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Please Educate Me: Charging in a 120v Outlet - iPhone versus Model 3

Can someone educate me on the physics of electricity? I have a Nema 14-50 in my garage that I use the UMC with on my Model 3, so I'm good there.

This is really an academic question about electricity. Assuming a USA 120 volt/15 amp standard outlet, would an EV pull more juice (aka cost more) than a phone or laptop computer plugged in for 1 hour? I'm confused because I thought EVs (maybe only Teslas?) were smart about how much amperage they draw, where a clothes dryer would not be, and just requires a 240 volt / 30 amp outlet.

My FIL is a security guard, and he has been instructed to unplug any vehicle while in the below-building parking garage. If an employer does not care if employees charge their cell phones or laptops at their desk, why should they care if you plug your EV into an outlet in the parking garage? I recognize there may be other issues of concern like tripping, fire hazards of extension cords, FUD about EVs.

I did some searching, unsuccessfully, so point me to any links if you know that this has been discussed.
 
Just because an outlet is rated 15 amps doesn't mean a device plugged into it has to draw the full 15 amps. Think about it. Would 2 computers plugged into the same outlet (via a power bar) draw the same amount of power as only one computer on that outlet?

An EV that's charging will draw far more power on a 120V outlet than a computer or cell phone would. If the employer has an issue with electricity costs, the cost of charging an EV would be quite a bit more than your average laptop computer or dozens of cell phones.
 
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Big Earl

bnkwupt
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Jul 12, 2017
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An electric car will consume about 1,500 Watts while an iPhone will consume about 25. In addition, most parking garages have shared circuits that are not designed for EV charging. If two vehicles inadvertent plug into the same circuit, best case scenario is it trips the circuit breaker.

Your time and resources would be better spent encouraging them to install dedicated EV charging.
 

Sophias_dad

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Jul 29, 2018
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Costs far more to charge a car than a phone. Power used for a Model 3 cruising for 1 mile could power a phone for a couple months.
Your first statement is correct, your second is a little optimistic.

Various sources say that the iphone 12(taken randomly) has around an 11wh battery, so one mile would equate to about 23 full battery charges of an iphone 12. I was actually pretty surprised it was that high.
 
Thanks for the feedback.

Of course there are other ways for people to charge their cars than plugging into a random outlet somewhere. My FIL was talking about this at Thanksgiving, and it really gave me pause. As a security guard, I wasn't going to tell him not to listen to his boss, I just was trying to work out the science in my head.

I guess I still don't know exactly how electricity works. I thought the kilowatt cost was associated with how long you drew power from 120v/15a. So I though 1 hour from a laptop was the same as 1 hour from a microwave, and the same for 1 hour from an EV. It makes sense that some devices would use less power.

My former employer installed 6 EV chargers in the parking lot, but I never used them because I charge at home. Also there was a SuperCharger within 3 miles. Of course I still have signed up for ChargePoint cause you never know when you might need it.
 
I guess I still don't know exactly how electricity works. I thought the kilowatt cost was associated with how long you drew power from 120v/15a
Think of it as an analogy to a water pipe in your house. 120V is the like the water pressure, such as 40 PSI (so 240V would be 80 PSI). The higher the pressure (voltage), the more gallons per minute could flow from a pipe of a given diameter. 15A is like the diameter of the pipe. The larger the pipe (maximum amperage), the more gallons per minute can flow at a given pressure (voltage).

Now, something connected to that pipe doesn't have to draw the maximum amount of water that the pipe can provide. The attached device could have an internal valve, either a fixed size or variable, that limits the actual amount of water that the device uses. A hypothetical water charged cell phone would have a much smaller valve than a hypothetical water charged vehicle. The amount of water that the vehicle uses per hour would be much greater than the cell phone, and thus cost more per hour, even though one or the other is attached to the same pipe.

The main thing to understand is that 15A is the maximum current (thus power) that a 120V/15A outlet can provide but any particular device plugged into that outlet doesn't have to draw the full 15A (and probably won't).
 
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Think of it as an analogy to a water pipe in your house. 120V is the like the water pressure, such as 40 PSI (so 240V whoud be 80 PSI). The higher the pressure (voltage), the more gallons per minute could flow from a pipe of a given diameter. 15A is like the diameter of the pipe. The larger the pipe (maximum amperage), the more gallons per minute can flow at a given pressure (voltage).

