Solar and Wind aren't fossil fuels. That said, we need a whole heck of a lot more of them to replace fossil fuels. But, as other commenters have stated, if you

Solar and Wind aren't fossil fuels. That said, we need a whole heck of a lot more of them to replace fossil fuels. But, as other commenters have stated, if you can charge these batteries during the day and use the power at night, then yes, you can replace the 24 hour nature of gas/coal.