Charger math - how does it all work?

Discussion in 'Charging & Batteries' started by daveo4EV, Feb 4, 2019.

  1. daveo4EV

    daveo4EV Member

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    EV batteries are measured in kWH (Kilowatt hours) or 1000 watt-hours - a watt hour is a unit of measure of 1000 watts of a electricity for 1 continuous hour. The Taycan will come with a 90,000 watt-hour battery - or 90 kWh battery. Charging a Taycan battery from empty requires 90,000 watt-hours - or 90 kilo-watt-hours (kWh)

    How do I calculate watts?

    Turns out the math is simple: you need to know 2 factors about your electrical supply (or charger).

    Volts
    AMPs

    watts = Volts * AMPS
    watt-hours = (Volts * AMPS) & time

    In the US electrical circuits are typically either 120v - 120 volts or 240 volts. 120 Volts are a typical wall plug in your home or kitchen or room, most home appliances are 120 volt devices, and 240 volts are typically: ovens, electrical water heaters, electric dryers, air conditioners, hot tubs

    in the US volts vary for 120 volt from 100-130 volts, and 240 volts vary from 200 to 250 volts - it all depends on your electrical provider, your houses main panel, time of day, overall load on the grid and so on and so forth). For purposes of this posting we're going to use 120 volts and 240 volts which is the "ideal" goal of the US electrical system - your actual watts will vary based on your actual home supply characteristics. If you can measure your home's voltage you can adjust the math below to actual volts and the watt calculations will change slightly.

    Electrical Circuits are installed in panels with am AMP rating for different voltages (120 or 240). Typically breakers are rated at peak/temporary load, and continuous load. For continuous load you have to de-rate the breaker by 20% - so a 10 AMP breaker is only rated to deliver 8 AMPs of continuous load. Loading a breaker at more than 80% of it's rating continuously is not recommended and can cause problems (fire and death)

    Charging an EV is considered continuous load (any load for more than 3 hours) so the 80% rule is in effect.

    so let's figure out how do we calculate charging time for a given charger for a Taycan.

    Typical home Plug - 120 volts on a 15 AMP circuit - derate by 20% - 12 amp continuous load
    120 volts * 12 AMPs = 1440 watts or 1.44 kw - do this for one hour at you get 1.44 kilowatt hours

    charge the Taycan from empty with this charging rate and you divide 90 kWh / 1.44 kW = 62.5 hours - that's best case and there is about a 10% loss with the chargers so 62.5 * 1.1 = 68.75 hours - we add a 10% fudge factor to deal with various in-efficiencies in the whole charging infrastructure.

    so it will take 68-72 hours to fully charge an empty Taycan from a normal house hold plug @ 12 amps on a typical home 15 AMP circuit.

    some homes/garages have 120 volt / 20 AMP circuits - these are typically used for engine block heaters and use a different plug from the normal household plug - 20 amps * 80% - 16 amp continuous load (these plugs are called NEMA 5-20, normal household plugs are NEMA 5-15)

    120 volts * 16 amps = 1920 watts or 1.92 kilowatts

    90 kwh / 1.92 kw = 46.875 hours * 10% fudge factor = 51.5625 hours or about 52-56 hours to charge from a home 120/20 amp circuit

    as you can see 120 volt house hold circuits really don't do the job - to properly charge an EV you typically want to use a 240 volt circuit. To do this there are lots and lots of choices for various EV charges are all sorts of different AMP Ratings. Typically you want to buy an EV charger who's maximum AMP rating matches or exceeds your EV car's charging capability (Taycan should be at least 40 amps or more - I"m hoping for 72 or 80 amps for the L2 charger onboard the Taycan)

    The Taycan should be able to handle at least a 40 AMP circuit, with a 32 AMP continuous draw. So any EV charger up to 40 AMPS will charge your Taycan much faster than your typical 120 volt house hold circuit. Typical 240 volt breakers are: 16 AMP, 24 AMP, 30 AMP, 32 AMP, 40 AMP, 50 AMP, and in increments of 10 AMPS up to 200 AMP circuits (typically a 200 AMP breaker is on most homes for the entire house, most US homes have 80, 100, 125, 150, 200, 400 AMP main whole-house breakers)

    Math for charging at 240 volts for various AMPS are:

    16 AMP 240 volt breaker - 240 volts * 12.8 AMPS = 3.072 kw
    20 AMP 240 volt breaker - 240 volts * 16 AMPS = 3.84 kw
    30 AMP 240 volt breaker - 240 volts * 24 AMPS = 5.76 kw
    40 AMP 240 volt breaker - 240 volts * 32 AMPS = 7.68 kw
    50 AMP 240 volt breaker - 240 volts * 40 AMPS = 9.6 kw

    Typical public chargers on the West coast from Chargepoint network are 208 volts at 30 amps charge rate or 6.2 kW.

