Rhode Islanders will finally get some relief on their electricity bills.

National Grid has proposed reducing rates by about 13.7 percent as of Jan. 1, trimming almost $13 a month from the bill of a typical customer.

The proposal, which was part of a 255-page filing with the Public Utilities Commission on Friday, reflects the steep decline in the cost of crude oil and natural gas that has occurred over the past four months.

Crude oil futures are down 62 percent from the record high set in July, and are at the lowest price in almost two years; natural gas futures have fallen about 53 percent during the same period.

Though very little electricity is produced from oil in New England, many of National Grid’s long-term contracts with suppliers require it to pay extra for power if the market prices of natural gas or crude oil rise above a certain point.

The decrease is the first change in rates since an unprecedented rate increase of 21.7 percent in July.

National Grid is Rhode Island’s dominant utility, providing electricity to 477,000 customers in 38 communities and natural gas to about 245,000 customers in 33 communities.

The filing is National Grid’s annual reconciliation report in which the company “trues-up” its expenses with the money it received from customers during the previous 12 months. After taking those figures into account, the company projects its cost to buy power for its customers in 2009, and proposes a new rate to match those projections.

If the PUC adopts the proposal without change, a typical customer who uses 500 kilowatt hours of electricity per month will pay $80.60 each month, compared with the current charge of $93.44 a month. The current rate for power went into effect on July 15, a time when energy prices were at their peak.

(Natural-gas rates may decline as well, depending on the outcome of two proceedings now before the PUC. National Grid has proposed lowering the rate for the natural gas itself, but also wants to raise delivery charges, in part, to speed up the replacement of aging gas mains. The PUC is expected to decide both issues by Nov. 30, with the new gas rates going into effect on Dec. 1.)

The new electricity rate proposed by National Grid reflects a substantial decrease in its projected cost to buy electricity, offset by a smaller, but relatively high increase in its transmission charges.

Specifically, National Grid wants to lower the “standard offer” rate to 9.5 cents per kilowatt-hour, a decrease of 2.9 cents from the current rate of 12.4 cents per kilowatt-hour.

That rate represents the cost of the electricity itself and is passed on to customers without markup. National Grid does not profit from the standard offer rate.

That change alone would cut $14.50 a month off a typical customer’s bill. But the company said it needs to increase total transmission charges to 1.5 cents per kilowatt-hour from the current rate of about 0.98 cents per kilowatt-hour, a 56 percent increase.

National Grid said transmission rates must go up because it expects its own transmission expenses to rise by $32 million through the end of next year, mainly because of additional investments being made to the power grid in various parts of New England.

The company said it expects its transmission costs to total about $104 million next year. The vast majority of that expense –– about $77 million –– will go to ISO New England, the regional power grid operator.

The three largest projects ISO New England plans to work on next year are: a new transmission loop in southwest Connecticut; the installation of three transmission cables in Massachusetts from Stoughton to Boston; and a new substation in Wakefield, Mass.

tbarmann@projo.com