Ten times last year, Judi Kinch, a geologist, got e-mail messages telling her that the next afternoon any electricity used at her Chicago apartment would be particularly expensive because hot, steamy weather was increasing demand for power.

Each time, she and her husband would turn down the air-conditioners — sometimes shutting one of them off — and let the dinner dishes sit in the washer until prices fell back late at night.

Most people are not aware that electricity prices fluctuate widely throughout the day, let alone exactly how much they pay at the moment they flip a switch. But Ms. Kinch and her husband are among the 1,100 Chicago residents who belong to the Community Energy Cooperative, a pilot project to encourage energy conservation, and this puts them among the rare few who are able to save money by shifting their use of power.

Just as cellphone customers delay personal calls until they become free at night and on weekends, and just as millions of people fly at less popular times because air fares are lower, people who know the price of electricity at any given moment can cut back when prices are high and use more when prices are low. Participants in the Community Energy Cooperative program, for example, can check a Web site that tells them, hour by hour, how much their electricity costs; they get e-mail alerts when the price is set to rise above 20 cents a kilowatt-hour.