Hawaiian Electric Co. had signed up 671 customers for its new time-of-use rate program as of a week ago, according to its website. The program will charge residential customers less for power used during the day — when solar energy production is highest — and more at night. The program began a few weeks ago. The program, developed under the direction of the Hawaii Public Utilities Commission, encourages customers to use power when solar energy is abundant and enables cost-effective integration…