This isn't a fantasy, there's several products that do this and they've been on the market for a few years.

Microsoft provide the api for knowing in advance when energy will be the greenest: https://api.watttime.org/faq/

Some utilities will give you rebates if you use a smart charger to charge at these times (since green energy is both cheaper and time sensitive).

https://emotorwerks.com/products/juicenet-software/juicenet-...