This SAE Information Report establishes a "Utility Factor" (UF) curve and the method of generating these curves. The UF is used when combining test results from battery charge-depleting and charge-sustaining modes of a Plug-in Hybrid Electric Vehicle (PHEV). Although any transportation survey data set can be used, this document will define the included UF curves by using the 2001 United States Department of Transportation (DOT) "National Household Travel Survey" and a supplementary dataset.

In use, the fuel and energy consumption rates of a PHEV vary depending upon the distance driven between charge events. For PHEVs, the baseline assumption regarding any UF is that operation starts fully charged and begins in battery charge-depleting mode. Eventually, the vehicle must change to a charge-sustaining mode. The vehicle miles traveled between charge events determine how much of the driving is performed in each of the two fundamental modes. A second assumption is that charging occurs every day after the day's driving is complete, i.e. once per day. In the absence of PHEV driver behavior data, the two unknown driver behavior issues of (1) how often charging occurs during the day ("opportunity charging") and (2) how often a driver will forget to charge, are assumed to be an equally offsetting, thus the baseline assumes one charge per day of operation.

Given the previous assumptions, a UF describes the fraction of driving in each of the fundamental modes using a given set of recorded in-use driving data. Driving statistics from the 2001 National Household Travel Survey and a supplementary dataset are used as inputs to the UF creation to provide curves applicable to a vehicle's charge-depleting mode results.