For Gold Rush prospectors in 1860s California, profits came before the planet. Money-hungry miners devastated fields and waterways with hydraulic mining technology as they covered fertile farmland with silts and sediments. Forests were hacked down and rivers dammed dry — all for the prize of precious gold nuggets.

Fast forward 150 years, and some Silicon Valley successors are treading similarly mucky tracks. Bitcoin, the upstart digital currency that has surged to a high price of $15,000 on Thursday, is under scrutiny for its exorbitant electricity bill.

Critics call the currency dirty for a simple reason. Each bitcoin — a virtual nugget buried under near-impenetrable algorithms — takes huge computational energy to 'mine'.

After the value of a bitcoin shot up tenfold during 2017, analyst website Digiconomist estimated its annual electricity demand to be about 32 terawatt-hours (TWh). That puts it in the same league as entire nation states, sandwiched somewhere between Serbia and Denmark.

Digiconomist's analysis is not the first to compare bitcoin's electricity draw to a country — mathematicians at Maynooth University pegged this as high as Ireland's in 2014 — but it is perhaps the most-cited.

Bitcoin mines consist of hundreds or thousands of humming computers stacked on shelves

Yet estimates vary wildly. Last month, software company SetOcean calculated electricity consumption to be about half of Digiconomist's figure. Back in July, entrepreneur Marc Bevand had it at around a quarter. And in January, Harald Vranken of the Open University put it even lower still.

So how much electricity does bitcoin really use?

Decentralized and anonymous

Energy estimates are tricky because bitcoin has no central authority.

Individual miners — not governments or banks — use specialist hardware to chip away at algorithms recording transactions across the network.

When miners strike gold, and add a new record to the "blockchain" ledger of transactions, they are rewarded with fragments of a freshly-minted bitcoin.

For each miner, the energy required to do this depends on two things: the complexity of the algorithm and the efficiency of their machines.

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Digiconomist analyst Alex de Vries, whose Bitcoin Energy Consumption Index model has faced criticism for some of its assumptions, said a more "optimistic" estimate can be simply calculated: Divide total computational power by that of the most efficient mining machines, and see how many fit in the network.

Analysis by DW using this approach produced a lower-bound estimate of more than a gigawatt of power across the entire bitcoin network for all of these calculations.

"This method ignores significant factors like cooling in large-scale operation, and older generations [of machines] completely," de Vries said, "but still leaves us with about 100 [kilowatt-hours, or kWh] per transaction instead of the 250 kWh I'm reporting."

Put into perspective, about 300,000 bitcoin transactions take place every day. Over a year, an electricity bill of 100 kWh for each equates to half of all the electricity consumed by Nigeria last year.

Not keeping pace

Vranken said his January estimates of less than half a gigawatt were "definitely not as much as a country consumes — but since then, things have changed."

Part of the confusion comes from bitcoin's meteoric price jumps, which have rendered many energy estimates out-of-date.

As the price of bitcoins rises, so too does the economic incentive to cash in on the craze. The resultant frenzy — which has been described as both a gold rush and a bubble waiting to burst — leads to more computer calculations, or hashes, per second.

"The hash rate in 2014 was about 300,000, and at the start of 2017 it was more like 2 million," said David Malone of Maynooth University.

In that period, top-shelf hardware got about five times as efficient at computing hashes, Malone said, outweighing a significant fraction of the increase in energy costs.

But now, the hash rate has leapt up to almost 12 million — but there "doesn't seem to have been big hardware advances in efficiency this year."

Clean or coal?

There are also concerns over bitcoin's electricity demand being supplied by fossil fuels.

More than half of bitcoin "mining pools" are based in coal-reliant China, according to a study by the Cambridge Centre for Alternative Finance (which was sponsored by Visa).

Coal still makes up about two-thirds percent of China's energy mix

A Digiconomist analysis of a bitcoin mine in Inner Mongolia — where electricity was sourced from coal-powered plants — estimated one bitcoin transaction could have the same carbon footprint as a passenger flying on a Boeing 747 for an hour.

But some analysts note that the economic incentive for low-cost energy leads many to opt for renewables. "A significant concentration [of Chinese mining pools] can be observed in the Sichuan province, where miners have struck deals with local hydroelectric power stations to access cheap electricity," wrote Garrick Hileman of Cambridge University.

Troubling scale of impact

Clean or dirty, Ireland or northern Nigeria — regardless of the difficulty in tracking down exact figures, many fear the order of magnitude is troubling enough.

Even though bitcoin is only eight years old, it already accounts for somewhere between 0.05 and 0.15 percent of global electricity demand.

For a service used by just 3 million people, bitcoin is vastly less efficient than the current global banking system.

"It's a ridiculous number either way," said de Vries. "Bitcoin isn't sustainable, no matter how you measure it: We'd only be arguing whether it's 10,000 or 20,000 times worse than Visa."