A Maryland utility that charges higher per-gallon rates for homes that use more water discriminates against larger families, a Montgomery County resident alleged in a recent complaint to a state panel.

The Washington Suburban Sanitary Commission, one of the country’s largest water and sewer utilities, prices water based on consumption — a “tier-based” system that many utilities use to encourage conservation. The more water people use, the higher price they pay for each 1,000 gallons.

However, unlike other water utilities, WSSC applies the most expensive rate to the entire bill — going back to the first drop — rather than only for the water used in the costlier tiers. That unusual approach can leave a household with six people paying almost twice as much per 1,000 gallons as a neighboring home with one occupant, critics say.

Richard Boltuck, a Bethesda resident and retired economist, said he filed the complaint July 8 with the Maryland Public Service Commission, which regulates utilities, because he said he believes WSSC’s rate structure violates the state standard for “reasonable” rates. Sure, more people in a home will use more water, he said, but they shouldn’t have to pay higher prices per gallon.

Boltuck said he and his wife pay about $155 for their quarterly WSSC bill. But if they lived in separate homes, he said, they would pay only $119 combined for the same amount of water because they would each be paying lower rates. That disparity gets multiplied over thousands of homes that WSSC serves, he said.

“We’re talking about massive amounts of discriminatory charges based solely on the size of a family, which social policy doesn’t seek to punish,” Boltuck said in a Tuesday conference call with reporters, organized by Del. Alfred C. Carr (D-Montgomery).

Carr said the higher rates also lead to disproportionate jumps in water bills, such as during harsh winters when people home on “snow days” run faucets and flush toilets more than usual.

Because people must pay more for the entire quarter once they bump into a higher-priced tier, Carr said, “A modest increase in usage can result in a major increase in your bill.”

Officials for WSSC, which serves nearly 2 million people in Montgomery and Prince George’s counties, say the utility will reexamine the 16-tier rate structure it has had since the late 1970s. However, WSSC spokesman Jim Neustadt said that won’t happen for another three years because WSSC first must upgrade its antiquated billing system.

Neustadt said WSSC realizes that water conservation efforts — particularly federally mandated low-flow toilets and shower heads — have been so successful that consumption nationwide is dropping, even as the population grows. WSSC also appreciates lifestyle changes that have resulted in larger households, he said.

“We realize it’s time to look at things,” Neustadt said.

But he added, “If someone is going to be paying less, then someone else will be paying more — who’s that going to be?”

Neustadt noted that a 2013 study by an Annapolis-based consultant, Municipal & Financial Services Group, found that WSSC’s rate structure was “markedly unique” among U.S. utilities by applying the highest-tier rate to all water used. However, the consultant recommended keeping the pricing as is, saying it had “not been able to identify any glaring issues” with it.

Utilities’ rate structures differ significantly, even in the Washington region.

D.C. Water charges residents one rate for the first 3,000 gallons per month and then a higher rate for anything over that.

Fairfax Water charges one base rate, regardless of the amount of water used. However, customers are charged a higher “peak use” rate from June to November if they exceed their average winter usage by a certain amount. The peak rate only applies to the additional water used, a spokeswoman said.