daveo4porsche

another post from other forums that's been well received - again some of you have this nailed and fully understood - others might find this illuminating - and it's basic EV knowledge to understand how volts/amp and EV chargers translate to battery charging duration math…

nothing in this post is unique or insightful, but it's all in one place instead of being spread out as morsels of information inside other EV related threads…

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:

15 AMP 240 volt breaker - 240 volts * 12 AMPS = 2.88 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 -

daveo4porsche

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.

whiz944

Change the above 16 amp to 15 amp (so max continuous power is 240*12=2.88 kW) and then it will be correct.

And yes - good point about 208v. Commercial and institutional properties almost always have three-phase wiring. So L2 charging stations running at 208v are quite common when charging in the wild. Derate charging speed accordingly.

daveo4porsche

@whiz944 post updated - thanks for the catch!

jhbrennan

Thanks for the info - I'm going to cut and paste so I can save it somewhere easy to find. Here's a question for the electricians here - I have 150 amp service into the house feeding a 100 amp sub panel in the garage. What is the max circuit I can run for a level 2 240 V circuit - 50A, 75A, or 100A? Any reason to install the max circuit to take advantage of size increases in batteries/chargers even if current needs would indicate a 40A or 50A circuit. Thanks.

daveo4porsche

@jhbrennan I don't know the answer due to lack of knowledge about local codes - a 150 amp main breaker certainly could support a 100 amp charger infrastructure - but the real issue is how often would the 100 amp load be active while other large items in the house were also active - for me most of my other high-amp household appliances tend not to be in use from 11 pm - 7 am (my off peak EV charging time) - so I'm probably "over-spec" in my panel but it's never been an issue since I've never had concurrent load that would cause the overload and the main house breaker to trip

now a city/building-code inspector might not find that argument compelling and that's the real issue, what do they think the concurrent loads are and how big of a breaker can you dedicate to EV charging - that will depend on other 240 breakers in your panels…

I'm sure someone on the forums with knowledge of building codes can weigh in

keep in mind a 100 amp EV charger load - will only EVER result in 80 amps of actual usage, and if you EV can only pull 48 amps, it will never use the full 80 amp capacity of the charger - so that makes things a little complicated.

I was charging 3 EV's last night after heavy driving days - I was using the full 80 amp capacity of my circuit, but each car was only getting about 24 amps worth of charge (Bolt, Model 3, Model X) - it was an epic 115 kWh charge session for 3 cars - $15 of electricity for about 430 miles worth of driving across the 3 cars…

whiz944

The J1772 charging standard is spec'ed to allow up to 80 amps on a 240v circuit - which is 19.2 kW. This would require a 100 amp circuit to provide 80 continuous amps (per the NEC 80% rule), and a high power EVSE (Electrical Vehicle Service Equipment - the 'charging station') to match.

An earlier Model S with the dual charger option could do L2 charging at the full 80 amps. Frankly, most folks have little need for that much charging capacity in their homes. Though in the absence of Superchargers or other high speed DC charging, they are nice to have on the road. Since the Model X came out, the S and X on-board chargers have been rated at a max of either 72 or 48 amps. The Model 3 are either 48 or 32 amps.

My google-foo hasn't been very successful at finding what the North American Taycans L2 on-board charging max will be. I was rather surprised that the Jag i-Paces charger is only 32 amp (7 kW) max - which is the same as the Chevy Bolt, Nissan Leaf, and a bunch of others. At a 7 kW power level, you'd get about 20-25 miles/hour charge rate.

daveo4porsche

a 60 amp breaker will support a 48 amp charge rate - which is likely to be more than the Taycan can actually charge at

I would do the following

install 60 amp breaker(s) but pull wire for 100 amp "future circuit"
wait for Porsche to announce their in home L2 charger - if it's cool buy the Porsche charger - if it's just an overpriced J-1772 charger, buy JuiceBox/Clipper/Creek/Tesla charger for a 60 amp circuit (48 amp charging rate, ~12 kW)

12 kw * 8 hour charging window = 96 kW or about 326 miles/daily driving with a full charge fitting into an 8 hour overnight charging window...

