Author: James

Cabin Preconditioning

How much energy does it take to have a toasty warm cabin before getting into the car and driving off?  Here is a view of energy consumed (kWh) and rate of energy (kW) to precondition the cabin (not battery) in 43 degree (6 C) ambient temperature garage.  20 minutes and 1 kWh later the car is warm and ready to go.  It cost me 14 cents to warm the car while parked in the garage and plugged into my Juicebox Level2 charger.   I also avoided losing about 4 miles of range since I didn’t have to use the main battery to warm the cabin while driving.
If we get another cold spell and the garage gets into the 30’s, I’ll re-test to compare the energy consumption at lower temperatures.

precondition graph

precondition table

First week with a fully electric car

After 1 week with the 100% electric BMW i3 I calculated and compared the energy costs to my previous gas car and our existing gas minivan.

In one week:
I saved  $68.76  vs. driving the premium unleaded fuel BMW 335i.
I saved  $49.64  vs. the regular unleaded fuel Toyota minivan.

Calculations:
Total miles driven:  478
Electricity cost including taxes and other fees:  0.1364 cents per kWh
Gas cost:  $3.40 per gallon of premium unleaded fuel
Gas cost:  $2.64 per gallon of regular unleaded fuel
Both gas cars average 19 MPG

What I actually paid driving the 100% electric BMW i3:
Electric used from all charging sessions: 123 kWh

As an added bonus, I’ve produced zero emissions during driving AND electric charging.   I switched my electric generation provider to 100% renewables years ago.

Fuel Costs Per Mile – EV’s vs. Gas

Here is a quick comparison showing only fuel cost differences between gas and electric cars.  This does not include any maintenance savings or other benefits of EV’s.  Comparing Gas cost of two different MPG rated cars versus the average MPkWh rated electric car.  This assumes home charging the EV with typical electric rates after all taxes, generation, distribution, and other fees in Pennsylvania.

Charts use the average efficiency of 3.27 Miles per kilowatt-hour (MPkWh) of an EV with electrical rate of $0.1364 cents per kWh.  For Gas,  $2.453 per gallon taken from today’s average in PA from www.gasbuddy.com

You can see from the graphs below,  a 30 MPG gas car will cost Double the EV costs for every mile driven.  A 19 MPG gas car such as a minivan will cost more than Tripple!    EV saving are even higher since maintenance is minimal with no oil changes, no brakes (regenerative braking basically eliminates brake wear), and minimal moving parts to break.

Calculations for charts above:

Quantifying EV Energy Consumption

I want to better understand the energy consumption of electric cars.  I’m not even going to discuss the EPA’s MPGe (miles per gallon equivalent) ratings for electric cars.  I completely ignore this figure.  I will never drive along in my pure electric car thinking…  “I’m averaging 95 MPGe on this trip to the beach!”  Especially when everything related to the EV will be in kilowatt-hours (kWh).  Sizing of the car battery pack is in  kWh.   I’ll charge the car at home, then  pay the electric bill which is based on kWh.   Also while driving the car it will show instantaneous watt-hour per mile consumption.

I want to think in electric / energy terms and have a good understanding of relational consumption.  The easiest way to understand energy consumption is to relate it to stuff we already understand.  Charging a cell phone, a 60 watt light bulb, a 1400 watt coffee maker etc.  We all have a pretty good idea of how much power these devices consume.  Let’s relate this knowledge to EV’s.

Consider the energy consumption of the following cars.   The kWh rating per 100 miles is pretty common to find now.  Based on that number we can calculate the Watt-hours per mile:

watt-hour = 1000 × kilowatt × hour
abbreviated:  Wh = 1000 × kW × h

We can also calculate Miles Per Kilowatt-hour (MPkWh):

kWh / 100 mi data source: www.fueleconomy.gov

The average watt-hour per mile of these electric cars is 312 Wh/mi.  We can now relate that energy consumption against some common household devices:

Showing the math for the Keurig coffee maker for example:

Another fun comparison…  how many times would you need to charge your phone to equal the same energy consumption of driving an EV 1 mile??

You would need to fully consume the S7 battery 21.33 times to equal the same energy usage of an electric car going 1 mile.

312 Wh / 14.63 Wh = 21.33

 

Electricity Primer

The following is a basic overview of electricity.  This terminology and theory can be helpful for understanding anything electric.  Solar energy, electric vehicles, household electric, batteries etc.

What is Electricity?

In most basic terms, electricity is the flow of electrons along a conductor.

How can electrons be a source of energy?  They travel at nearly the speed of light (186,000 miles per sec), and billions of them can move at once through a wire.  The combination of speed and concentration together produce significant energy.

Primary Units of Measure – Volts & Amps:

Ampere (often shortened to  Amps)

  • one amp of current flow is approximately equivalent to 6.2415093×1018 electrons moving past a given point in one second.
  • For electrons to move in a particular direction, it is necessary for a potential difference to exist between two points
  • The same number of electrons stored on an object (static charge) and not moving is one Coulomb .  Coulomb is the quantity of electrons vs the rate of electron flow (amps)

Voltage (measured in Volts)

  • Voltage is electric pressure exerted on electrons in a circuit.  The force is known as electromotive force (emf) which is measured in volts.  Electric pressure, potential difference and emf mean the same thing

Electric Power

Power is measured in watts and is the rate at which work is done.

  • Power (watts) = Voltage (in volts) x current (amps)
  • 1 watt represents the amount of power consumed when the difference in potential of one volt produces a current of one amp

Electric Power related to Electric Vehicles

  • Power can be electrical or mechanical
  • When a mechanical force is used to lift a weight, work is done.  The rate at which the weight is moved is called power. Horsepower is defined in terms of moving a certain weight over a certain distance in one minute (e.g. 33,000 lb lifted 1ft in 1 min = 1 hp)
  • 746 W of electric power is equal to 1 hp
  • Hp rating of electric motors is arrived at by taking the voltage and multiplying it by the current drawn under full load.  Power = volts x amps

What is Work?

  • The unit of work or energy is the Joule (J)
  • 1 Joule is the work required to produce 1 watt for 1 second
  • Watts = J/s
  • Work is done whenever forces causes motion – including movement of electrons through a conductor

Energy Capacity – Total Energy

Power consumed over time is quantified by the watt-hour.  Multiplying watt’s by a number of hours is watt-hours.  1000 watt-hours is equivilant to a kilowatt-hour. The kilowatt-hour is commonly used as a billing unit for energy delivered to consumers by electric utility companies.  watt-hours and kilowatt-hours is used for energy consumed as well as energy available, such as energy stored in a battery.

  • watt-hours=watts x hours
  • watt-hours = amp-hours x volts
  • watt-hours  = Wh
  • 1000 Wh = 1 kWh  (1 kilowatt-hour)
  • Kilowatt hour (kWh) = 1000 watts used for a period of 1 hour
  • A household electric bill, the entire month’s time is equated to one hour’s time

Terminology:

  • Current flow – a flow of electrons moving along a conductor
  • Conductor – A material which electricity passes easily because it has free electrons
  • Circuit – A circuit is a pathway for the movement of electrons
  • Coulomb – 6.2415093×1018  electrons stored on an object (static charge) and not moving is equal to one Coulomb .  A Coulomb is the quantity of electrons vs the rate of electron flow (amps).

A good source for electrical formula conversions:
http://www.rapidtables.com/

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