The hardest part is finding the raw data. I only considered the energy costs of actually producing the currency itself (mostly imputed by operations cost, since I couldn't find any reports on the actual energy consumption of the mints or engraving presses). I realize purists will object to not including the amortized cost of the facility, etc., but the number that's being compared against for Bitcoin focuses only on the cost of the proof-of-work and not the similar facility costs.