Tariff schedules are getting more complex, thanks in part to the Energy Policy Act of 2005, which was passed in August 2005. One clause in particular reads:
"Not later than 18 months after the date of enactment of this paragraph, each electric utility shall offer each of its customer classes, and provide individual customers upon customer request, a time-based rate schedule under which the rate charged by the electric utility varies during different time periods and reflects the variance, if any, in the utility's costs of generating and purchasing electricity at the wholesale level. The time-based rate schedule shall enable the electric consumer to manage energy use and cost through advanced metering and communications technology."
Time of Use rates (TOU) charge you for your electricity based upon the time you consume it. For example, PG&E's schedule E-7 has a base rate of 8.664 cents/kWh for summer off-peak usage (6pm - noon) and 29.372 cents/kWh for summer peak usage (noon - 6pm). So the energy you use 3pm costs you more than 3x the same amount used at 7pm.
BTW, TOU rates have been around in the commercial/industrial sector for over 25 years.
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