Now, something connected to that pipe doesn't have to draw the maximum amount of water that the pipe can provide. The attached device could have an internal valve, either a fixed size or variable, that limits the actual amount of water that the device uses. A hypothetical water charged cell phone would have a much smaller valve than a hypothetical water charged vehicle. The amount of water that the vehicle uses per hour would be much greater than the cell phone, and thus cost more per hour, even though one or the other is attached to the same pipe.
Nice analogy! Thanks for taking the time to explain it. :cool:
 
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jjrandorin

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Thanks for the feedback.

Of course there are other ways for people to charge their cars than plugging into a random outlet somewhere. My FIL was talking about this at Thanksgiving, and it really gave me pause. As a security guard, I wasn't going to tell him not to listen to his boss, I just was trying to work out the science in my head.

I guess I still don't know exactly how electricity works. I thought the kilowatt cost was associated with how long you drew power from 120v/15a. So I though 1 hour from a laptop was the same as 1 hour from a microwave, and the same for 1 hour from an EV. It makes sense that some devices would use less power.

My former employer installed 6 EV chargers in the parking lot, but I never used them because I charge at home. Also there was a SuperCharger within 3 miles. Of course I still have signed up for ChargePoint cause you never know when you might need it.

Plugging in my phone (any phone, my iPhone or my Nexus 6 Pro), into a 15amp outlet in my home doesnt even move the needle on my home power usage (I can see exactly what my home is using because I have powerwalls).

Running my toaster or microwave uses as much power while its running as everything else in my house does (my power usage goes up from 1.1kW to like 2.9kW or something.

A car charging would max out that circuit for hours at a time. My microwave is on its own circuit, my toaster ( for example ) isnt. I suspect that the parking lot / garage where your FIL works has shared circuits, which would be fine with the random things people plug in like phones, but not for running something that maxes out the circuit for hours at a time.
 
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LoudMusic

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Jul 21, 2020
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Get a Kill-A-Watt and have fun learning how much power each of your devices pulls.


I believe it's required these days that everything sold in the US have printed on it the maximum power it can draw from an outlet. But that doesn't mean they draw that much all the time. Fans have multiple speed settings for example.
 
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Your first statement is correct, your second is a little optimistic.

Various sources say that the iphone 12(taken randomly) has around an 11wh battery, so one mile would equate to about 23 full battery charges of an iphone 12. I was actually pretty surprised it was that high.
Didn’t say at what crushing speed. Didn’t say if HVAC was on. Didn’t say what kind of phone. Didn’t say a full battery would be consumed per day. Didn’t say it “would;” said it “could.”

I stand by my statement. 😁
 

Rocky_H

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Feb 19, 2015
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I guess I still don't know exactly how electricity works. I thought the kilowatt cost was associated with how long you drew power from 120v/15a. So I though 1 hour from a laptop was the same as 1 hour from a microwave, and the same for 1 hour from an EV. It makes sense that some devices would use less power.
Oooh, I see why you have some confusion about this, and it's not just you. We use a very convoluted and confusing unit for energy.

The official metric unit for energy is a Joule. But a Joule is really small, so electric companies don't want all of the numbers coming out in millions and billions, so they have created a different unit that is kind of formed from another one.

Watt or kilowatt is a unit for amounts of energy per time. So if you multiply that by time, you cancel out the denominator and get the energy amount, right? So a kilowatt is a rate of energy per time, multiply by hours, and you get kilowatt-hours, which is an amount of energy. So that may be where you were thinking cost was based on time used instead of amounts of energy.

So now here's how we apply this:
Your Tesla pulls 120 Volts x 12 Amps continuous = 1440 Watts
Your iPhone pulls 5 Volts x 2 Amps = 10 Watts. Compare the Watts, and it's not even 1% as much.
These are accurate rates of power for these kinds of devices: 1,440 Watts and 10 Watts. Let me convert those to kilowatts first:
1.44 kilowatts and 0.01 kilowatts
If you used each of those for 1 hour, you multiply by 1 and get 1.44 kWh and 0.01 kWh as the actual amounts of energy used. So you used them for the same amount of time, but one is drawing energy much faster, so it uses more total energy in that same time.

So yeah, the units on this stuff are kind of confusing and annoying, but that is why you do sometimes see people being nitpicky about making sure people are using the right units. kW is a rate of energy usage and kWh is the amount of energy.
 

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