The batteries needed are a lot less than you might think. Solar doesn't work at night and the wind doesn't always blow, but we have tons of regional weather data about how they overlap. From that, it's possible to calculate the maximum historical lull where neither are providing enough. You then add enough storage to handle double that time period, and you're good.

Getting 95% coverage with this is a very achievable goal. That last 5% takes a lot more effort, but getting to 95% would be a massive reduction in CO2 output.