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Consumption vs Efficiency
Efficiency is a relative term, indicating how much energy is used per distance or time interval, and is measured in units such as miles per kWh or kWh per 100 miles. Consumption, on the other hand, is an absolute term, describing simply how much energy was used. For an ICE vehicle, the consumption is how many gallons of gasoline were used, and for an EV, how many kWh of electricity were used. In ICE vehicles, consumption and efficiency are very closely related because gasoline is used directly to move the vehicle, whereas generated electricity is used for the auxiliary components (e.g. lights, radio).
In an EV, on the other hand, electricity is used for all functions as it’s the only source of energy. You can see in the image below, from a Chevrolet Bolt EV, that only 97% of the energy used is for driving, while another 3% goes to climate controls.
In most EVs, the high voltage battery also powers climate control, lights, and on-board systems. This means that a fraction of your “fuel” is used to support the vehicle and driver, and will not be directly related to miles driven (i.e. fuel efficiency). In an ice vehicle, gasoline is used to move the vehicle, while the 12V battery is used to operate the auxiliaries. As a result, the fuel efficiency is nearly directly proportional to the number of miles driven, with the exception of a small portion of fuel going towards the alternator system.
Additionally, some electricity is lost while the battery is charging - this is called charging loss, and includes electricity lost to heat, and that is used by your battery management system to regulate the battery’s temperature and rate of charge. You may see when you charge that you add 20 kWh, but only have an additional 18 kWh of drivable range. The 20 kWh is part of the energy efficiency calculation, while you can only consume the 18 kWh that is actually added to the battery.
Consumption vs. efficiency also captures energy lost to phantom drain, or the electricity required to keep your EV maintained when not in use. For some vehicles, such as Chevy Bolt, there is very little phantom drain and what you see available when you turn your car off is what you see when you turn it back on. Teslas, on the other hand, have onboard diagnostics and telematics systems that use a fair amount of electricity even when not being used.
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