from the paper:

> Training proceeded [...] using 5,000 first-generation TPUs

> We evaluated the fully trained instances of AlphaZero against Stockfish, Elmo and the previous version of AlphaGo Zero [...]. AlphaZero and the previous AlphaGo Zero used a single machine with 4 TPUs.

So the 5000 TPUs processing power or energy consumption should be compared with those spent in devising Stockfish, not running it.