The “utility factor” is phrase that comes up regularly when discussing Plug-in Hybrid Electric Vehicle (PHEV).
The utility factor (UF) is a measure of a vehicle’s ability to use its electric charge as fuel as opposed to gasoline. This is generally a proportion of the range on electric vs. the range on when gasoline-powered.
The utility factor might seem like a fuzzy metric, but is has a standard definition which was determined by the Society of Automotive Engineers J2841. This standard says that utility factor is officially the fraction of vehicle miles traveled on electricity power (abbreviated as “eVMT”) vs. overall miles traveled (“VMT”). The document was created in 2009 and revised in 2010.
Logically, a PHEV has a given range on electric power (officially known as “battery charge-depleting mode”). Once this is used, the vehicle must switch to gasoline and electric power (charge-sustaining mode) where power is supplemented by the engine as well as features like regenerative braking. The longer the drive between charges the more one mode will be used versus the other.
Utility factor is essentially the proportion of driving in one mode versus the other based on actual GPS data or larger surveys of household travel data.
Complexities Of Utility Factor
Utility factor is dependent on many factors including:
- Battery capacity (most importantly)
- Charge speed
- Vehicle weight
- Individual driving pattens
Utility factor is generally calculated on an individual basis as usage patterns will vary from driver to drive. It is devised by assuming a day where the car’s battery is fully charged in the morning and returns from the trip with the battery fully depleted.
Utility factors as delivered by FuelEconomy.gov, NHTSA, and other agencies tend to look at groups of dozens to hundreds of drivers of a specific car, measuring their activity with GPS data, and then calculating the electric usage vs. the gasoline usage. This does mean that certain factors will cause large variations in utility factor, such as the ability to charge the vehicle at an mid-day stop (usually a workplace).
FuelEconomy.gov uses utility factor in a number of PHEV measurements including:
- City UF – Utility factor (share of electricity) for a PHEV when city driving
- Highway UF – Utility factor (share of electricity) for a PHEV when highway driving
- Combined UF – Utility factor (share of electricity) for a PHEV when combined city and highway driving