State regulators this month approved a $1.1 million annual “low-carbon” surcharge for Black Hills Energy’s Wyoming customers — an overall 0.67% increase that will be added to monthly bills beginning in February.

That’s just the beginning of what could add up to $1 billion in additional costs for the company’s 145,000 customers in the state, and more than $2 billion for Rocky Mountain Power’s Wyoming customers, according to preliminary filings with the state, if the utilities move forward on a state-imposed mandate to retrofit coal-fired power plants with mechanisms to scrub carbon dioxide from smokestacks.