Charging Losses and MPGe: What the EPA Test Accounts For
- EPA notes MPGe includes charging losses to better reflect energy use from the outlet.
- FuelEconomy.gov's dataset notes MPGe and kWh values include charging losses.
- For budgeting, pair MPGe with kWh/100 miles and your $/kWh.
What We Know (Sourced)
EPA's fuel economy and EV range testing overview explains that plug-in electric vehicle efficiency is reported using metrics like MPGe, and it notes that MPGe values include charging losses to represent energy use from the outlet.
FuelEconomy.gov's download page also notes that for plug-in vehicles, MPGe and kWh values include charging losses in its datasets.
EPA provides EV label documentation (interactive and text versions) that helps consumers interpret MPGe and kWh/100 miles on the label.
Why Charging Losses Exist
When you charge an EV, not every kWh drawn from the wall ends up as stored battery energy. Some energy is lost as heat in the charging process (in the vehicle and/or the charging equipment). Those losses matter for budgeting because your utility bill is based on what you draw from the outlet.
How to Use MPGe and kWh Metrics Correctly
MPGe is useful for cross-fuel comparisons, but kWh/100 miles maps more directly to electricity cost.
If you're comparing EVs (or an EV to a gas vehicle), a practical workflow is:
- Use kWh/100 miles and your effective $ / kWh to estimate cost per mile.
- Use MPGe for a single "efficiency" number when comparing across fuel types.
Related guides:
What's Next
- For accurate budgeting: compute your effective $/kWh based on where you charge (home vs public).
- For accurate efficiency: track your real kWh/100 miles across seasons, especially winter. Related: EV range in cold weather.
- For data pipelines: use FuelEconomy.gov downloads or web services, noting that plug-in MPGe/kWh include charging losses per their documentation.
Frequently Asked Questions
Does MPGe represent energy from the outlet or from the battery?
EPA notes MPGe values include charging losses to represent energy use from the outlet. FuelEconomy.gov's dataset notes likewise state that plug-in MPGe and kWh values include charging losses.
If MPGe includes losses, why do my real costs still differ?
Real costs depend on your $/kWh (time-of-use, demand charges, public charging fees), temperature, speed, and accessory use. Track your real kWh/100 miles and your effective charging price to get a realistic cost-per-mile estimate.
Should I use kWh/100 miles instead of MPGe?
For budgeting, kWh/100 miles maps directly to electricity use and cost. MPGe is helpful when comparing across fuel types as a single efficiency metric.