Energy costs for running Hive are quite low, because it uses proof-of-stake instead of proof-of-work to determine who gets to produce the next block in the blockchain. Energy costs for running a regular server in the US are probably around $10/month. Witness computation requirements are very low, so these can even be run on very low power servers at much cheaper rates, whereas an API server will consume more energy because it has to handle all the API traffic (so probably around $10/month for those). So I'd guess that less than $20K per year is spent on energy costs for operating the Hive network, including API servers, and the cost could be lower if anyone was trying to optimize that cost.