Many of the plug-in hybrid (PHEV) drivers in the Recurrent community are diligent about plugging in at night and using “all-electric mode” as frequently as possible. Many report that they rarely fill up the gas tanks on their PHEV, essentially using them as a short range EV, with 20-40 miles of battery range.

For some other drivers, the “electric” part of the PHEV is rarely used. In this case, they are actually worse than regular, mild hybrids, and as bad as some gas cars. Why is this true? The battery packs in PHEVs make them heavier than gas cars and HEVs, so they require more fuel - aka gas - to run in “ICE” mode. 

Many studies agree: PHEV’s green credentials don’t match what is advertised. Whether or not PHEVs are green vehicles depends heavily on who is driving the car and how they think about the technology. 

Why do people like PHEV?

Plug-in hybrids dispel many of the anxieties new EV drivers have about going all-electric, and can offer an easy entry to an electric drivetrain, including:

  • Cheaper to buy than a battery electric vehicle (BEV), but cheaper to operate than a gas car
  • Smaller batteries use fewer mined materials (e.g. lithium) and have a smaller initial carbon footprint than a BEV
  • Car makers can leverage existing factories, unions, materials, so they can be cheaper to produce en mass
  • No range anxiety or charging worries for drivers
  • The potential to drive emission free most of the time

But the last selling point is the rub. While PHEVs are considered “clean vehicles” per federal regulations and tax code (i.e. they are eligible for the federal tax credit programs), PHEVs are only clean if you actually plug them in. 

You have to drive PHEVs like a BEV in order to get the benefits of a BEV. 

The thing is – most of the time, people don’t. Even the testing procedures that calculate PHEV emissions and fuel economy grossly overestimate how “green” PHEVs are. And those calculations can lead people astray about environmental impacts and cost of ownership. Moreso, they may make “green car” rankings, such as those by the American Council for an Energy-Efficient Economy (ACEEE), less accurate, since they rely on reported emissions and fuel economy data. 

Even the most diligent PHEV drivers may not realize that there are certain driving modes for many cars, and that the specifics of your transmission may dictate whether your car is ever really emissions-free. 

  1. Charge depleting (CD) mode - this is the mode that is closest to a real electric car. The battery is propelling the vehicle. In some models, there is help from the gas motor to accelerate quickly or go uphill. 

As per EPA guidelines, cars should have minimal fuel consumption in CD mode, but several studies have found that automakers often underestimate how much fuel is actually used (Bieker et al., 2022; Dornoff, 2021a). In these cases, CD mode is “dirtier” than advertised. 

  1. Charge sustaining (CS) mode - this is when the gas motor recharges the high voltage battery while driving. The gas engine, rather than the battery, is the primary propulsion system. While the battery may assist the fuel economy of the vehicle, the car is acting as an ICE, or at best, a mild hybrid. 

In some cars, the driver can switch between CS and CD mode, while in many cars, putting on the “sport” or “transmission” mode will switch it into CS mode. Things like ambient temperature may make emissions higher, too. 

The new Volvo PHEVs have five drive modes. These cars were not part of the recent study.  

One worthwhile note is that models like the Chevy Volt or BMW i3 can actually be as green as advertised. This is because the engine is designed as a “range extender” system, and not a primary propulsion system, so they run exclusively on battery power (pure CD mode) until the battery is depleted. These models often achieve their EPA ratings for emissions and fuel use. 

How much less clean than advertised are PHEVs? 

Quite a bit. In early 2024, The European Commission issued a report on real world vehicle emission and fuel consumption numbers, based on data from on-board monitors in European vehicles. These devices measured data from gas, diesel, and plug-in hybrid vehicles and compared them to the official, WLTP-advertised values. The WLTP is an emissions and efficiency rating agency, like the EPA. 

The problem comes down to the testing protocols not being aligned with how people actually drive and fuel their PHEVs. 

“For the new plug-in hybrid electric cars registered in 2021, the average real-world CO2 emissions (139.5 g CO2/km) were only 23% lower than for conventional cars (180.3 g CO2/km), and 3.5 times (100 g CO2/km) higher than what the WLTP test indicated (39.5 g CO2/km)” 

The findings were shocking and in response, the WLTP tests are being updated and will go into effect in 2025, with revisions if required. 

This new study builds on previous work done in the US by the International Council on Clean Transportation (ICCT), which found that

“real-world electric drive share may be 26%–56% lower and real-world fuel consumption may be 42%–67% higher than assumed within EPA’s labeling program for light duty vehicles.” 

The ICCT found similar results in a 2022 study in the EU,

“for PHEVs owned by private individuals, the real-world fuel consumption is on average three times higher than the official WLTP values, while for company car PHEVs the fuel consumption is on average five times higher.”