According to one widely cited website that tracks the subject, the Bitcoin network is consuming power at an annual rate of 32TWh—about as much as Denmark. By the site's calculations, each Bitcoin transaction consumes 250kWh, enough to power homes for nine days.
Naturally, this is leading to concerns about sustainability. Eric Holthaus, a writer for Grist, projects that, at current growth rates, the Bitcoin network will "use as much electricity as the entire world does today" by early 2020. "This is an unsustainable trajectory," he writes.
Global energy production obviously can't double in two years, and it would be an environmental disaster if it did. Fortunately, while the Bitcoin network consumes a ridiculous amount of energy, particularly on a per-transaction basis, the situation isn't as dire as critics like Holthaus claim.