That's because the cost for often exorbitantly priced imported power the province needs in times of high demand is a floating rate charged separately from domestically produced power.

"You can get a very mistaken impression of what you're paying for power during those periods when we are importing," said Tom Adams of Energy Probe.

"It is right now impossible for consumers to see what price they're actually paying for the juice."

Ontario is highly dependent on its neighbours during peak periods - such as occurred during recent heat waves.

At those times, the province routinely went outside its borders for about 10 per cent of its daily power, a substantial amount.

That power routinely ranges between $1 and $2 a kilowatt-hour, 25 to 50 times the usual going price.

On a customer's bill, the extra cost for imported power is rolled into a range of charges known as "uplift" charges.

Consumers now pay a fixed 0.62 cents a kilowatt-hour for those charges based on a projected estimate of what the various costs will be over a year.

However, that price was set before the decision was made to separate the costs of imported and domestic power and the actual uplift will only be known at the end of the year.

Although, theoretically, the uplift cost might end up being lower than expected, Charlie Macaluso of the Electricity Distributors Association said people on fixed-price contracts could well be in for shock.

"There are significantly high uplift charges that are accumulating that are probably beyond what many may have expected," said Macaluso.

"Those people who are on a fixed-rate contract will potentially see a significant increase in their bill . . . because the import charges were not protected with that fixed rate."

The Independent Electricity Marketing Operator - or IMO - the provincial agency that manages the amount of power flowing through the province, said it's monitoring the uplift costs.

"So far, they're tracking to be within that 0.62," said spokesman Ted Gruetzner.

"But it was always understood that it was a flexible cost."

It's impossible to know what the imported power component of the uplift charge is at any given moment because of the time lag in calculating the final price.

However, to address criticism about a lack of transparency, the IMO on Thursday began listing an estimate on its Web site.

Bob Hunt of the Ontario Energy Marketers Association said marketing companies are concerned about a potential customer backlash.

"They recognize that they will be asked about additional charges and unfortunately it's not something that's within (their) control," said Hunt, adding that many large industrial users are also unhappy with the pricing system.