120v vs 240v charging

120v vs 240v charging

Considering having a 240v outlet installed in my garage to charge a Tesla 3. Length of charging time aside, which outlet, 120v or 240v, would result in a lower electric bill? And will it be appreciably lower? Thanks!

kevin_rf | 04. Januar 2019

Charging at 240v had a higher efficiency, people have indicated roughly 70% charging efficiency at 120v. When I compared my meter to the car Which I charge at 240v I get roughly 89%.

M3phan | 04. Januar 2019

I’ve heard battery guys say the slow trickle charging from 120 when done too often (solely?) can harm battery life. Just like - but in the opposite extreme - always (only) charging at a supercharger.

CST | 04. Januar 2019

Never heard that, but efficiency is at 240V.

Magic 8 Ball | 04. Januar 2019

Hardly any difference. Any additional loss would be from wire heating up. Shorter charge times (220v) result in less heat lost through wire but the heat loss is a very small amount if the wiring is to code.

RES IPSA | 04. Januar 2019

I would just put in a 240v outlet. Hook it up to a 40 amp breaker and charge the car at 32 amps (or less) using the mobile charger that comes with the car. Offers a quick enough charge to be convenient and practical.

M3phan | 04. Januar 2019

Couple of interesting links in this regard:

(From the above link:) “According to Battery University ( as well as several other sources, these are three best practices:

-10% to 90% is a good range to keep your battery within
-Charging speed should not be too slow or too fast, but ideal
-Small charges are better than big charges”

Tâm | 04. Januar 2019


To save electric bill, you might want to convert your bill to Time of Use so you can charge at off peak time when it's the cheapest.

Thus, Time-Of-Use billing might necessitate faster 240V so you don't have to pay higher rate with slower 120V that might cost higher cost during peak time.

However, if you don't have Time-Of-Use billing then you can charge any time you want without worrying about finishing your charge quickly before the cheapest billing is done.

However, as others pointed out above, 120V is not as efficient as 240V. That means you pay more to charge in 120V and get less energy due to efficiency problem.

Coastal Cruiser. | 04. Januar 2019

lmat, not to disagree with kevin_rf, but we have had an owner who charges at both 120v and 240v report their numbers here, and the difference came out to about 3%. In other words, charging at 240V is about 3% more efficient. At least in that example.

Coastal Cruiser. | 04. Januar 2019

M3phan, I'm not seeing the caution about slow charging in that Battery University article you were kind enough to link to. I'm sure it is because I'm blind. Do you recall where in the article slow charge rate is discussed? The words "low" or "slow" are not used in the body of the article.

Brian | 05. Januar 2019

If you can swing it, and since you just bought a Tesla, you should be able to work it into the budget, get a 240v receptacle installed. I was using 120v for a little while since my commute is under 40 miles and I would be able to be fully charged (90%) every morning. BUT WAIT. This is a great car, you will want to drive it more, and even a couple more small trips and you won't have a fully charged (90%) battery the next morning. You can certainly start with less than your max charge in the morning the following day, but if you keep doing this, your state of charge at the end of every day will eventually be too low and you will have to park for a day or find a supercharger.
I don't even charge at home most of the time. I pay for parking at a lot in Boston, and it has free charging for 4 hours where I can get 80 miles added. So I try to charge there, but as much as I drive my 3, I still have to charge at home, and 120v wouldn't be enough for weekend use.

Brian (The one from Massachusetts)

kevin_rf | 05. Januar 2019

If thinking about a TOU (Time of use electric plan) read the fine print. Not every plan works for everyone, not is every plan is the same.

For instance, my towns electricity is supplied by a town owned (Sterling Municipal) electric company. They buy power in bulk from the grid, meaning during peak use hours electricity is really expensive for them, and off peak, not as much. It is in the companies interest to get you to shift as much of your consumption as possible off peak. They therefore offer a very good TOU plan, 1.2 cents a kwh plus 5 cents a kwh delivery between 11pm and 7am. (The down side is if you use more than 550 kwh a month peak 7am to 11pm, they charge you a whopping 30 cents a kwh. Super Chargers are cheaper) They actually installed a 4 mwh battery in town to try to offset some of the extra costs associated with customer peak usage. Savings to the rate payer last year was $700,000.

National Grid, the local grid operator and power company for most of the rest of the state. No incentive to do that, my understanding is they don't have a TOU that they offer to the average consumer. You have to use in excess of 2500 kwh's a month to qualify for the TOU plan. That's double our usage, on a house with an EV, hottub, swim in place pool, and AC. It is not designed for the consumer.

So look at your usage and determine if a TOU and what your electric company offers. Then determine if the plan will work for you.

To take advantage of the TOU, we ended up installing a third meter for the car (Our, power company required a separate for the solar panels).

jordanrichard | 05. Januar 2019

To illustrate the efficiency difference between 120 and 240, here are the numbers for my Model S.

120v gets me 4 miles of range per hour into the battery
240v gets me 29 miles of range per hour.

