Even before state regulators have considered a proposed 15.6-percent increase in electricity rates, National Grid now says it wants to increase rates even more.

National Grid told the Public Utilities Commission yesterday the increasing price of crude oil and natural gas has forced it to revise its proposed rate increase to 21.7 percent.

The request is the largest single rate increase the company has ever sought, and would put rates at their highest level.

National Grid has asked that the new rates go into effect July 15.

A typical customer, which National Grid defines as one who uses 500 kilowatt-hours of electricity each month, would pay a total of $93.44 a month, an increase of $16.67 a month from the current monthly bill of $76.77.

The revised proposal was necessitated by the continued rise in energy prices, the company said. Since its original rate hike request in mid-May, National Grid said that both natural gas and crude oil prices have increased more than 10 percent. Since November, when the current rates went into effect, the cost of both fuels have increased by more than 60 percent, the company said.

The rate increase would add to the already strained budgets of families who are coping with record-high gasoline prices and rising food costs.

In addition, National Grid has proposed raising natural gas rates by 10 percent.

National Grid is Rhode Island’s dominant utility company, providing electricity service to 477,000 customers in 38 communities and natural gas to about 245,000 customers in 33 communities. The company filed its revised rate-increase proposal with the state PUC yesterday afternoon.

The company acts as a buying agent for the vast majority of its customers, purchasing electricity and natural gas on their behalf through long-term contracts. It makes no profit from these rate increases.

The contracts that National Grid has with its electricity suppliers call for the company to pay more for power if crude oil and natural gas rise above a certain trigger point.

Crude oil futures are in record territory. Crude oil for August delivery rose yesterday by $1.72, or 1.2 percent on the New York Mercantile Exchange, to settle at $145.29 a barrel.

Regulators have little discretion when it comes to rate-increase requests that are based on rising fuel prices. State and federal laws allow a utility company to recover from customers what it pays for electricity or natural gas, as long as the company makes “prudent” buying decisions. The PUC could cut the proposed rate increase, or delay it. But eventually, customers will have to pick up the increased costs.

In yesterday’s filing, National Grid cautioned against delaying the increase, or implementing a lower one. It said that doing so may mean customers would face an even higher rate increase later.

“The company is concerned about the impact on ratepayers of a substantial increase in the standard offer service rate,” National Grid said in its cover letter accompanying the filing. “But it is also concerned about the prospect of accumulating significant under recoveries by the end of 2009, which will then need to be included in the calculation of the January 2009 standard offer service rate.”

The standard offer rate is the charge on electricity bills that cover the cost of the electricity itself. (The other charges –– transmission and delivery of the power –– will remain unchanged.)

About 85 percent of all electricity delivered by National Grid is billed under the standard offer rate, currently at 9.2 cents per kilowatt-hour. Yesterday’s filing seeks to increase that rate to 12.4 cents.

If rates remain unchanged, National Grid said that the amount of money it collects from customers will fall short of its own cost for electricity by $104 million by the end of this year. If the PUC were to approve the earlier rate increase of 15.6 percent, customer payments would still fall short by a total of $27.9 million by the end of the year.

Last month, in response to the initial 15.6-percent rate increase, Attorney General Patrick C. Lynch suggested that the PUC should consider revisiting National Grid’s entire rate structure to cut the company’s allowable profits in order to provide some relief to electricity customers.

“Ratepayers should not have to bear the full economic impact of spiraling fuel costs by themselves,” William K. Lueker, special assistant attorney general, in a position paper filed with the PUC.

In its own filing, National Grid said it disagreed.

“Although it is understandable that consumer advocates would find it very attractive to have the commission simply order the company to absorb electric power costs to mitigate bill impacts, the company is compelled to point out that applicable legal and constitutional standards do not allow for ratemaking to be determined based on a principle of ‘sharing the pain.’

“The company has a legal right to recover the cost of electric supply that it has purchased for the direct benefit and use of standard offer service customers.”

tbarmann@projo.com