Fleet managers are no strangers to fuel efficiency. Years of experience with gasoline and diesel vehicles have given them a strong sense of how much fuel is needed per mile and per route. But the transition to electric vehicles (EVs) brings a new set of variables: different units of measure, unfamiliar sources of energy loss, and a new way of thinking about fueling costs.
With liquid fuels, nearly every gallon pumped ends up in the tank—the EPA considers fueling losses near zero in its analyses. EV charging, by contrast, always involves some degree of energy loss. Not all the electricity drawn from the grid makes it into the battery, and that difference has real implications for operational costs.
From Grid to Usable Battery Energy
To capture this reality, industry standards such as SAE J2953/4 use the term Usable Battery Energy: the energy that is actually stored in the battery and available for driving. Looking at charging sessions through the lens of overall efficiency—from grid input to usable battery energy—helps fleets avoid underestimating their true electricity needs.
Consider a concrete example. According to Car and Driver, in Tesla’s 2021 EPA certification tests for the Model Y, the vehicle consumed 87.868 kWh of input energy to store 77.702 kWh in the battery when charging from 0% to 100% on a Level 2 AC charger. That translates to an charging efficiency of 88.4% from charger output to usable battery energy.
Where the Energy Goes
So where does the remaining 11.6% go? Two main factors stand out:
- AC-to-DC conversion losses. AC-to-DC conversion happens either in the vehicle’s onboard charger during AC charging or within the external charger during DC charging. Top-performing onboard chargers achieve around 92% efficiency, while newer DC fast chargers can reach about 97%.
- Battery resistance and thermal management. Some energy is lost as heat inside the battery due to electrical resistance. More energy is also used by the thermal management system, which warms the battery in cold weather and cools it in hot weather to keep charging safe and efficient. A study of electric school buses found that about 96% of the energy stored could be used later on average—but temperature and charging speed can shift that percentage.
Why EV Charging Efficiency Matters for Fleets
These losses don’t erase the cost advantages of EVs, but they do matter for planning. The electricity a fleet purchases is always greater than the energy ultimately available for driving. Ignoring this gap can lead to under-budgeting, mis-sizing infrastructure, or miscalculating operational costs.
At Electrada, we model charging efficiency in detail. By accounting for real-world energy losses, we help fleets forecast electricity requirements and costs with accuracy. We also monitor vehicles in operation to flag outliers—which can point to performance or equipment issues—and apply these insights when planning new sites with similar vehicle profiles.
For fleets, the takeaway is clear: EV charging efficiency is an operational factor you can’t afford to overlook. Understanding where energy goes ensures accurate budgeting, smarter infrastructure decisions, and reliable fleet performance.
Vince Wyborski is a Solution Intelligence Engineer at Electrada. To learn more about Electrada Electric Fuel, the most reliable and cost-effective way to power an EV fleet, click here. Or get in touch to discuss how Electrada can tailor an Electric Fuel solution for your fleet.
