy Certificates (also known as Green Tags or Green Certificates) are a simple way to effectively power a business (or your home) on 100% renewable energy resources, without changing electricity providers. The idea is to pay for green energy to be put into the power grid equal to the amount of energy you use."