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Leaving it on the charger: wasted watts

wcalvin

Member
Apr 3, 2013
310
16
Seattle WA USA
In trying to figure out what I owe my condo for electricity from the 120 VAC outlet near my parking space, I discovered that you cannot use measures of energy gained by the MS battery with a 85% efficiency factor.

At the top end of the charging curve, for example, only one-third of the energy ended up in my battery over a 17hr session; the rest was wasted as heat. And that is with limiting current to 5A; at 7A, it is far worse. The charger just spills the excess rather than reducing the draw.

A Kill-a-Watt power meter inline with the 120 VAC extension cord will do a better job, but even then you must correct for the voltage drop before it gets to the car. If you start charging from the 17" screen and watch the first voltage displayed (when it still says 0A), you will see it drop until amps settles down. The ratio Vo/Vload times the kWh reading on the meter tells you what you are really using by including the heat dissipated in the wiring.

Anyone worked this out in better detail? Tesla really should document the power throughput for AC charging, to give us better guidance on avoiding wasted watts.
 
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FlasherZ

Sig Model S + Sig Model X + Model 3 Resv
Jun 21, 2012
7,024
1,013
I don't understand how it's worse at 7A vs. 5A. Tesla's site has the cost to charge based on various outlet types (charging currents), but it's overhead is inversely linear to current - 120V @ 12A is the most expensive because of the higher overhead.

Do you know the rough number of hours your car was charging? If so, I'd probably just recommend taking the number of hours and calculating the cost based on it. 12A * 120V = 1440W, or 1.44 kW. Each hour on the charger at that rate, then, is 1.44 kWh. If you charge on average 5 hours per day, then 5 * 1.44 = 7.2 kWh, then multiply it times your rate (in my case, .085) to get the cost per day. 7.2 kWh * $0.085 = $0.612 per day or $18.36 for a 30-day month.
 

David99

Active Member
Jan 31, 2014
4,850
7,021
Brea, Orange County
I guess battery temperature/heating/cooling/cabin climate during charging are all variables that add to the total losses. Just as much as they eat up energy that isn't used to actually drive the car. A 'Kill-a-Watt' unit at the outlet will account for all of it since it just measures what goes through. How much of it ends up in the battery depends on all these things above. I doubt Tesla will make it transparent how much energy is used vs how much ends up available in the battery. Just as much as they don't expose how much power is present at the motor/wheel vs how much is drawn from the battery.
In the end all that maters is how much you draw from the grid and the Kill-a-Watt should give you a pretty accurate measure as it monitors voltage and amps and time.
 

FlasherZ

Sig Model S + Sig Model X + Model 3 Resv
Jun 21, 2012
7,024
1,013
In the end all that maters is how much you draw from the grid and the Kill-a-Watt should give you a pretty accurate measure as it monitors voltage and amps and time.

The setting that you have placed in the car is all-inclusive of pack heating, etc. So if you were to use the total amperage draw that you use and multiply it by the hours, it would work as well, if they will allow you to estimate such.
 

VolkerP

EU Model S P-37
Jul 6, 2011
2,464
27
Germany
Why do you think your condo has to pay for the energy wasted in the line up to the 120V socket at your parking slot? You make the utility meter spin for drawing amps, so you should pay your share of meter rotations.
 

JohnQ

Active Member
Jan 1, 2012
1,612
75
Redding, CT
Why do you think your condo has to pay for the energy wasted in the line up to the 120V socket at your parking slot? You make the utility meter spin for drawing amps, so you should pay your share of meter rotations.

He doesn't think that the condo should pay for the energy loss. He's on a shared meter so can't measure directly and the energy stored in the MS battery would be a significantly low approximation of the energy being used. He wants to pay his fair share.
 

EarlyAdopter

Active Member
Jun 24, 2012
2,818
2,047
Redmond, WA
I took a series of measurements last year on 120v to work out efficiency. You can find all the numbers here:
Charging rates, current draw, and efficiency data for 120V charging

At only 5A or 7A 120V as you mention doing, which is only 600W or 840W, your energy use will be dominated by idle load draw (300W) for the car computer systems which all wake up during charging and pack thermal management (anywhere from 0W to 1500W depending on temperature), which will all draw from shore power when plugged in. This leaves close to nothing left to go into your battery.

