We didn't have means to convert it into electricity for the majority of human history or machines to run with that electricity that made it possible to produce enough to support all those people. We're able to do a lot of shit with just electricity now that used to require engines running fossil fuels

Please describe how sunshine alone will sustain this unchanged.

Because you're ignoring the hugely inefficient baseline of how much sunlight we have historically been able to harvest.

From this page, sunlight-to-biomass efficiency of wild plants is less than 1%. For crops specifically bred over thousands of years to grow as much useful biomass as quickly as possible in each season, the number ticks up to 2%. But even that number also fails to account for the periods in which fields lay fallow between seasons, or for crop rotation.

Then, once you have that biomass, converting it into a useful form of fuel for burning, or food for biomechanical energy, has huge efficiency losses along the way.

Meanwhile, solar panels achieve approximately 25% solar efficiency.

That's how we can collect far more energy from the sun today than at any point in the past. Biology is inefficient compared to photovoltaics.

Best to think of fossil fuels as a shot in the arm of the development of human civilization. On that timescale, It was always going to be temporary, whether due to climate change or simply the limits of availible fuel. We were always going to need to use it to bootstrap onto something else within a couple hundred years of the invention of the steam engine. The math is reasonable and yes, total solar power isnt a problem (storage and distribution is, but total availability isnt hard).

Who said we need to sustain it? Personally, I think we should reduce our energy use. Fewer private vehicles and more public transit, for example.