Re: I've been pointing this out for years.
As with all other electricity "retailers" the green ones balance supply and demand. In the case of the green ones, their total supply to customers is balanced by an equal supply from renewable generators *over time*. At any given instant they may be either buying non-renewable energy from suppliers to cover a renewable shortfall, or supplying surplus renewable energy back to the grid.
It is a fallacy that there is no point in "going green" unless you can get 100% of your energy that way. Even 1% helps.