Electrification of our driving is a key step in reducing greenhouse emissions.
Reports in the US and the UK found that for every mile driven with electricity,
it produces around 65% fewer CO2 emission than driving with gas.
And the reductions will only improve
as the electricity grid includes more renewable sources over time.
As we transition to fully electric transportation,
Plug-in Hybrid Electric Vehicles (or PHEVs)
combine all-electric driving with a traditional gas engine
and can be effective options to electrify more of our driving with much smaller batteries.
With the battery materials from one full Battery Electric Vehicle,
you can produce batteries for about 4 to 5 PHEVs.
A good scenario for owning and using a PHEV would be
for you to plug your car in to charge every day overnight and in the morning you will have
about 20 to 50 miles [30 to 80 km] of all electric range for commuting
to and from work or running your errands.
For many owners, this range is sufficient for their routine driving to be completely all-electric
(with no tailpipe emissions). And they don't need the much bigger battery and
200+ miles of range of a typical BEV.
However, if you exceed the electric range of the PHEV,
the gas engine will turn on and the car will operate as a gas hybrid car,
generating tailpipe emissions until you return home or plug it in again.
So the emissions of a PHEV very much depends on user behavior,
in particular, how often you charge it and drive it in electric mode.
Previous reports from Europe highlighted
how real-world greenhouse emissions from PHEVs were higher than expected,
indicating that owners were not plugging them in as often as expected.
For example, PHEV drivers in Germany plugged in their car
only about 3 out of every 4 days they drove their car.
But this study included company cars with subsidized fuel and
car models that were older and had smaller electric ranges than PHEVs you can buy today.
Prior studies looking at the US produced mixed results,
some studies showing that owners frequently plugged in,
while others showed that it could be much improved.
So we set out to get a more modern snapshot of how people in the US use their PHEVs.
So our research questions were, first
What routines and habits for charging have owners in the US established?
Next, how do the factors like charging infrastructure, environmental issues,
and cost concerns impact charging behavior?
And how do owners drive their PHEVs?
To answer these questions, we use a mixed method approach
combining an online questionnaire with 488 PHEV owners
who drove modern PHEVs
to have them self-report their charging access, frequency, effort, and habits.
From these participants, 40 of them were selected
to participate in a diary study where they completed daily activities
that captured their experiences of driving and charging their PHEV.
15 of the diary study participants
were selected for one-hour long semi-structured interviews that dug deeper
into the reasons behind their behaviors
We used a "prototype as conversation" approach where we generated storyboards of prototype interventions
for charging and efficient driving and used them as probes
to better understand owners’ needs and also for owners to co-design new solutions with us.
For details on the analysis method, check out the paper.
Now we’ll just jump right into the results...
For charging, owners reported they charged frequently.
In fact, 89% of owners reported charging at least once per driving day,
which is the frequency needed to electrify most routine trips.
Charging also was reported as a strong habit,
with a median Self-Reported Habit Index score of five and a half out of seven.
This was also supported by interview participants
who explained that they built a habit of plugging in every time they returned home,
especially those who had a consistent parking space near an outlet.
It had become so automatic and routine
that it required little conscious thought or effort to plug-in everyday.
When we showed intervention concepts for reminding people to charge at home,
they reacted negatively saying they didn't need reminders for something they already did.
So even though we went into this study assuming that people would forget to plug in at home,
we found home charging was actually something of a pretty strong habit.
Access to home charging infrastructure played a big role in this,
it was associated with being seven times more likely to charge daily
and is correlated with a lower perceived effort of charging
than those who did not have reliable home charging.
Also stronger habits were found for those with access to home charging and workplace charging,
but not with access to public charging.
In fact, those who primarily relied on public charging infrastructure perceived charging
as being much more effortful.
In fact, our study identified many issues with public charging for PHEV owners,
despite recent infrastructure investments in the US.
Most owners perceived that charging infrastructure was lacking in the areas they regularly visited.
And even if they find a charger, it might be broken or occupied by another car, which is pretty frustrating.
Plugging in would only gain a few additional miles, and it wasn't worth the inconveniences,
such as the charger being far away from the entrance
or feeling unsafe because it's in a dark place,
or dealing with setting up accounts across different charger networks.
Another interesting finding was how much owners really enjoyed driving
in the all-electric mode as much as possible to avoid using gas.
69% of the miles driven were reported to be in electric mode.
Many owners drove in a hypermiling sort of way
by defaulting to EV mode, driving gently, avoiding high speeds,
even taking local roads (adding time to their drive)
to make it home before their electric range ran out.
It was a kind of self-competition
to squeeze the most miles out of a single charge.
Some owners liked the intervention concepts that showed streaks of electric only driving.
So in many ways, they were learning how to drive an electric car efficiently
but without the actual threat of being stranded with a dead battery
like they would be with a BEV.
Another interesting finding was that usage of PHEVs
was strongly influenced by financial factors,
much more so than by pro-environmental reasons.
For example, owners would choose the energy source that is cheaper.
If running on electricity was cheaper, they would charge it.
If running on gas is cheaper, they would drive with gas.
The owner in this quote did the math and found
that driving on electricity from away charging
was about one and a half cents more expensive per mile than driving with gas,
so he just drove with gas
And this was a participant who described himself as
being "very passionate about environmentalism...",
he works in renewable energy,
and yet wasn't willing to pay just a little more to run on electricity.
So financial factors are strong influences for usage behaviors.
Based on these findings, we identified some future opportunities for interventions.
We found that forgetting to charge at home is not really a common problem
for PHEV owners, if they had access to a power outlet at home.
Instead, we highlight two opportunities for supporting owners in
navigating public charging and highlighting the cost benefits of charging.
We can help owners become more familiar and comfortable
with ever-improving public charging infrastructure.
For example, the prototype app we developed can proactively point out new public chargers
when drivers arrive at the destinations they regularly visit
and also point out the amount of range they can gain while they shop or dine,
and this can apply to both new PHEVs owners
as well as potential owners even before they purchased an EV.
Also providing the precise location and availability of a charger
with navigation instructions within a shopping center
can also help owners actually find these public chargers.
Another opportunity is to leverage owners' cost sensitivity to motivate more green driving and charging.
For example, instead of just showing the cost of electricity after each charging session,
this prototype app can show the cost savings of driving the recharged range on electricity vs gas,
to reinforce that using electricity is not only greener but also cheaper and more convenient.
The app can also amplify people’s sense of self competition
to see how long they avoid visiting a gas station or even
friendly competitions within households.
To wrap up, the key takeaways from our work are that
owners charged regularly if they had reliable home charging
so we should support away-from-home public charging for new and prospective EV owners
owners were particularly sensitive to the costs of operating and purchasing their PHEV
so we should build systems that illustrate the costs savings of charging and driving efficiently
and finally owners strongly preferred driving in all-electric mode as often as they could,
so PHEVs can be a good stepping stone to BEV ownership.
Before we go,
we’d like to acknowledge the coauthors
and others who contributed to this work.
Thank you.