Welcome to Tesla Motors Club
Discuss Tesla's Model S, Model 3, Model X, Model Y, Cybertruck, Roadster and More.
Register
  • Want to remove ads? Register an account and login to see fewer ads, and become a Supporting Member to remove almost all ads.
  • The final cut of TMC Podcast #29 is available now with topics time-stamped. We discussed the Tesla Cybertruck's expected 1 MW Ultra-Fast Charging capability, the Tesla Semi Delivery Event, the coming Model 3 refresh (project "Highland"), and more. You can watch it now on YouTube.

My Condo EV charging station...

In 2019 I purchased a 2020 Prius Prime after Strata Council OKed me to use a 110V outlet beside my (secure) underground parking spot, as a charging station. Here's how I did it:

I agreed to keep the charge rate to 8amps to avoid overheating/overloading the circuit. I purchased a simple 'kilowatt' meter for $12, and I keep that plugged into the outlet, and plug my Level 1 charger into the meter and I keep the meter and charger permanently in place. I informed my parking neighbours to simply unplug the meter if they need access to the 110v plugs. Strata reads the meter quarterly and bills me for power consumed at their charged rate (~10cents/kwh).
Obviously there's some degree of an honour system here but, over time, the quarterly billing is quite consistent, and we're not talking big bucks when charging at 8amps/110v.

I kept the same setup for my 2022 M3 RWD, and I get about 1.3% charge/hour at 110v/8 amps which is enough to cover most of my daily driving and if I need more I'll make a quick stop at a Level 2 charger or a local DCFC. There seems to be about a 200w overhead in the M3 AC charging system, so 880w results in about 700w/hr into the battery.
Level1_metered_charging_station.jpg
 
Last edited:
  • Informative
Reactions: KenC

AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
14,357
18,569
San Diego
There seems to be about a 200w overhead in the M3 AC charging system, so 880w results in about 700w/hr into the battery
700Wh/hr. (Aka 700W)

Anyway keep in mind the display of “kWh added” on the screen is not what was actually added to the battery; the actual amount added is 95.5% of this number (and it is the net energy added, so if you preheated or whatever if you reduce this kWh number added or increase it more slowly). So it is the 200W (or whatever) overhead plus any other extra usage.

The “energy added” displayed on the screen is not a measure of energy added. It is just displayed rated miles added (added = current value - initial value) * charging constant (but displayed rated miles contain 95.5% of the energy of the charging constant, hence the discrepancy).

I’m curious what your Tesla app tells you for a given charge session vs. what is displayed on the screen. I need to do this experiment myself to see if the car truly measures energy used (which it can do).
 
Last edited:
700Wh/h.

Anyway keep in mind the display of “kWh added” on the screen is not what was actually added to the battery, the actual amount added is 95.5% of this number (and it is the net energy added, so if you preheated or whatever if you reduce this kWh number added or increase it more slowly). So it is the 200W (or whatever) overhead plus any other extra usage.
I used the percentage of range added, averaged over time, to deduce the range added per hour, and the overhead, The temperature is quite stable in the underground parking so I never use preconditioning,
 

AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
14,357
18,569
San Diego
I used the percentage of range added, averaged over time, to deduce the range added per hour, and the overhead,
I see. Not ideal because % is not a fixed energy value. You’d need to know your rated miles at 100% (and calculate kWh for the pack), and this would still not be correct if above the degradation threshold.

Anyway your answer is close. And curious if the Telsa app tracks wall energy perfectly (I know it accounts for charging losses for sure, but not sure exactly whether it calculates them or estimates them). You’d have to have a very large charge session or deduce rounding error (by watching carefully for the “tick” to the next kWh, both in the app charging stats and on the car “energy added” display) to get accurate results for efficiency this way though, since it does not display fractional kWh.
 
  • Like
Reactions: DuncanM
I see. Not ideal because % is not a fixed energy value.

Anyway your answer is close. And curious if the Telsa app tracks wall energy perfectly (I know it accounts for charging losses for sure, but not sure exactly whether it calculates them or estimates them). You’d have to have a very large charge session or deduce rounding error (by watching carefully for the “tick” to the next kWh) to get accurate results for efficiency this way though, since it does not display fractional kWh.
I used the average % increase during several 20+ hr charging sessions. It's not perfect but it seems close enough.

