William F. Pickard has published an article in “Energy Policy” titled “Where renewable electricity is concerned, how costly is “too costly”? (Hidden behind paywall).
One interesting point he makes is an estimate on how much work it would take for human muscle power to generate the equivalent of 1 kWh. His result is that eight hours of a person weighing 65 kg working a treadmill at the equivalent of climbing four 3.75 meter storeys of stairs per minute would only result in 1.27 kWh. That means that one person could generate by very hard work not more than one kWh per day.
Whatever energy system the future brings, it will be quite impossible to run modern society on muscle power. The Bible says that King Solomon had 70.000 workers to carry stones for his Temple, which would work out to 70 MWh a day. Yesterday, Germany used 654 GWh, or about by a factor of 10.000 more energy than those 70.000 workers could provide.
Another way to see this is that the 30.200 MW solar capacity Germany now has can provide as much energy at peak as around 30 million people working at full capacity for eight hours.