Do you not get charged different rates based on usage amounts? This is common across the world - where the first X KWH of usage per month cost a lot less than the next X.
Commercial contracts generally don't work that way, they generally pay the market rate (the current most expensive producer).
It means there is no way for an individual consumers behaviour to drive (or be driven by) the green energy proportion, as if the collective demand is too high then you pay the market rate for coal anyway.
