More Detailed Information and Definitions

Power: expresses how fast energy is being transferred; it is measured in kW (1 kiloWatt = 1000 Watts = 1.36 horsepower); this can describe

  • The maximum power output of a motor (transferring energy from gas/diesel/battery to a transmission or the wheels); note that a gas or diesel engine has much higher losses than an electric motor because most of the energy in the gasoline or diesel fuel is lost to heat rather than converted to mechanical energy to move the vehicle

  • The power consumption of electrical appliances

    • Household LED light bulbs: 5-15W 

    • Small water kettle: ~1kW

    • Hot tub: ~5-10kW

    • Small EV going 60mph on flat surface at 70F: ~15kW

  • Charging speed: this may be limited by

    • How fast a charging station can deliver energy to a vehicle 

    • How fast a vehicle can accept the energy

  • Energy: measured in kWh, which is the energy required to run

    • A 1kW small water kettle for 1 hour

    • A 3kW heat pump for 20 mins

    • 100 5W LEDs for 2 hours

    • An electric motor at 15kW power output for 4 mins; this may correspond to a small/medium sized EV going 60mph on flat terrain at 70F. That vehicle would consume 15kWh every hour to cover 60 miles. If it had a 60kWh battery, it would be able to go ~240 miles under those conditions.

    • On average, US homes use almost 900 kWh per month; this number will increase as electrification progresses and transportation and home appliances move increasingly away from fossil fuels.

  • Battery capacity: typically expressed in terms of the stored energy, measured in kWh;
    Note that most manufacturers reserve a buffer to reduce battery degradation associated with fully charged or completely empty batteries. For instance, the Hummer EV has 212 kWh of “usable” battery capacity, as some of the 246 kWh capacity is always charged (to avoid a completely empty battery) and some is never charged (to avoid a fully charged battery). It is not always easy to determine if the quoted battery sizes refer to actual battery capacity or usable battery capacity.

    • 24 kWh for many early EVs (early Leaf, e-Golf, Fiat 500e)

    • 40 or 62 kWh for recent Leaf

    • 66 kWh for Chevy Bolt

    • 98 or 131 kWh for Ford F-150 Lightning

    • 246 kWh for Hummer EV

  • Vehicle efficiency: expresses how much energy the vehicle expends per distance at conditions defined by the EPA. 

    • Watt-hours per mile (Wh/mi): in line with many countries’ metric (liters/100km); lower is better; for instance, a Tesla Model 3 LR expends 260 Wh/mi, or 26 kWh to cover 100 miles.

    • Miles per kWh (mi/kWh): in line with US metric (miles/gallon); higher is better; for instance, a Tesla Model 3 LR is rated at 3.8 mi/kWh (=1 mile per 260 Wh)

    • MPGe (Miles per gallon equivalent): since a gallon of gasoline contains ~33.7 kWh of energy, this metric is used to show how far an electric vehicle can drive on 33.7 kWh of electric energy. Since electric motors convert energy much more efficiently than gasoline or diesel engines, they can travel much farther on the energy contained in a gallon of gasoline. A vehicle that uses 260 Wh/mile can go ~130 miles on 33.7 kWh (33700/260=130), so it would be rated at 130 MPGe.

  • Charging

    • Charging will provide energy [kWh] to the vehicle. The range provided by that energy depends on vehicle efficiency and driving conditions: the same energy will provide much lower range for an EV driven at 80 mph than at city speeds, and heavier EVs will require more energy for the same distance than smaller, lighter EVs. Typical range per kWh varies from 2.5 - 4 (mi/kWh).

    • Level 1 (L1) charging uses a regular household outlet (120V), ideally on a separate circuit with no other appliances. The vehicle’s inverter converts residential AC to DC to charge the battery. Typical power is up to 1.44 kW (12A x 120V), though some vehicles allow lower limits. For efficient vehicles, that translates into ~5 miles of range for each hour of charging.

    • Level 2 (L2) charging uses a 240V wall unit, typically on a dedicated circuit. The vehicle’s inverter converts residential AC to DC to charge the battery. Typical power is 7 - 10kW (30 - 40A), but can reach as high as 19 kW (~80A); some vehicles have tighter limits (early Leafs accept either 3.3 kW or 6.6 kW, depending on trim level). An efficient EV charging at 7.2 kW will gain 25-30 miles of range for each hour of charging.

    • DCFC (direct current fast charger): often referred to as “Level 3” or DCQC (direct current quick charger); these are non-residential stations with an internal inverter to provide direct current at higher voltage; they can provide between 40kW (early stations) to 350kW of power, with higher-power installations in the future. Different vehicles have different maximum charge rates that can dramatically affect how much energy and range is added in 30 mins. Charging costs vary by location and charging network, and sometimes by time of day to account for cheaper off-peak electricity rates.

  • Time of Use (ToU): this is a tariff offered by some electric utility where the electricity rate varies throughout the day and week. In most cases, overnight power is cheaper since there is excess power available (more generation than demand), but rates increase above the normal rate during times of high demand (on-peak, the most expensive rate, is often from 5-8pm, but it varies by season and location). Weekends are often mid-peak or off-peak. Different utilities use different names for similar schemes (Time of Day, Peak tariff, EV charging plan, etc.).