The current energy consumption of the planet is 113,000Twh (according to Wikipedia). Since every single Joule of renewable energy is some derivative of solar energy (solar, wind, tide, hydro, but not geo I suppose) the maximum energy we can derive from renewables is 765,000Twh.

The problem with that, is if we start to consume 10's of percent of the total solar radiation through "renewables" that would otherwise go into generating weather and other natural events, well I'm sure you can see the potential problems.

So, we have to get away from carbon intensive electricity generation, but we can't physically rely solely on renewables. Therefore we need fission/fusion.

There's obviously the case of our current economic system causing us to overuse energy in the name of profit (oil is so important because it makes energy cheap and thus easier to make profits), and a change in production/consumption/distribution priorities would likely cause huge decreases in energy needs globally. But we can only really consider energy needs based on what we know.

Whoops, I forgot the "achtually".