National Grid yesterday proposed lowering electricity rates by about 6.6 percent, which would trim the bill of a typical residential customer by about $5.11 a month.

In a filing submitted to the Public Utilities Commission, National Grid said that declining natural gas and crude oil prices have lowered the projected costs of buying electricity for its customers next year, allowing it to lower rates.

The company proposed that the new rates become effective Jan. 1.

A typical customer, which National Grid defines as one who uses 500 kilowatt-hours of electricity per month, would pay $72.71 a month under the proposal, a savings of $5.11 over the current charge of $77.82.

National Grid has also proposed extending through next year an additional discount on its low-income rate, which cuts a total of $17 off the bill of a customer with typical usage. There are about 35,000 customers in Rhode Island who receive the low-income rate, the company has said. The extra discount was implemented last year to help counter the effects of big rate increases at the time.

National Grid provides electricity to about 477,000 customers in 38 of Rhode Island’s 39 communities.

The rate changes were submitted as part of an annual filing National Grid prepared for state regulators in which the company reconciles its costs over the past year with revenues it received from customers. It also takes into account anticipated costs and what it expects to receive from customers next year.

If approved by the PUC, the proposal would lower rates for the second time in four months. In September, National Grid decreased rates by 3.9 percent, which lowered the bill of a typical customer by about $3.12 a month.

Even with the proposed decrease, rates are still well above where they were before an unprecedented series of rate increases last year following damage to the oil and gas infrastructure in the Gulf of Mexico caused by Hurricanes Katrina and Rita. The combined effect of two increases — one in October, and the other in January — pushed up rates by 29 percent, or $18.18, for a typical customer. The rate decrease in September and the proposed decrease in January would still leave rates 16 percent higher than they were before the hurricanes.

Specifically, National Grid proposed lowering its “standard offer” charge to 8.3 cents per kilowatt hour, from 9.4 cents per kilowatt-hour. That charge represents the electricity portion of a customer’s bill. The proposed rate was based on futures prices on the New York Mercantile Exchange over a three-day period in late October.

National Grid said its projections show that it could have lowered the standard offer rate even further — to 7.9 cents per kilowatt-hour. Doing so would have trimmed an additional $2 a month off the bill of a typical customer.

The company said it chose not to lower the rate to that level in case energy prices spike again. The proposal will “provide customers with a significant reduction in monthly bills but takes into account the fact that fuel prices can change dramatically during the winter months,” National Grid said in its filing.

The company added that if prices remain stable, it will consider seeking another rate decrease after the winter.

The extension of a bigger discount on its low-income rate would be paid for by part of a settlement that National Grid reached last year with state regulators over two separate accounting disputes that date back to 1999. That settlement called for National Grid to return $8 million to its customers. About $2 million of that money was used to provide a bigger discount for low-income customers last year.

Yesterday’s proposal would use another $2 million from that pool to continue the larger discount next year.