According to Ars Technica:

One estimate suggests the Bitcoin network consumes as much energy as Denmark.

There are other sources that make similar claims:

The Bitcoin Energy Consumption Index tracker puts Bitcoin’s current estimated annual energy use at almost 32 terawatt-hours, roughly equivalent to Serbia’s yearly electricity consumption and ahead of at least 159 other countries, including Ireland and most nations in Africa.

The BitCoin Energy Consumption Index is apparently a common source. However, it seems that this article uses a lot of assumptions.

Does the Bitcoin network use about as much energy as Denmark or Ireland, as of late 2017?