Temperatures won't be the only thing rising this summer. O'ahu homeowners will be paying more for air conditioning as well.

They can expect to pay at least one-fifth more to maintain their cool if pricing trends remain constant in coming months. For some people with large central air conditioning systems, the increase could mean more than $50 in added costs per month. It's because most O'ahu electricity is from petroleum-fired generators, and oil prices are way up.

Summer officially begins today, bringing with it higher temperatures and a jump in electricity usage as people flip on air conditioners. During the past quarter century, the number of homes with some form of cooling systems — ranging from window units to central air conditioning — has more than tripled to 54 percent of O'ahu homes, according to Hawaiian Electric Co.

Jenny Miranda, 35, came to Hawai'i two years ago from Oregon and bought five window units for her 'Aiea home. "We moved in the middle of summer, and it was just horrible," said Miranda.

Last summer, Miranda's utility bill was as much as $275 a month during the hottest weather, or about $75 more than the rest of the year. That $275 would probably jump by about $60 this year if she wasn't moving to a house with central air conditioning.

Hawaiian Electric's generators typically work the hardest during August, September and October, driven in part by rising air conditioner use. The company expects the same this year, even with the higher costs.

"It's not a luxury item anymore. It's more of a necessity," said Aldrin Villahermosa, president of Honolulu-based AMV Air Conditioning Inc. People want to avoid hot summer temperatures and also see air conditioning as a way to boost their home's value, he said.

Yet people also are becoming more mindful of air-conditioning costs. In May, the typical residential bill was $124.04, or 21 percent higher than a year earlier mostly because of climbing fuel costs that are passed directly through to customers. Hawai'i residents, on average, pay more for electricity than any other state and more than twice the national average, according to U.S. Department of Energy data.

"They want air conditioning, but they want me to approximate how much it's going to cost to operate," Villahermosa said. "They're more aware of these things than before."

So much so that many people are electing to go with systems costing $300 to $600 more than comparable units that are less efficient. Systems with higher Seasonal Energy Efficiency Ratios don't use as much electricity, and thus may have be cheaper in the long run to own and operate.

"We're selling more energy- efficient units than we ever have," said Neil Weidemann of the Aire Conditioner Shoppe in Waipi'o. Drew Santos, president of Admor HVAC Products on Sand Island, also has seen the switch. In the past year, he's seen an increase in people looking at energy ratings.

"It used to be whoever had the lowest price sold the job," Santos said. "Now people are looking for price but also efficiency and features."

Hawaiian Electric also is encouraging use of energy-efficient air conditioning as part of its conservation program. It has produced a brochure with tips about saving electricity while lowering home temperatures.

People can learn about how solar-powered attic fans reduce heat, or how to get the right-sized system. For now, the brochure is available at the utility's offices and through Heco.com.

Hawaiian Electric also is offering rebates to businesses that install more efficient air-conditioning systems.

Reach Greg Wiles at gwiles@honoluluadvertiser.com.