You need to bump up to 12A (1500W) if possible to get over that base load draw and start seeing some energy go into the battery.
 
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tezco

Sig P85
Nov 9, 2012
819
4
Colorado
Anyone worked this out in better detail? Tesla really should document the power throughput for AC charging, to give us better guidance on avoiding wasted watts.

The energy can go such places. But this is a big 2/3s loss, and it scales up with higher current.
As Flasher mentioned, the Tesla Charge Time and Cost Calculator shows that 120 V charging is the most inefficient and most costly, but the amps are what they are. The Tesla figures seem to match what most of us have observed.
 

AmpedRealtor

Well-Known Member
Jun 30, 2013
6,351
3,346
Phoenix, AZ
The problem is that you are charging at 120v. At 120v, most of the energy is used to maintain the battery temperature rather than storing the current in the battery. Charging efficiencies are much higher at higher current levels, because a much smaller fraction of the total current is being "wasted" for battery maintenance.
 

sms

Member
Feb 17, 2013
9
1
Bellevue WA
Doc: what specialty do you practice?

What possible pertinence does a M.D.'s specialty have to Tesla information that is going to improve my ownership experience?
 

FlasherZ

Sig Model S + Sig Model X + Model 3 Resv
Jun 21, 2012
7,024
1,013
What possible pertinence does a M.D.'s specialty have to Tesla information that is going to improve my ownership experience?

That's why it's in the "introductions" part of the forum and not this section, which is "battery & charging", until Nigel moves these two posts. :)
 
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wcalvin

Member
Apr 3, 2013
310
16
Seattle WA USA
Minimizing AC draw when charging Tesla Model S

The object here is to create a recipe for minimizing your (electric bill/emissions) over many recharge cycles. It is not about minimizing time or maximizing range.

I have shown that topping up is terribly wasteful. Current draw from the wall remains high, even when little arrives in the battery because of the taper.

What Tesla ought to be giving us is a procedure for minimizing kWh drawn from the wall, and with enough detail that we can adapt it to our useage style. We don't know how they regulate charging or what waste it produces.

in the meantime, has anyone worked out this problem? (I'd do it myself but I cannot charge faster than 7A on 119VAC no-load because of a 2ohm series resistance that drops the voltage to 105 at the car.)
 

AmpedRealtor

Well-Known Member
Jun 30, 2013
6,351
3,346
Phoenix, AZ
The object here is to create a recipe for minimizing your (electric bill/emissions) over many recharge cycles. It is not about minimizing time or maximizing range.

I have shown that topping up is terribly wasteful. Current draw from the wall remains high, even when little arrives in the battery because of the taper.

What Tesla ought to be giving us is a procedure for minimizing kWh drawn from the wall, and with enough detail that we can adapt it to our useage style. We don't know how they regulate charging or what waste it produces.

in the meantime, has anyone worked out this problem? (I'd do it myself but I cannot charge faster than 7A on 119VAC no-load because of a 2ohm series resistance that drops the voltage to 105 at the car.)

You are drawing an erroneous conclusion from your limited ability to charge at either 5A or 7A. Those are inherently wasteful, low current charging rates that put very little charge into the battery while wasting most of the current on battery heating, cooling, and operating other vehicle systems during charging. As I and others have said, to improve efficiency and reduce overhead, you should charge at 40A or higher.

Because you charge at 120v, you haven't really shown or proven anything that we don't already know to be true about charging at such low current levels. Get yourself a NEMA 14-50 outlet and you'll greatly reduce your charging overhead. The solution is simple. You're making this far too complicated, and instead, should use the tools available to accomplish your goal - higher amperage charging.
 

tezco

Sig P85
Nov 9, 2012
819
4
Colorado
The object here is to create a recipe for minimizing your (electric bill/emissions) over many recharge cycles. It is not about minimizing time or maximizing range.

I have shown that topping up is terribly wasteful. Current draw from the wall remains high, even when little arrives in the battery because of the taper.
I set my charge limit to 80% or less and never see a current taper like one sees when approaching 100%. As everyone has said, the best efficiency gain for you would be to find a way to charge at 240V at 20 amps or higher, and stop short of a 100% top-off.
 

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