60 amp circuit will probably be more than enough for any EV in the next 3 years...

why pull the bigger wire? well if you want to upgrade later, no labor or wire change, just swap breakers (20 min. operation typical) and buy a bigger charger - but use the same wire you installed

the pulling of wire, and the cost of wire is the major expense of installing a charger - the rest is just parts cost once you have the wire, and parts are easy to swap (charger and breakers) but pulling wire over and over again gets pricy and is actually hard work - it also depends on where your home main panel is vs. the charger location - if you main home panel shares a wall with your garage it's really really easy and cheap - if your main panel is not he other side of the house from your garage - well then I'd suggest only doing that job once, which means do it over-spec and then forget about it.

whiz944

Definitely epic!

We charge our Volt nightly - as my wife has a fairly long commute (30+ miles/day each way) and the Volt has a somewhat small battery pack (14.4 kWh usable). However I work from home and don't put as many miles on my Model 3. Plus it has a fairly large Long Range battery pack. So I only bother to charge it a couple times a week. And even then, to the 80% level. I only charge it to 100% just before a road trip.

Like Dave, I time our charging to take advantage of the PG&E EV-A rate. One nice thing that GM did was implement a very extensive delayed charging system. It is flexible enough that one can basically program the entire EV-A rate schedule, both summer and winter, into it. So once plugged in, the car will automatically start and stop charging at optimal times for lowest costs. The Tesla delayed charging is surprisingly much more primitive. I'd guess the Taycan will offer delayed charging at least as good as the eGolf - which is somewhere in between the two.

whiz944

Heavier wire is definitely better than 'just enough'. In this case, #6 THHN in conduit should be fine for up to a 60 amp circuit. (There are some distance/temp calculations that could be done. But with a subpanel in the garage presumably very close to where the EVSE will be, it probably isn't an issue.)

On the breaker size, it really depends on what is on the other end of the wires. If it is a simple receptacle, the breaker must match the receptacle. For example, the common clothes dryer receptacle in newer homes is a 14-30 - so one would use a 30 amp breaker pair. For a 14-50, use a 50 amp breaker pair - though there is an exception in the NEC that allows a 14-50 receptacle on a 40 amp circuit. If you have a hard-wired EVSE, then the breaker pair would need to match the EVSE.

earl pottinger

Since it has the range and you rarely go out, is there a reason you drive the Tesla and your wife the Volt. It sounds to me like you two should switch cars.

Earl Colby Pottinger (Tesla, Bollinger, Rivian and other BEVs fan)

whiz944

It's ... complicated. (She recharges at work. And I do go out more than 'rarely'. So it is all good.)

earl pottinger

I guess there is a reason I never could find a woman to marry me.

Earl Colby Pottinger (Tesla, Bollinger, Rivian and other BEVs fan)

daveo4porsche

another bump

we now know the Taycan can charge at 9,600 watts or 9.6 kW - which 240 volts @ 40 amps - this requires at 50 amp 240 circuit to be installed in your home - and a NEMA 14-50 plug - although porsche sells other adapter cables - it won’t change the maximum 9.6 kW charge rate.

this places the Taycan firmly in the middle of shipping EV maximum charge rate

Model 3 LR/P = 48 amps
Taycan = 40 amps
Bolt/Leaf = 32 amps

W8MM

I'll be using a previously installed NEMA 6-50 receptacle with the Taycan. It was installed to match the supplied 240V plug attached to the charger that came with my Panamera PHEV.

Conveniently, the Taycan charger looks in press photos to be the very same housing configuration so that I may be able to merely detach the Panamera charger from the existing mounting frame and snap in the 9.6 kW device. Nice if true.