It's an accounting trick but it does direct funds to "green" power. However keep in mind that wind turbines require a certain amount of idle power sources that can be brought online quickly to make up for any loss of power and winds tend to blow mostly at night where there is the least demand of power. This means that wind can never be more than a certain percentage of the grid without adding additional, small, expensive and inefficiently sized generation plants, usually small natural gas turbines that one would hope remains turned off. You have to remember that fossil fuels are energy from the sun stored by nature over millions of years. Yes using hydrocarbons that have been sequestered for millions of years does destabilize our environment so fossil fuels isn't a good idea and we are using the fossil fuels at a rate that's many times that at which nature creates it, indeed we're literally the fossil fuels of the distant future. But there are more reliable sources of energy than current wind and solar, there is Thorium. Here you will get more information about electricity suppliers.