    Charging times will be less for actual daily usage since most people only drive 40-60 miles a day - you can do the math for kWh usage - at 3.4 miles per kWh for the Taycan - typical 60 mile/day usage = 18'ish kWh to recharge at the end of the day

    to estimate your daily charging duration - you can approximate your kWh usage by:

    daily_miles / 3.4 = estimated_kWh consumed

    estimated_charge_time_hours = (estimated_kWh / charge_rate_kw) * 1.1 - and round up

    DC Fast Charging is another whole ball of wax - and the typical math there is 400/800 volts * some number of AMPS - the wattage ranges from 24,000 watts (charge point) to 50,000 watts (evGO), 150,000 watts and and some DCFast chargers as high as 350,000 watts. The math is a little complicated because you can not charge the battery at the same rate during the entire charge cycle, the closer to full the battery is the slower the charge, you can only pump maximum wattage into the battery when it's close to empty.

    But if you're planing to buy a Taycan or any EV understanding the charger math and your battery size will allow you to make an informed decision regarding how big of a charger you can install in your garage to make your daily charging needs a simple and painless process. I would size any charger you plan to install to allow you to fully charger the car in an 8 hour "off-peak" electricity billing Time-Of-Use rate window. In Northern California PG&E has an EV charging rate where off-peak rates of $0.1275/kWh are between 11 pm and 7 am (8 hours)

    charging a 90 kWh battery in 8 hours requires a charge rate of at least 11,250 watts (or about 48 amps which would be a 60 amp breaker). Now again most daily usage is well below the full battery capacity and most daily usage should be less than 20 kWh - but using the 8 hour time window as a yard stick and sizing your charger to "fit" into that window for a full charge means you can minimized your charging costs in a TOU rate window even on high demand days where you nearly exhaust your battery, and it means the car will charge faster for the light duty days meaning turn around time from plugging it in until it's done is also minimized.

    I hope you've found this posting useful and I _KNOW_ the internet will correct any mistakes (people on forums are ruthless that way -
     
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  2. daveo4EV

    daveo4EV Member

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    #2 daveo4EV, Feb 4, 2019
    Last edited: Feb 4, 2019
    knowing your you kWh usage also lets you estimate cost to charge your Taycan - and your cost per-mile.

    We know the Taycan will have a 90 kWh battery.
    We also know the Taycan should go 310 miles according to Porsche
    to go 310 miles with 90 kWh - that means the ideal/EPA/rated consumption for the Taycan is about 3.4 miles per kWh - or 294 watt-hours (wh) per mile.

    If you're electricity rate is $0.1254 per kWh (the PG&E EV off-peak rate in Northern California)

    1 kWh cost $0.1275

    $0.1275 / 3.4 miles = $0.0375 per mile driven in electricity cost

    let's compare that to a 28 mpg Panamera at $3.75/gallon for gasoline

    $3.75 / 28 = $0.1339 per mile driven

    so the Taycan is about 3.57 times cheaper per-mile to "fuel" at PG&E off-peak rates per mile driven.

    A daily drive of 60 miles would be about 18 kWh = 18 * $0.1275 = $2.295 /day for a 60 mile round trip commute

    vs the gas car for 60 miles at 28 mpg = 2.1428 gallons * $3.75 = $8 /day for a 60 mile round trip commute

    now your actual kWh on your meter to charge the Taycan will be about 5 to 10% more than the kWh consumed to charge the battery - so add 10% overhead to your electrical usage while charging the car to slightly adjust your per-mile chargers higher for actual kWh consumed for charging the car, but still way cheaper than a gas car for the same miles driven.

    If you look up your local utility bill and your kWh cost, you can very accurately estimate your electricity bill based on daily/weekly/monthly miles driven and compare it to your current gasoline bill to see what makes sense for you.

    efficiency for EV are very very very sensitive to driving style and speed (faster speeds are deeply expensive efficiency wise) - so the 3.4 miles/kWh is an ideal consumption rate for "typical" usage - I would suggest your range of efficiency for the Taycan will vary from 2.5 miles/kWh to 4.5 miles/kWh depending on speed, climate, driving style, acceleration - even at 2.5 miles/kWh EV's compare favorably to gas cars, but EV's are also tempting to accelerate from stop lights - using the Taycan's ample acceleration will NOT achieve 3.4 miles/kWh and you'll learn to optimize efficiency when it matters, and have fun when you want to....

    after over 180,000 miles driven in various EV's I can safely state I routinely do about 3 miles/kWh and don't alter my driving style that much - I can do much better, and much worse, but overall I'm in the 2.8-3.2 miles/kWh range and simply drive the way I want to drive.

    3 miles/kWh is a good "rule of thumb" for estimating EV range and consumption and will put you in the ball park for estimating costs and kWh impact on your monthly electrical bill...

    once you get an EV and start thinking in terms of kWh - it also becomes very evident there is a natural synergy with grid-tied solar systems which produce kWh during the day for you to consume at night while charging your EV - an ample solar system will easily cover your EV kWh daily usage and you can cover both your home and personal transportation needs with a roof top solar system on your home…it's gets kinda cool when you're charging your Taycan from sunlight falling on your roof and your electricity bill is near zero…but that's another post.
     
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  3. Earl Colby Pottinger

    First Name:
    Earl
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    Note: Here in Ontario power is $0.068 per kWh at night. That will really cut down to costs to drive.
     

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