You could in theory supplement it with rooftop solar and batteries, especially if you can get customers who can curtail their energy use easily. Datacentres have a lot of roof space, they could at least reduce their daytime energy costs a bit. I wonder why you don't see many doing solar, do the economics not work out yet?

I'd have to do the math, but I doubt that makes sense given the amount of power these things are drawing. I've heard of DCs having on-site power generation, but it's usually in the form of diesel generators used for supplemental or emergency power. In one weird case, I heard about a DC that used on-site diesel as primary power and used the grid as backup.

Compared to their volume they absolutely do not: you get about ~1kW / m^2 of solar. Some quick googling suggests a typical DC workload would be about 50 kW / m^2, rising too 100 for AI workloads.