As the legal marijuana market expands, so grows the energy consumption associated with an industry that depends on growing facilities with high-wattage lights and powerful cooling systems.

In 2012, a study from Evan Mills, Ph.D.—a scientist with the Lawrence Berkeley National Laboratory—found that legal indoor marijuana growing facilities accounted for 1% of national electricity use at a cost of roughly $6 billion per year, which compared to just $1 billion in energy costs for the pharmaceutical industry. Mills recently told Bloomberg that some of the bigger growing facilities operating today can use up to $1 million in power every month.

In Colorado, where residents have voted to legalize both medical and recreational marijuana, Bloomberg adds that more than 1,200 licensed growing facilities eat up roughly half of the state’s new power demands and, last year, those facilities combined to use about the same amount of energy as 35,000 households.

As Bloomberg notes, most marijuana growing facilities make use of powerful, environmentally unfriendly lighting systems that allow growers to churn out fresh crops year-round. In fact, the heat from the lights is often so strong that growers also need high-power air conditioning systems to keep facility temperatures at temperate levels. With no industry-wide regulators, growers have no standards for energy efficient design in their facilities, which creates unnecessary energy waste.

And, the marijuana industry’s energy consumption is only going to become a bigger problem in the coming years. Already, 23 states have legalized medical marijuana, while recreational pot is legal in four states and the District of Columbia. Next year, that number could grow even larger as roughly half a dozen more states (and maybe more) are expected to vote on bills to legalize recreational pot.

The increase in power consumption by the growing legal marijuana industry has led some lawmakers to demand that pot growers pay various types of special fees or taxes to balance the strain their high-consuming ways put on the environment. Last year, lawmakers in Colorado’s Boulder County said they would charge marijuana growing facilities a little more than 2 cents per kilowatt hour consumed. A similar tax has been put in place in Arcata, California, where Bloomberg notes “officials are banking $300,000 a year from an ‘excessive energy use tax’ that went into effect in October 2013.”