besides cars mass energy storage isnt available for all countries yet, so you need a way to balance grids. green energy resources doesnt have the ramp up required when energy usage spikes(e.g people get back from work). we dont suddenly put out more sun, make faster rivers or purposely make stronger winds for power. you balance the spike with the dirtier energy. so you often run into a situation where once green, a majority of your energy can be green, but you always have to reserve space for spike usage.