At a computer store in downtown San Francisco, a five-shelf cabinet normally filled with graphics cards — popular add-ons that soup up PCs — stands bare.

It’s a strange but true sign that the cryptocurrency craze is bleeding into the real world — with speculators taxing electrical grids and silicon supplies in their quest to make digital money.

“We need as many as we can find,” said Donnie Williams, 36, a Novato resident, as he and his business partner David Miller, 47, of Pleasanton, paid $1,000 for the only graphics card in stock one day last week at Central Computers near the Moscone Center. The two needed the card to add to five they had already installed in their computing rig inside Miller’s home.

People who help mine, or create, bitcoin and other cryptocurrencies are snapping up the processors as the currencies have soared in value. Whenever people buy and sell these digital coins, the transaction is verified through a series of complex mathematical computations. Whoever does that math earns some currency themselves. The high-end chips in graphics cards process those computations faster, accelerating miners’ earnings.

The mining is also taxing real-world energy systems: The Energy Institute at UC Berkeley’s Haas School of Business notes studies that show bitcoin mining around the world is consuming more energy per year than the country of Bulgaria.

College students are using free dorm electricity to make a fortune. On Wednesday, Stanford University issued students a stern warning against using campus power or computers to mine cryptocurrency, reminding students, “Stanford resources must not be used for personal financial gain.”

The skyrocketing demand for graphics processors is good news for companies like Santa Clara’s Advanced Micro Devices and Nvidia, which make them. But shortages and price rises have angered other users, like video game enthusiasts, who rely on the cards to make sure the latest graphics-intensive games scream across their monitors.

Dan Sherman of St. Louis said he planned to build a new gaming computer, which he needed for his job as a video game commentator and streamer for an e-sports organization.

“It was super shocking to see the prices go up,” Sherman, 32, said in an email. “I was not expecting this at all. It’s really disappointing that I’ll have to wait to move forward with my plans, or maybe won’t get a chance to at all.”

The video game news site Kotaku has blamed miners for causing “the great graphics card shortage of 2018.”

“Gamers are resentful of miners,” said Craig Yeung, general manager of the six-store Central Computers chain, based in Santa Clara. His stores are capping sales at four per customer to try to keep some inventory available for regular customers, like businesses or video game players.

Caught by the shortage are engineering and architectural firms, which use increasingly sophisticated 3-D and virtual reality software for computer-aided design work. Video game and movie special-effects studios also rely on the graphics processor units inside high-end workstations.

But Yeung said he receives email from cryptocurrency miners around the world seeking as many graphics cards as they can get.

The recent run-up in cryptocurrency prices — bitcoin is now worth $11,000, up from $1,000 a year ago though down from a recent peak near $20,000 — has created a gold-rush atmosphere for small-scale miners like Williams and Miller, who have spent about $12,000 on equipment in the last month to cash in on the cryptocurrency craze.

Typically, miners will connect several cards together to work as one custom-built machine, with a motherboard, cooling fans, power converters, monitor and software. The rig will run all day and all night, processing the complex transactions that combined, make up the community-run accounting system for transactions in bitcoin, Ethereum, ripple and other new currencies.

These rigs plug into an elaborate online ecosystem designed to keep decentralized cryptocurrency transactions honest. The miners play a vital role in verifying transactions. For their efforts — if their computers are fast enough to keep up — they are rewarded in cryptocurrency.

“For me, it’s like investing in stocks and gambling at the same time,” Miller said. “It’s got that excitement. Ever bet on a horse race? You root for your horse, if it comes in, it comes in. If it doesn’t …”

And so, the top-rated video graphics cards using processors made by Advanced Micro Devices and Nvidia are hard to find in stores, and the shortages have caused prices to rise. Miller and Williams, for example, bought an Nvidia GeForce GTX 1080 card that used to cost about $600, said Central Computers store manager Candy Kwong.

