Jul. 28--It's not often a company asks customers to use less of its product.

But doing so is common practice for NorthWestern Energy, said Tom Glanzer, communications coordinator for the utility company.

NorthWestern has plenty of capacity for customers to keep cool during summer's hottest stretches, Glanzer said. But, he added, it's still better when people conserve energy, especially during peak demand in the afternoon.

"Anytime there's a long, extended period of heat, we're going to see an increased demand on the load that people are using because more people are running air conditioning and more people are staying inside, where it's cool and comfortable," Glanzer said.

The National Weather Service office in Aberdeen forecasts highs of 100 degrees or higher through the weekend.

Cory Huber, resource mode dispatch coordinator for NorthWestern, said the utility company's base demand is usually 200 megawatts per hour. About 4 p.m. Thursday, demand was 290 megawatts per hour, he said. That's 45 percent above normal and even higher than when temperatures topped 100 degrees a couple of weeks ago. Then, Glanzer said, demand was 287 megawatts per hour.

Of the 290 megawatts used per hour Thursday, Aberdeen used between 56 and 57, Huber said.

Generally, he said, electricity demand on a Saturday or Sunday is 80 percent to 90 percent of that on a weekday, he said.

Huber said in an hour's time a megawatt can supply 400 to 500 homes with electricity. One megawatt is 1,000 kilowatts. An average home uses two to three kilowatts per hour, Huber said. In comparison, he said, a Wal-Mart store uses roughly a full megawatt in the same amount of time.

NorthWestern's system has a normal maximum capacity of 310 megawatts a day. When that total is exceeded, the utility study doesn't stop providing electricity. Rather, it has to buy more power at a higher price, Huber explained.

While hot days lead to high cooling bills for South Dakotans, things are worse in California. There, a lack of infrastructure has led to rolling blackouts -- times when there is no electricity available to consumers. There's no danger of anything similar in South Dakota, Glanzer and Huber said.

Glanzer said it's smart for people to keep their lights down and close their shades during the day. He also suggested keeping the thermostat at 78 degrees to conserve energy. That might seem warm, but when somebody enters a 78-degree house and it's 100 degrees outside, the home will feel cool, he said. Turning the thermostat up 2 degrees results in a 5 percent to 10 percent savings on an energy bill, he said.

A NorthWestern release offers other tips to help control cooling costs:

--Use a programmable thermostat, or set the thermostat warmer when nobody is home.

--Do not use ranges, stoves, dishwashers, clothes dryers or other heat-producing equipment during the mid-day.

--Adjust ceiling fans to turn counter-clockwise.

--Change your air filter. A dirty filter can use 5 percent to 10 percent more energy than necessary.

--Stay on the lowest level of your home.

High temperatures this month are driving record-breaking demand for electricity among NorthWestern customers in Montana, the company said. That's a change for NorthWestern, which typically sees its peak usage during winter.

"With the weather we've seen recently (in Montana), it's not really surprising," NorthWestern spokeswoman Claudia Rapkoch said. "Every little thing adds up to how much energy is used across our system."

The Associated Press contributed to this report.