Ethereum, the world’s second-largest cryptocurrency by market value, is often lauded for its potential to enable more efficient and more secure transactions than its rivals. But a key question has been how much energy each transaction on the Ethereum network uses.

A new study provides some answers. Based on data from the past year, the research estimates that the average Ethereum transaction uses about 193 kilowatt-hours (kWh) of electricity—enough to power more than four U.

S. households for a day—and results in around 780 kilograms (kg) of carbon dioxide emissions.

That makes each Ethereum transaction about 40,000 times more energy-intensive than a Visa credit card transaction, and produces emissions equivalent to those from driving a car nearly 500 miles (800 kilometers).

The study’s authors say the findings underscore the need for Ethereum and other cryptocurrencies to find ways to reduce their energy use if they are to scale up and achieve widespread adoption. “It is hard to see how cryptocurrencies could play a significant role in global payments without a radical improvement in their energy efficiency,” says Camilo Mora, an environmental scientist at the University of Hawaii at Manoa and lead author of the new study, published in Nature Sustainability.

To be sure, Ethereum isn’t the only cryptocurrency with a large carbon footprint. A previous study estimated that Bitcoin transactions used about as much electricity as all of Ireland in 2016.

Other estimates put Bitcoin’s energy use even higher. And while Ethereum may be more efficient than Bitcoin, it is still far from being green.

The new study is based on data collected by Alex de Vries, founder of Digiconomist, a website that tracks the energy use and environmental impact of Bitcoin and other cryptocurrencies. De Vries estimates that each Ethereum transaction uses about 53 kWh of electricity—enough to power two U. households for a day—and produces around 200 kg of carbon dioxide emissions.

De Vries’s data come from two sources: estimates of the total amount of electricity used by all computers running Ethereum software, and estimates of how much processing power is needed for each transaction. The researchers used these data to estimate the electricity use and carbon dioxide emissions associated with an average Ethereum transaction.

The study also looked at other cryptocurrencies besides Ethereum, including Bitcoin, Litecoin, Monero, and Zcash. Overall, the research found that cryptocurrency transactions use as much electricity as Ireland does in a year—about 22 terawatt-hours (TWh)—and result in around 107 million metric tons (MT) of carbon dioxide emissions per year.

That’s equivalent to the annual emissions from 5 million passenger cars in the United States.

Cryptocurrencies have been criticized for their high energy use before. In 2018, for instance, reports surfaced that Bitcoin mining operations in China were using so much electricity that they were driving up power prices in neighboring countries.

And last year, researchers warned that Bitcoin could single-handedly increase global warming by 2 degrees Celsius by 2033 unless its energy use is curbed (see “What Bitcoin Is Really Doing to Our Planet”).

But Mora says he was surprised by just how energy-intensive cryptocurrency transactions are when he started looking into it himself. “I was not expecting such large numbers,” he says.

“I think it is important that we start paying attention to this now so we can avoid making decisions that will lock us into high-carbon infrastructure for many years to come.”.