Where the Energy Goes: Electric Cars
About 74%–94% of the energy used to power an electric car is used to move it down the road, depending on the drive cycle. Electric cars are more efficient than comparable conventional vehicles, especially in stop-and-go driving, due to the use of regenerative braking and start/stop technologies—see All-Electric Vehicles for details.
Still, some of the energy is lost in charging the battery, braking, and powering accessories such as air conditioning and heating.
![Energy Requirements for City (Stop and Go) Driving: Energy Lost in Charging the Battery (16%), Parasitic Losses (4%), Net Regenerative Braking Energy Returned to the Battery and Subsequently to the Road (32%), Power to Wheels (62% + 32% [recovered] = 94%), Electric Drive System Losses (18%), Idle Losses (near 0).](images/energy_reqs_city_ev.jpg)
Energy requirements in this diagram are estimated for stop-and-go city driving using the EPA FTP-75 Test procedure.
The electric drive systems of electric cars are much more efficient than the engine and transmissions of conventional vehicles. Still, some of the vehicle's energy is lost through drivetrain inefficiencies.
When charging the battery, energy is lost in converting alternating current (AC) from the electrical grid to direct current (DC) for use in the battery, as well as in overcoming the battery's resistance to charging, which increases as the battery reaches its capacity.
Power steering, air conditioning, heating, and other accessories use energy. This estimate does not include losses from heating or cooling, which can be significant in extreme temperatures.
Braking Losses
When you apply the brakes in a conventional vehicle, energy initially used to overcome inertia and propel the vehicle is lost as heat through friction at the brakes.
Elecric cars use regenerative braking to recover some energy that would otherwise be lost in braking, making them more efficient than comparable conventional vehicles, especially in stop-and-go traffic.
Wind Resistance (Aerodynamic Drag)
A vehicle expends energy to move air out of the way as it goes down the road—less energy at lower speeds and more as speed increases.
This resistance is directly related to the vehicle's shape and frontal area. Smoother vehicle shapes have already reduced drag significantly, but further reductions of 20%–30% are possible.
Rolling Resistance
Rolling resistance is a resistive force caused by the deformation of a tire as it rolls on a flat surface.
New tire designs and materials can reduce rolling resistance. For cars, a 5%–7% reduction in rolling resistance increases fuel efficiency by 1%, but these improvements must be balanced against traction, durability, and noise.
Electric cars reduce idling by turning the engine off when the vehicle comes to a stop and restarting it when the accelerator is pressed.
This makes them more efficient than comparable conventional vehicles in city driving, which includes a significant amount of idling.
Electric cars use regenerative braking to recover energy typically wasted in braking. Since more braking takes place in stop-and-go traffic, they are most efficient in city driving.
When you apply the brakes, the vehicle's inertia turns an electric motor-generator, producing electricity that is then stored in a battery. The electricity can later be used to power the electric motor, which supplies power to the wheels.
![Energy Lost in Charging Battery (16%), Parasitic Losses (1.5%), Net Regenerative Braking Energy Returned to the Battery and Subsequently to the Road (6%), Power to Wheels (68% + 6% [recovered] = 74%), Electric Drive System Losses (14%), Idle Losses (none). Highway driving does not include significant idling.](images/energy_reqs_hwy_ev.jpg)
Energy requirements in this diagram are estimated for the EPA Highway Fuel Economy Test procedure (highway driving with an average speed of about 48 mph and no intermediate stops).
When charging the battery, energy is lost in converting alternating current (AC) from the electrical grid to direct current (DC) for use in the battery, as well as in overcoming the battery's resistance to charging, which increases as the battery reaches its capacity.
The electric drive systems of electric cars are much more efficient than the engine and transmissions of conventional vehicles. Still, some of the vehicle's energy is lost through drivetrain inefficiencies.
Power steering, air conditioning, heating, and other accessories use energy. This estimate does not include losses from heating or cooling, which can be significant in extreme temperatures.
Braking Losses
When you apply the brakes in a conventional vehicle, energy initially used to overcome inertia and propel the vehicle is lost as heat through friction at the brakes.
Electric cars use regenerative braking to recover some energy that would otherwise be lost in braking.