Yes it is double the voltage but it gets you 7 times the number of miles per hour.

mrburke | 05. Januar 2019

@jordanrichard - Sorry but that is not a measure of efficiency. You need to measure both volts & amps. The 120v is either 12 or 16amps. The 240v would have much higher amps.

Watts = volts * amps.

jjgunn | 05. Januar 2019


I'm assuming the amps are 15 Amps (charge at 12 A) for the 120v & 40 Amps (charge at 32 A) for 240v? Just curious.

Coastal Cruiser. | 05. Januar 2019

Good to see jordonrichard chime in because he is the example I referring to earlier. However, richard, as I have tried to point out to you before, your example only demonstrates that 240v is capable of charging the car at a higher rate. At 240v you charge may be charging your car at 7 times the rate of 120v outlet, but you are using 6.7 times the electricity.

I was in error before when I stated that 240v is 3% more efficient than 120v. I looked up thread again and it is in fact 5%. Based on jordon's example 240v charging is 5% more efficient than 120v charging. Here's the math if anyone wants to double check me:

Note: jordon has claimed he charges at 40A at 240v.

120v times 12a is 1440 watts.
240v times 40a is 9600 watts.

Now if you divide 1440 watts by the 4 miles of range per hour you get 360.
If you divide 9600 watts by the 28 miles of range per hour you get 343.

The percentage between 343 and 360 is 5%.

(note: jordon originally claimed 28 miles of range per hour at 240v in the original thread on this. That is why I used 28 in the math. He is now stating 29 miles. Seems like a small difference but at 29 miles of range per hour the 240v efficiency jumps to 8%.)

Original thread

kevin_rf | 05. Januar 2019

@Coastal Cruiser, very, very large error bars on how you did the math.

It's better to look at how many kwh on the meter, or monitoring system and divide that into how many kwh hours was added to the battery.

Charging efficiency = 100 x kwh added to battery / kwh on meter.

Magic 8 Ball | 05. Januar 2019

@kevin Yup and do an apples to apples experiment. Make sure both runs are at same temperature and account for any differences and running other things besides just charging battery.

JAD | 05. Januar 2019

I don't think the cost will actually be a significant factor as it will cost more to install the 240v line than you are likely to save in efficiency.

Now convenience is another matter. You can get by on a 110v in theory without an issue, but you will have to worry about range before and after every big trip and may need to use Superchargers in your area more.

After years of owning Tesla's, it actually bothers me a bit to wake up and not have the cars charged to 80% even with no big drives planned. It is so nice not to deal with having to go to a gas station every week when the car gets low, it just seems wrong when things aren't ready to start the day.

Long way of saying, just get the 240v installed and enjoy the car. You will likely need the 240v for your next car, and next car, so it isn't just an investment in this car. Having that installed in the house will probably raise the value of the house as it really appears EV's are here to stay.

sheldon.mike1010 | 05. Januar 2019

Was in same predicament: after obsessive research, am happy with resulting 240V NEMA 14-50 outlet using
Tesla connecter from frunk.
Switched to TOU and 3 comparable monthly bills have been less, even with charging a car. The combination
of no gasoline expense and less expensive electric utility makes me a verrry happy Tesla owner. Electrical upgrades and new outlet will pay for themselves in short order.

M3BlueGeorgia | 05. Januar 2019

Run a cable capable of 240v-60A, since the cable run normally is your most expensive item and a high wattage cable is excellent future proofing. Regardless of what socket you use initially.

If you can put another 240v (double) breaker on your breaker box, then go with 240v.

If you are in the weird situation of only being able to add a single (120v) breaker, then use the maximum amperage connection possible with a 120v, which is either going to be 30A (11mph) or 50A (17mph).

ICEMELT | 05. Januar 2019

My understanding is that its just a difference of time to charge what you need. The cost is per KW so its going to cost the same if you pump in 12KW in one hour OR 6 hours.

I could be completely wrong :)

tscats | 05. Januar 2019

From what I understand 240V is always more efficient. The car needs power for more than just charging -- pumps, electronics, etc. I personally don't have any data, but others on this forum have noted them.

Also in winter the batteries need to be warm for charging. Hence if the battery is too cold, power is needed just to generate heat. At 120V more of the generated heat dissipates before the battery comes up to the required temperature. I read in this forum that if it gets really cold little if any of the power at 120V gets into the battery.

jordanrichard | 05. Januar 2019

Yes, the 240 outlet is drawing a constant 40 amps versus the 12 amps from a standard 120 outlet.

What most people focus on is what can charge their cars the fastest. The minutia of efficiency differences between 120 and 240 is not what most people care about. Often deep diving into stats and figures is just deep diving into stats and figures....

