Indoor marijuana cultivation consumes enough electricity to power 2 million average-sized U.S. homes, which corresponds to about 1 percent of national power consumption, according to a study by a staff scientist at the Lawrence Berkeley National Laboratory.

Researcher Evan Mills' study notes that cannabis production has largely shifted indoors, especially in California, where medical marijuana growers use high-intensity lights usually reserved for operating rooms that are 500 times more powerful that a standard reading lamp.

The resulting price tag is about $5 billion in annual electricity costs, said Mills, who conducted and published the research independently from the Berkeley lab. The resulting contribution to greenhouse gas emissions equals about 3 million cars on the road, he said.

Narrowing the implications even further reveals some staggering numbers. Mills said a single marijuana cigarette represents 2 pounds of CO2 emissions, an amount equal to running a 100-watt light bulb for 17 hours.

"The added electricity use [to an average home] is equivalent to running about 30 refrigerators," Mills wrote. "Processed cannabis results in 3,000 times its weight in CO2 emissions. For off-grid production, it requires 70 gallons of diesel fuel to produce one indoor cannabis plant, or 140 gallons with smaller, less-efficient gasoline generators."

Mills went on to compare an average pot-growing facility to the electric power intensity of a data center. In California, which is the top producing state and one of 17 states to allow medical use of marijuana, cultivation accounts for 3 percent of all electricity use and 8 percent of household use, he said.

Mills added that he completed his research with no external sponsorship, insisting that he does not mean to pass judgment on the merits of cannabis cultivation or use. He also suggested that the minimal amount of information available and the almost complete lack of regulation of the industry mean energy consumption could easily be lowered.

"If improved practices applicable to commercial agricultural greenhouses are any indication, the energy use for indoor cannabis production can be reduced dramatically," he said. "Cost-effective efficiency improvements of 75 percent are conceivable, which would yield energy savings of about $25,000/year for a generic 10-module growing room."

Mills, a member of the U.N. Intergovernmental Panel on Climate Change, drew his data from open literature and interviews with horticultural equipment retailers.

By some estimates, marijuana has long been the largest cash crop in the United States -- ahead of corn, soybeans and hay. The industry has been pegged at about $40 billion in value, with California, Tennessee, Kentucky, Hawaii and Washington the top five production states.

Water consumption is also an issue when it comes to environmental impact, with each marijuana plant said to need between 3 and 5 gallons of water per day to grow to fruition.

Click here (pdf) to read Mills' study.

Sullivan is based in San Francisco.

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