Because of the rush, Central Computers limits purchases to four cards per customer. Nvidia itself has placed a two-per-customer limit on orders online. Both Nvidia and AMD declined to comment for this story, citing SEC regulations requiring a “quiet period” before they release quarterly earnings reports.

Prices online for top-range AMD and Nvidia cards have almost doubled in the past two months, said Phillip Carmichael, founder of PCPartPicker.com, an Austin, Texas, company that tracks prices of parts for customers who build their own computers.

“Our users are certainly not happy with the current situation, but there’s not much to do right now besides wait it out,” Carmichael said in an email.

Eugene Chung, CEO of San Francisco’s Penrose Studios, said his startup relies on graphics cards to create virtual reality animation stories. He doesn’t see the shortages as having an immediate impact on his business, since capital costs can be spread out over time. But he does wonder whether rising graphics-card costs will affect his business in other ways. To run virtual reality headsets, consumers need PCs with powerful graphics cards. If those get more expensive, it could tamp down growth in the virtual reality market.

“My biggest concern is for consumers,” Chung said.

But it’s also hard to predict what will happen in the future because the cryptocurrency market “is very transitory,” said Gregory La Blanc, a lecturer at UC Berkeley’s Haas School of Business.

Small-scale miners like Miller and Williams have to contend with large-scale mining operations in places like China, Russia and Iceland, where electricity is cheaper and the cool climate keeps the chips from overheating, he said. The recent surge in bitcoin’s price — it reached $19,783 in mid-December before falling sharply — has drawn more people into the market despite tough competition.

“Ultimately, it sounds like the Gold Rush in California in the 1840s, when you could show up with a pickaxe and a pan and make money in the first couple of years,” La Blanc said. “But the big mining companies showed up with hydraulics and heavy-duty machinery and steam engines and put these little guys out of business fairly quickly.”

Such prospects are not putting off Rod Mohammed, 25, of Riverside, who dropped by Central Computers the day after Miller and Williams. The only GTX 1080 card in stock, which a clerk had placed in the otherwise barren graphics card section moments earlier, was already priced $50 higher.

He said he’d been scouring computer stores for those elusive cards: “Fry’s, Best Buy, I mean everywhere, and nobody’s got them.” Mohammed has built a rig with six graphics cards he bought last month, when they were $670 each.

“When I saw (the value of) bitcoin shoot from a thousand bucks to $20,000, I said, ‘I don’t want to miss this, I don’t want to be that guy who’s slapping himself in the face for not jumping in at the right time,’” he said. “And if you figure you can make over a thousand bucks a month extra profit, why not, right?”

Ultimately, he didn’t buy the lone card available. “One doesn’t do me any good, you’ve got to get six of these,” he said. “I don’t know why anyone would want to buy just one, unless they’re just gaming.”

Benny Evangelista is a San Francisco Chronicle staff writer. Email: bevangelista@sfchronicle.com Twitter: @ChronicleBenny

Bitcoin mining explained

A bitcoin is a digital token that you can send electronically to anyone in the world. A bitcoin is a type of cryptocurrency — the general term for any currency whose value derives from computer code rather than the backing of a physical good or government guarantee.

No single person or company runs the network on which bitcoin is exchanged. Instead, transactions are recorded by computers on a digital ledger. Because the transactions aren’t centrally stored, people have to verify them by running complex mathematical equations — essentially, solving a puzzle.

Solving the puzzle is known as “mining,” and it yields a prize in the form of more bitcoin, the reward for verifying transactions. The cost of mining is computing power; generally speaking, the more computing power a miner uses, the better the chance of solving the puzzle and getting the prize.

That computing power has a price — the cost of the hardware and the electricity to run it. Crescent Electric estimates that it costs $6,200 in California to mine one bitcoin, based on the current price of electricity in the state. A bitcoin is currently worth almost twice that, making for a nice profit margin.

The microprocessors in standard PCs aren’t as good at solving equations as the specialized processors found on graphics cards. By rigging graphics cards together, miners improve their odds.

— New York Times and Chronicle staff