How Plug-In Hybrids Are Rated: Utility Factor Explained

A plug-in hybrid (PHEV) has two sets of numbers on its EPA label: fuel economy in MPGe when it runs on electricity, and MPG when the battery is empty and it runs on gasoline. A single “combined” value is built by weighting the two, using what the EPA calls a utility factor. The result depends on how much of your driving can be done on electricity.

Key takeaways

• The left-hand label box gives fuel economy on electricity in MPGe, the right-hand box gives MPG on gasoline once the battery is emptied.
• EPA tests two modes: charge-depleting (starting with a full battery) and charge-sustaining (starting with an empty one).
• A utility factor estimates the share of driving done in the electric mode, and it is used to weight the two results.
• The more of your miles you can cover on electricity, the closer your result is to the electric numbers.

What is on the label

The EPA’s plug-in hybrid label describes fuel economy when operating on electricity in miles per gallon of gasoline-equivalent (MPGe), and fuel economy once the battery is emptied in MPG. It also shows kilowatt-hours per 100 miles, which the label explains as the number of kilowatt-hours the vehicle would use to travel 100 miles. The all-electric range is an approximate number of miles that can be traveled in combined city and highway driving when fully charged. Charge time is how long it takes to charge a fully empty battery using 240-volt service.

For the units themselves see what is MPG and MPGe and kWh per 100 miles and EV efficiency.

Charge-depleting and charge-sustaining

The EPA’s testing summary describes two operating conditions:

• Charge-depleting. The test starts with a fully charged battery and ends when the battery is discharged. It follows the same kind of procedure as for electric vehicles. If the gasoline engine runs during this mode, both the electricity and the gasoline used are included in the MPGe value for this mostly electric operation.
• Charge-sustaining. This test starts with a discharged battery and the vehicle is tested like any other conventional hybrid, with the engine doing the work and the battery level held roughly steady.

fueleconomy.gov notes that plug-in hybrids come in configurations. In one, only the electric motor turns the wheels and the gasoline engine generates electricity. In a parallel or blended configuration, both the engine and the motor are connected to the wheels, and electric-only operation usually occurs only at low speeds.

What the utility factor does

Drivers do not use the electric mode for every mile, and a vehicle with a short electric range cannot cover long trips on electricity. To produce one combined number, the EPA uses a “utilitized” value, which its label guidance describes as a weighted combination of the charge-depleting MPGe and the charge-sustaining MPG. The weighting factor is the utility factor: an estimate of the fraction of miles driven in the charge-depleting mode. In general, a vehicle with a longer electric range has a higher utility factor, because more typical trips fit within that range.

Think of it as a share. If a PHEV’s utility factor were 0.5, half of the assumed miles would be driven on electricity and half on gasoline. EPA’s plug-in hybrid testing follows SAE Recommended Practice J1711, and the combined values on the label come from that standardized method rather than from a driver’s own habits.

How Gallonly turns this into a cost

Fuel cost for a plug-in depends on both fuels. Gallonly multiplies the utility factor by the electricity cost per mile, adds the remaining share times the gasoline cost per mile, and then applies the annual mileage. When a model uses gasoline in charge-depleting mode too, that gasoline is added. The inputs come from the EPA label data and a dated price for each fuel. The method is on the methodology page, and how annual fuel cost is calculated covers the basic arithmetic.

Where the real world differs

The utility factor is an assumption about typical driving, and your own pattern may differ. A driver who charges every night and travels short distances can use electricity for far more of their miles than the average. A driver who rarely plugs in will rely on the gasoline numbers, and fueleconomy.gov adds that PHEV fuel economy can be sensitive to driving style, driving conditions and accessory use. fueleconomy.gov also says using electricity is usually cheaper than using gasoline, and charging times vary: a 120-volt outlet can take several hours and a 240-volt charger about 1 to 4 hours, with some fast charging to 80% in as little as 30 minutes.

If you charge rarely, treat the MPG on gasoline as the more relevant number. If you charge daily and your trips are short, the electric figures matter more. The hybrid savings calculator and the EV vs gas calculator let you try different assumptions.

How it differs from other vehicles

A conventional hybrid never plugs in and is rated in MPG only; an electric vehicle has no gasoline numbers at all. The differences, and what each means for cost and convenience, are compared in hybrid vs plug-in hybrid vs EV. Range questions are covered in EV range explained.