Since there is little braking in highway driving, regenerative braking offers little advantage over a conventional vehicle on the highway.
Wind Resistance (Aerodynamic Drag)
A vehicle expends energy to move air out of the way as it goes down the road—less energy at lower speeds and more as speed increases.
This resistance is directly related to the vehicle's shape and frontal area. Smoother vehicle shapes have already reduced drag significantly, but further reductions of 20%–30% are possible.
Rolling Resistance
Rolling resistance is a resistive force caused by the deformation of a tire as it rolls on a flat surface.
New tire designs and materials can reduce rolling resistance. For cars, a 5%–7% reduction in rolling resistance increases fuel efficiency by 1%, but these improvements must be balanced against traction, durability, and noise.
Highway driving includes little to no idling. The EPA highway driving cycle (HWFET) includes no idling.
Electric cars use regenerative braking to recover energy typically wasted in braking.
When you apply the brakes, the vehicle's inertia turns an electric motor-generator, producing electricity that is then stored in a battery. The electricity can later be used to power the electric motor, which supplies power to the wheels.
However, since there is little to no idling on the highway, regenerative braking provides less benefit in highway driving.
![Energy Requirements for Combined City/Highway Driving: Charging Losses (16%), Parasitic Losses (2.5%), Net Regenerative Braking Energy Returned to the Battery and Subsequently to the Road (17%), Power to Wheels (65% + 17% [recovered] = 82%), Electric Drive Losses (16%), Idle Losses (near 0).](images/energy_reqs_comb_ev.jpg)
Energy requirements in this diagram are estimated for 55% city and 45% highway driving. See the estimates for city and highway driving for more information.
When charging the battery, energy is lost in converting alternating current (AC) from the electrical grid to direct current (DC) for use in the battery, as well as in overcoming the battery's resistance to charging, which increases as the battery reaches its capacity.
The electric drive systems of electric cars are much more efficient than the engine and transmissions of conventional vehicles. Still, some of the vehicle's energy is lost through drivetrain inefficiencies.
Power steering, air conditioning, heating, and other accessories use energy from the battery. This estimate does not include losses from heating or cooling, which can be significant in extreme temperatures.
Braking Losses
When you apply the brakes in a conventional vehicle, energy initially used to overcome inertia and propel the vehicle is lost as heat through friction at the brakes.
Electric cars use regenerative braking to recover some energy that would otherwise be lost in braking.
Wind Resistance (Aerodynamic Drag)
A vehicle expends energy to move air out of the way as it goes down the road—less energy at lower speeds and more as speed increases.
This resistance is directly related to the vehicle's shape and frontal area. Smoother vehicle shapes have already reduced drag significantly, but further reductions of 20%–30% are possible.
Rolling Resistance
Rolling resistance is a resistive force caused by the deformation of a tire as it rolls on a flat surface.
New tire designs and materials can reduce rolling resistance. For cars, a 5%–7% reduction in rolling resistance increases fuel efficiency by 1%, but these improvements must be balanced against traction, durability, and noise.
Electric cars experience negligible energy losses from idling in combined city-highway driving.
In city driving, the electric motor stops when the vehicle stops. It doesn't waste energy idling like the engines in most conventional vehicles.
In highway driving, there is little to no idling.
Electric cars use regenerative braking to recover energy typically wasted in braking.
When you apply the brakes, the vehicle's inertia turns an electric motor-generator, producing electricity that is then stored in a battery. The electricity can later be used to power the electric motor, which supplies power to the wheels.
Estimates based on analysis of a 2012 Nissan Leaf.
Lohse-Busch, H., et al. 2013. Ambient Temperature (20°F, 72°F and 95°F) Impact on Fuel and Energy Consumption for Several Conventional Vehicles, Hybrid and Plug-In Hybrid Electric Vehicles and Battery Electric Vehicle. SAE 2013-01-1462.
Lohse-Busch, H., et al. 2012. Advanced Powertrain Research Facility AVTA Nissan Leaf testing and analysis.
Thomas, J. 2014. Drive Cycle Powertrain Efficiencies and Trends Derived from EPA Vehicle Dynamometer Results. SAE 2014-01-2562.