The amount of energy required to create a parallel (fake) chains is so huge that it makes more sense to express it in kilograms.<p>Some back-of-the-envelope calculations based on total sustained mining power consumption in 2017, extrapolating back, using the good old E=mc^2, yields the average weight of a bitcoin of about 0.2mg.<p>This makes Bitcoin the most expensive substance today.
That is not the only flow of or reservoir of energy and/or mass associated with bitcoins.<p>Consider, carbon added to the atmosphere to make that energy, kg of fission products to do the same, mass of all the servers running Bitcoin software, mass of party favors left over after Bitcoin conference, etc. Any of those is much greater than 3kg.