Most people take energy for granted and have trouble visualizing what exactly “1 kWh” means.
It is interesting to join 5 million people and look at this Youtube video of Olympic Cyclist Robert Förstemann spending three minutes on a bike generating enough energy to toast one slice of bread (0.021 kWh generated).
How many times would he need to do this to power one Bitcoin transaction?
That obviously depends on how much energy one transaction needs on average. The best estimate I could find was in a January 2016 blog post by Marc Bevand. He estimates that it took 51 kWh for one transaction in August 2014 and that this number went down to 18 kWh in January 2016.
The reason for improving these numbers is that mining hardware used less energy per hash as it improved over time and that the number of transactions per day increased.
Extrapolating these trends, Bevand estimates the energy cost per transaction go down to between 4.5 and 6.8 kWh until January 2018.
If we just call it 10 kWh right now, we see that the Olympic Cyclist would need to get on his bike around 470 times to get one transaction through right now. Or that you could toast 470 slices of bread for the energy cost of one transaction.