The next time I begin charging at a low SOC, I'll note the meter reading at the beginning and end of the charge, and compare it to the Tesla app,
 
Last edited:
  • Love
Reactions: AlanSubie4Life

AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
14,357
18,569
San Diego
I used the average % increase during several 20+ hr charging sessions. It's not perfect but it seems close enough.
Does the kWh delta on the Kill-A-Watt agree with the car “Charge Stats” (in the app, not the car!) for one of these long sessions?

Just curious; would spare me having to do my 120V experiment at some point. [EDIT: saw your edit - great!]
 
Last edited:
  • Like
Reactions: DuncanM

AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
14,357
18,569
San Diego
I edited my reply above, but I'll make some notes when I next do a long charge.
Yeah I think the key pieces are:
1) kWh delta Kill-A-Watt
2) rated miles added as displayed in car (toggle display while still plugged in by tapping battery icon in car or in app) - each displayed mile is about 212 Wh/drmi = 60.5kWh/272rmi*0.955rmi/drmi for your car.
3) kWh added in car (or in app, it displays it too for last charge session); will not agree with above, as it is 4.7% (1/0.955) high (0.955*displayed added energy = actual energy added).
4) Charge Stats (single session per day allowed, since data is just cumulative per day in app, charging has to occur in one day only, or need to be careful to make prior day have no charging (or make note of starting value for that day) if charging over a date boundary with some on that prior day and more on the day taking the reading).

Very large sessions of course offer more accuracy (or average over time as you have done).
 
Last edited:
  • Like
Reactions: DuncanM
In 2019 I purchased a 2020 Prius Prime after Strata Council OKed me to use a 110V outlet beside my (secure) underground parking spot, as a charging station. Here's how I did it:

I agreed to keep the charge rate to 8amps to avoid overheating/overloading the circuit. I purchased a simple 'kilowatt' meter for $12, and I keep that plugged into the outlet, and plug my Level 1 charger into the meter and I keep the meter and charger permanently in place. I informed my parking neighbours to simply unplug the meter if they need access to the 110v plugs. Strata reads the meter quarterly and bills me for power consumed at their charged rate (~10cents/kwh).
Obviously there's some degree of an honour system here but, over time, the quarterly billing is quite consistent, and we're not talking big bucks when charging at 8amps/110v.

I kept the same setup for my 2022 M3 RWD, and I get about 1.3% charge/hour at 110v/8 amps which is enough to cover most of my daily driving and if I need more I'll make a quick stop at a Level 2 charger or a local DCFC. There seems to be about a 200w overhead in the M3 AC charging system, so 880w results in about 700w/hr into the battery.
View attachment 868158
I wonder if 8 amps is more efficient than 12 amps — i had the same set up but was charging at 12 amps. The meter showed x kilowatts went into the car, and the car showed that it received 70% of x kilowatts. That inspired me to move to a level 2 charging solution, which (I believe) allows the car to end up with around 88% of the kilowatts sent to it.
 
  • Like
Reactions: DuncanM
I wonder if 8 amps is more efficient than 12 amps — i had the same set up but was charging at 12 amps. The meter showed x kilowatts went into the car, and the car showed that it received 70% of x kilowatts. That inspired me to move to a level 2 charging solution, which (I believe) allows the car to end up with around 88% of the kilowatts sent to it.
It's hard to know until I conduct better testing.

I have sentry mode off to try and reduce any unneeded loads on the car.
 

stopcrazypp

Well-Known Member
Dec 8, 2007
13,105
9,428
I wonder if 8 amps is more efficient than 12 amps — i had the same set up but was charging at 12 amps. The meter showed x kilowatts went into the car, and the car showed that it received 70% of x kilowatts. That inspired me to move to a level 2 charging solution, which (I believe) allows the car to end up with around 88% of the kilowatts sent to it.
I believe the issue with 120V charging is that the fixed overhead loads of the car (things like heating the battery or running the computer) makes up a greater portion of the power going to the car, so a smaller proportion (as a percentage) goes to the battery. I believe there are cases reported where in very cold climates, the 120V charging doesn't actually charge the car (because the energy is all going to heat the battery).
 
I believe the issue with 120V charging is that the fixed overhead loads of the car (things like heating the battery or running the computer) makes up a greater portion of the power going to the car, so a smaller proportion (as a percentage) goes to the battery. I believe there are cases reported where in very cold climates, the 120V charging doesn't actually charge the car (because the energy is all going to heat the battery).

My bad, I didn’t really complete the point of my post. If duncanm is putting in 880 and the car is getting 700 of that, then 79.5% of the power he’s putting into the car is actually ending up in the car. This is about 9% more than I was getting, and about 9% less than what I now get with level 2 charging. I would have thought that for every step down in amps, the efficiency would go down as well. But at 8 amps his efficiency is actually better than mine was at 12 Amps.
 

AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
14,357
18,569
San Diego
But at 8 amps his efficiency is actually better than mine was at 12 Amps.

His efficiency may be lower and he may not be measuring the same way.

Remember it is likely 4.5% lower than quoted, since presumably he was scaling the % by 60.5kWh. (When it should be scaled by 4.5% less than this of course.). Hard to know what was done without seeing the calculations.
 
His efficiency may be lower and he may not be measuring the same way.

Remember it is likely 4.5% lower than quoted, since presumably he was scaling the % by 60.5kWh. (When it should be scaled by 4.5% less than this of course.). Hard to know what was done without seeing the calculations.
One thing that I now remember, is that I assumed a usable battery capacity of ~56.5 kwh, and based my % of range based on that: 1% = ~.565 kwh. Which is about 4.5% less than the nominal full capacity. There was also a small variation in the AC voltage from 110v, which IIRC, varied from ~109v to ~112v.
 
  • Like
Reactions: AlanSubie4Life

AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
14,357
18,569
San Diego
One thing that I now remember, is that I assumed a usable battery capacity of ~56.5 kwh, and based my % of range based on that: 1% = ~.565 kwh. Which is about 4.5% less than the nominal full capacity. There was also a small variation in the AC voltage from 110v, which IIRC, varied from ~109v to ~112v.
Sounds about right. I guess we’ll see.

Guess I’d expect closer to 70% efficiency at 900W input but I really don’t know. Maybe LFP cars are different, etc.
 
  • Like
Reactions: DuncanM

AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
14,357
18,569
San Diego
consistently show 80% efficiency with TeslaFi. I am not sure how well that is measured, just thought I'd throw it out there.
It is not measured well by TeslaFi. And I think the result it gives is configurable. (I don’t use TeslaFi so the details for why it has errors I can’t explain). Obviously it is not way off but you can’t rely on it to tell you actual efficiency.
 
One thing that I now remember, is that I assumed a usable battery capacity of ~56.5 kwh, and based my % of range based on that: 1% = ~.565 kwh. Which is about 4.5% less than the nominal full capacity. There was also a small variation in the AC voltage from 110v, which IIRC, varied from ~109v to ~112v.
I just did a charge from 85% (est 84.8% or 363km) to 94% (est 94.4% and 403km at max = 428). The meter read 226.8 vs 219.6 (17:10 - 01:10 = 8 hrs) at start at = 7.2kw input to the charger. If battery capacity = 56.5kwh then 9.6% = 5.4 kwh and charging loss = 1.8kw or 225kwh.

so 900w average input for 8 hours = ~9.6% increase in range or 1.2%/hr . Unfortunately I forgot to read the car's charge stats.
 
Last edited:

AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
14,357
18,569
San Diego
I just did a charge from 85% (est 84.8% or 363km) to 94% (est 94.4% and 403km at max = 428). The meter read 226.8 vs 219.6 (17:10 - 01:10 = 8 hrs) at start at = 7.2kw input to the charger. If battery capacity = 56.5kwh then 9.6% = 5.4 kwh and charging loss = 1.8kw or 225kwh.

so 900w average input for 8 hours = ~9.6% increase in range or 1.2%/hr . Unfortunately I forgot to read the car's charge stats

Small charge so a lot of uncertainty here.

I get 403km being 94.2%, not 94.4%.

Your car would display 6kWh added (rounding 60.5kWh/272rmi*24.8mi = 5.5kWh), and 40km added.

It's ~132Wh per displayed rated mile. (60.5kWh/272rmi * 0.955/1.6093).

So that's 40rkm*132Wh/rkm = 5.3kWh.

So 5.3kWh/7.2kWh = 74%

Seems about right.

But yeah can read those other app stats on a future charge (note that as long as this was the only charge this day you can still read that in the app, should be 7kWh).
 
  • Like
Reactions: DuncanM

About Us

Formed in 2006, Tesla Motors Club (TMC) was the first independent online Tesla community. Today it remains the largest and most dynamic community of Tesla enthusiasts. Learn more.

Do you value your experience at TMC? Consider becoming a Supporting Member of Tesla Motors Club. As a thank you for your contribution, you'll get nearly no ads in the Community and Groups sections. Additional perks are available depending on the level of contribution. Please visit the Account Upgrades page for more details.


SUPPORT TMC
Top