Power

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There are a number of units used to describe power, energy and efficiency for electric vehicles.

These units are in three categories:

  • Power (Watts) describes the rate of flowing power at any moment - whether electrical power (charging a battery) or mechanical work (turning a driveshaft) - it is the rate at which work is done
  • Energy (kilowatts) describes a total amount of energy - whether stored (in a battery), supplied (from charging), or used up (when driving)
  • Efficiency (Wh/km or km/kWh) describes how much power is used to travel some distance (or vice-versa)


For example:

  1. If you plug your car into a charger that supplies 30kW of power, over the course of an hour, it will supply a total of 30kWh of energy into your battery.
  2. If you drive a Model X P90D, with a 90kWh battery, it will (hypothetically) take 3 hours to fully charge from this 30kW charger.
  3. If you then drive with an efficiency of 200 Wh/km, you will use 1kWh of energy every 5km, and use all 90kWh after 450km.

However: in the real world, systems are not 100% efficient or predictable. Energy is lost to heat; chargers vary in their output; different batteries have subtly different capacities; battery capacities change over time; charging slows down when you get closer to 100%. Likewise, the efficiency of a vehicle depends on a number of factors, including speed, driving style, weight/load, wheel size, road conditions, temperature and wind.


SI Units[edit]

SI units are the standard, scientific metric units. It's best to use these, as they are universal and easily converted.

SI Unit Description Example Equivalent to
kW Kilowatt 1kW of flowing power A supercharger can supply 120 kW of power 1 joule per second
kWh Kilowatt hour The total energy that 1kW of power supplies in 1 hour A Model S 90D can store approximately 90kWh of energy in its battery

The drive from London to Bristol used 37kWh of energy
While I was getting my coffee, the supercharger added 30kWh to my battery

3.6 × 106 joules
1,000 watt hour
2.247 × 1025 electronvolts
8.598 × 105 calories

Wh/km Watt-hours per kilometre The total energy used per kilometre of driving
(similar to miles-per-gallon)
When driving on the motorway, I use 211 Wh/km
km/kWh Kilometres per kilowatt-hour The distance you can travel for each kWh of energy stored in your battery When driving on the motorway, I use 4.7 km/kWh

Note that Watts per hour (W/h) is a unit of the rate of change of power per hour. For example, a power plant might ramp-up from 0MW to 2MW over the course of 30mins, representing a rate of change of 4MW/h. This is different to a "4MWh" energy total.

Multiples[edit]

These SI units can be increased/decreased by changing their prefix.

Prefix Power Energy
None 1 W 1 Wh
kilo 1kW = 1000 W 1 kWh = 1000 Wh
Mega 1MW = 1000 kW 1 MWh = 1000 kWh
Giga 1GW = 1000 MW 1 GWh = 1000 MWh
Tera 1TW = 1000 GW 1 TWh = 1000 GWh
Peta 1PW = 1000 TW 1 PWh = 1000 TWh

Horsepower[edit]

Traditional ICE vehicles measure their mechanical power output in horsepower.

Brake horsepower (bhp) is the power measured at the crankshaft just outside the engine, before the losses of power caused by the gearbox and drivetrain.

Two common definitions being used today are:

  • Mechanical /imperial horsepower - approximately 745.7 watts
  • Metric horsepower - approximately 735.5 watts.

The horsepower used for electrical machines is defined as exactly 746 W.

The motor in a Model S P100D in Ludicrous mode can produce 779 bhp (581 kW) of power, with 920 lb⋅ft (1,250 N⋅m) of torque.

For comparison, a BMW M3 F80 produces 317 kW; 431 PS (425 hp) between 5,500 and 7,300 rpm and up to 406 lb-ft (550 N·m) of torque between 1,850 and 5,500 rpm.


Other units[edit]

There are other units that you may see mentioned.

Note that some of these are also SI units, but for Tesla-related purposes, try to use the standard terms above to be consistent.

  • Joules
  • electronvolts
  • Calories
  • Volt-ampere