Electric costs at six-year low in Mass.

The size of the average ratepayer’s monthly electric bill in Massachusetts has shrunk to a six-year low, as utilities reduce rates because falling natural gas prices have made it cheaper to produce power, state energy officials said Thursday.

State data shows that, on average, the residential utility customer is now paying about $112 a month for electricity, down roughly 25 percent since 2006 when they were paying about $150 a month. The savings come as natural gas prices hover around their lowest point in about a decade. On Wednesday, natural gas closed just below $2.62 per million British thermal units, down almost 40 percent in the last 12 months.

Earlier this week, state utility regulators approved a nearly 16 percent decrease in electric rates for 1.1 million customers who get power from NStar, now a subisidary of Northeast Utilities. The cut is expected to save customers about $6 a month. At the beginning of the month, National Grid lowered its electricity supply charge, cutting a typical customer’s bill by an estimated $7.74.

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While state energy officials lauded the drop in fuel prices, state energy and environmental affairs secretary Rick Sullivan urged utility customers to support public investments in renewable energy sources such as wind and solar power. Currently, the state spends $22 billion a year on energy, according to the state, much of which is imported from outside Massachusetts.

“It is imperative that we take advantage of this breather in energy cost increases to redouble our efforts to bring on line new clean energy resources that do not rely on dirty fossil fuels,” he said in a statement.