TD747 | 05. Januar 2019

I cannot comment on the cost aspect, but similar to some other comments I recommend the 240V outlet. Like Brian above (<40 miles round trip), I relied on a standard outlet for about 6 weeks and then got the 240V installed. The main reason was that charging was taking 12-14 hours or more.

joe.lynn.atp | 05. Januar 2019

IMHO, 240v is definitely worth it to put an end to range anxiety. Before purchasing my Tesla, I thought through scenarios, such as what if we get a call that one of my daughters, who lives 130 miles away, was in an accident or seriously ill? How long before I can get on the road if at the end of the day and I already put 100 miles or so on the car? With the Tesla Wall Connector on. 60amp 240v circuit, I can add 44 miles per hour. I plug the car in when I get home from work, and it’s back to 80% in 1 to 3 hours, depending on how much I drove that day. I lived with 120v for about 10 days when I first got the car, and that is painfully slow. I averaged a little over 4 miles each hour, and that was when the weather was warm. It will be even slower in the winter if you are in a cold climate.

kevin_rf | 05. Januar 2019

Btw. A dedicated 120v outlet (no other outlets on the same breaker) can always be converted to 240v 15amp outlets by replacing single pole 15a breaker with a slim double pole 15a breaker and the outlet with a NEMA 6-15. Tesla makes an adapter, and it will give you 11 miles of charge an hour.

This assumes you have a modern breaker box and a dedicated outlet (which is a little rare). But it is an option for people that have full breaker boxes or pulling new wires would be prohibitive.

M3phan | 05. Januar 2019

@Coastal Cruiser.: I’m no battery guy but have to lean on what I read and see from others. So best I can see, at the 2:56 mark in the video is where the gentleman talks about slow and fast charging versus ideal (moderate) charging. He clarifies consistently charging slowly (as one might at 120 for several hours) isn’t harmful for the battery per se but is hard on the auxiliary systems. He then notes always fast charging is indeed harmful for the battery.
In the BU article he references in his video description, it contains a link to another BU article on charging dos and donts, , and on that page is a chart that notes for battery care the charging method should not be a trickle charge. My assumption is that this can include what effectively happens when plugged into a regular wall outlet for many hours.

SteveWin1 | 05. Januar 2019

I don't think anyone else mentioned this yet, but if you're charging with 110, your car won't be able to "sleep" during the night much. There is definitely better efficiency with 220V charging, but it also finishes charging sooner, which let's your car go into sleep mode and lowers "vampire drain," where with 110V, you're keeping your car "awake" for more time while not in use.

Coastal Cruiser. | 05. Januar 2019

M3phan, thank you so much!

robin | 06. Januar 2019

For the same wattage going to your batteries, a higher voltage can deliver more power at the same amps, and since the power lost as heat is calculated P= I^2xR, you want to keep current as low as possible. 240V will give you half the heat loss (in your wiring) for an equivalent amount of watts delivered by 120V. This is why utilities use insanely high voltages for transmission. The trouble is when we hit the battery where the voltage is determined by the cells and the faster you charge the hotter it gets (I^2R again, don't forget about internal resistances of the cells).

The most "efficient" system would be a very high voltage connection to a charger working extremely slowly, but everything else about it would be impractical and expensive.

TL:DR install 240V

Frank99 | 06. Januar 2019

M3phan -
Nothing you do at home would be considered "fast" charging. The industry standard charge for LiIon batteries is called "1C" - meaning that the highest charge current would charge the battery in 1 hour. For a 75 kwh LR Model 3, that would be a 75 kw charge current. Due to tapering (reducing charge current) as the battery approaches full charge), a full charge generally takes longer - around 1.5 hours. This is true for LiIon powered hand tools, phones, as well as Teslas.
Fast charging is trying to do a full charge faster than this - Tesla does this, for example, when the SOC is less than 50% - my Model 3 with a 75 kwh battery will show a 120 kw (1.6C) charge rate.
At home, the highest charge rate you can get (using an HPWC on a 60A circuit) is (240V*48A)=11.5kw, or about C/6.5. A standard 240V outlet will get you 32A, or almost precisely C/10, which is kinda the definition of a "slow charge" for LiIon.
The recommendation not to "trickle charge" references an old charging method used for lead-acid and NiCd batteries - a charge current of around C/100 is provided to the battery continuously (days, weeks, months, years) to keep the battery continuously fully charged. LiIon does not take kindly to this kind of charging.

syclone | 06. Januar 2019

Just charged at a supercharger in Plantation, FL. I started with 51 miles available and charged to 279 Miles in exactly one hour. Charge rates at the begining were ~460 MPH. I think that's the fastest charge that I've had since I've owned the car.

M3phan | 06. Januar 2019

@Frank99: thanks! yes, by fast charging I am thinking of superchargers, not anything we could do at home. : )

Patrick | 06. Januar 2019

It really is amazing to see 450-500 mph Supercharger charge rates during road trips with a new Tesla. They were rare during our outbound trip on the Christmas travel weekend as most (almost all) of the stalls everywhere were full, but were common on the return trip. At least until the battery hits about 65-70% charge.

After over 2,000 miles we now have zero range anxiety and would travel almost anywhere in the US. Just takes a little planning before and during the trip, which just adds to the fun!

As has been reported many times by others we saw 4-5 mph with 120VAC charging in remote destinations and 29-32 mph with 240V/50A charging.