There's a completely insane amount of energy available from solar power if you put the solar power collectors in orbit (which makes them harder to protect, but also increases the amount of energy input). There are still engineering/technical questions to be solved here, and even resource limitations (e.g., running out of the raw materials out of which to make the solar collectors) but I don't think that either energy budget or land surface area, per se, needs to be very much of a limitation in the long run.