Heads up: 120V charging less efficient

Heads up: 120V charging less efficient

While you can charge at 120V (and several people on this forum do this or have done this while waiting for a 240V plug to be installed), it turns out to be quite a bit less efficient in energy usage than charging using any 240V power source.

Tesla has recently updated their charging calculator (click here: and by playing with the dials you can see how much actual energy the car needs to charge using different plugs and power sources.

If you use a 120V plug to charge instead of a NEMA 14-50 240V plug, you'll use about 32% more energy (and money) to charge up your car.

Electricity is so cheap though, that that isn't the end of the world - if you drive 1,000 miles a month (12,000 miles a year) at $0.12/kWh, that'll be an extra $12.70 per month in electric expenses.

soma | August 29, 2013

Well, is part of this related to the vampire drain that happens when you have to leave the car sitting for so much longer?

cloroxbb | August 29, 2013

This just refers to the inefficiencies when charging. Of course vampire drain would account for an amount of kwhs also depleted from the battery.

soma | August 29, 2013

A modern transformer would be tossed out the window for losing 30% of energy at a different voltage. This is not that. I am sure this has to do with more realistic assumptions in the calculator about other drains during the extended charging period.

It would be helpful for someone with actual knowledge of the calculator assumptions to chime in.

Theresa | August 29, 2013

I am not positive about this but when I got my Roadster I was informed by the technician that 240 is more efficient charging because while charging the air conditioner is used to keep the batteries cool. So by charging using 120 it takes longer to charge leading to more air conditioner time which leads to more charge needed. It makes sense to me but I cannot confirm this assumption.

AmpedRealtor | August 29, 2013

Makes sense. The car needs a minimum level of current to perform basic maintenance while charging, i.e., maintaining the battery temperature and other parameters. This stays pretty much the same as you ramp up the current, so the higher the current the more efficient the charging because the current used by the basic battery maintenance functions becomes a much smaller percentage of the total current being used.

soma | August 29, 2013

Cooling could be part of it, but I doubt it could be that much. Less heat is generated at lower charging voltage, so if the cooling were running full blast as much as with 240v, the batteries would be getting actually cold?

AmpedRealtor | August 29, 2013

@ soma, a lot of what happens with the battery cooling also has to do with ambient air temps. So even at 120v, we are probably going to see variation between a Model S in Arizona and one in Minnesota.

soma | August 29, 2013

Um, but this issue is about a decrease in charging efficiency vs. voltage, independent of ambient temperature...

mjs | August 29, 2013

I think we're missing something here.
Here's what I know that helps me misunderstand this:

The line loss is related to the current by the formula P=I^2*R, where I is the current and R is the internal resistance.

If you double the current, to (nominally) double your charging speed, then the internal resistance goes up by a factor of 4x. Now, in this case the internal resistance is a relatively small part of the power usage, but it tells me that there should be a decreased efficiency as the current rises.

Of course, I am not correcting for the different 'R' values of the wires, and maybe the smaller 120V cabling makes more of a difference than I think.

I know enough to be dangerous. Anyone else know enough to be safe?

soma | August 29, 2013

yeah, the dangerous part is that that formula applies to DC resistive loads (and AC in absence of reactive elements), and I think the main question here is whether this efficiency loss is something about the AC transformer voltage. (or other things going on in the car during charging)

Brian H | August 29, 2013

Any constant load/loss would be a higher percentage of a lower total current.

jeff | December 11, 2015

When you design a high speed switching DC-DC converter (like what is used in the tesla charger) efficiency is typically maximized for a specific input voltage. This is based on the design of the converter and the specific components that are used to build it. It is likely to assume that tesla designed the charger to maximize efficiency at ~210-240 volts, because that is what will be used most often. The charger operates just fine at 120 volts because it has a feedback loop to keep the output voltage stable, but it is likely that efficiency is lost due to the wider gap between the input and output voltage. (Output voltage probably ranges from ~360v-410v.) My rough guess is 10% less efficiency when compared to 240v. This doesn't explain the entire difference, but the rest is probably from what has already been mentioned above.

NOTE: I realize the input waveform is 60z sin so the fact that a DC-DC converter is used might seem counter intuitive, but the input AC waveform is transformed with some variation of a diode bridge prior to being input to the DC-DC switching circuitry.

jeff | December 11, 2015

One more note: The tesla charger does not use an AC transformer, at least not the type people typically think about. If a legacy power supply like that was used, the power capacitors would probably need to be the size of grain silos! Ok, not THAT big -- but you get the idea, BIG.