How Should I Charge My Tesla?

If you are thinking of buying a Tesla or just purchased one, the most common question I get is “How should I charge my car?”. While Teslas come with the most flexible charging options of any EV, the plethora of options can be a bit daunting. Hopefully we can demystify this a bit and help you find the optimal charging solution for your situation.

First off, some background

It is important to understand that the “charger” is actually built into your car (unless we are talking about Level 3 Charging / DC Fast Charging / Supercharging – which is outside the scope of this article). The charger takes Alternating Current (AC) from the power grid and rectifies it to Direct Current (DC) in order to charge the batteries in the car.

The device most people typically think of as the “charger” (the connection from the wall to the car) is actually called an EVSE (Electric Vehicle Supply Equipment). An EVSE is a safety device that 1. Does not apply power on the cord to the car until it verifies a car is connected. 2. Provides Ground Fault Circuit Interruption (GFCI) to reduce the risk of shock (especially in wet areas). 3. Signals to the car how many amps of current are the max it should safely draw.

The limit of your charging speed will be governed by the lowest of the following: The limit of the onboard charger, the limit of the EVSE you are using, the limit of the circuit the EVSE is attached to, and if your battery is cold or near 100% full, the charge rate will also be limited due to that.

So the first question is: What Tesla model did you purchase? All current model “S” and “X” vehicles come with a 48a capable charger. The Model 3 LR comes with a 48a charger. The Model 3 SR and MR come with only 32a chargers. Some relatively recent “S” and “X” vehicles came with 72a chargers (if you bought the upgraded package – but they don’t currently offer it) and very old Model S cars came with a 40a charger by default, or (as an upgrade) dual 40a chargers for a total of 80a.

The next question is: What kind of EVSE do you have? Older Teslas came with the Universal Mobile Connector (UMC) Generation 1. This was capable of up to 40a charging speeds (on the right circuit). All new Teslas are shipping with the UMC Gen 2 which can only support 32 amps.

It is important to note that the National Electric Code (which most jurisdictions in the US follow) has defined Electric Vehicle Charging to be a “Continuous Load” (meaning that it may run for more than three hours continuously). Because of this, you must oversize any circuit it is connected to by 25%. So the max draw rate on a 15a circuit will be 12a. On a 20a circuit 16a, etc… Basically the EVSE/Car should only allow 80% of the circuit rated capacity to be utilized. (this is to avoid breakers blowing due to them heating up – what is called a “nuisance trip”)

The Tesla UMC units are really quite impressive as they sell different tips/adaptors for them that let them connect to different types of electrical receptacles. They then signal to the UMC which in turn signals to the car what the maximum allowable current draw is on that specific circuit type.

Historically, the UMC has come with each Tesla including a NEMA (National Electrical Manufacturers Association) 5-15 adapter (this is a standard wall outlet you see everywhere in homes and businesses) as well as a NEMA 14-50. A 14-50 is used for modern electric oven/ranges as well as large RV’s. Tesla has included the 14-50 since there is a wide installed base of that receptacle type at campgrounds across the country and it allowed for high amperage charging at home with the simple installation of a receptacle. As of May 2019 they are no longer shipping new cars with the RV Adapter (NEMA 14-50) as of May 2019. The adapter will still be available for purchase for $35.

It is also worth noting that all Teslas come with a small J1772 adapter that let you use all of the industry standard Level 2 (aka 208/240v) chargers out there in the world. Virtually all public charging stations have a J1772 connector on them so it is recommended that you keep this adapter with you in your car at all times. The J1772 adapter has nothing to do with the UMC, it is totally independent.

The other home charging option Tesla has provided is the “Wall Connector” (the previous version was known as the “High Power Wall Connector” or “HPWC”). This unit serves the same function as the UMC (it too is an EVSE), but it is hardwired (no plug) and it can provide up to 80 amps of charging current to the car on a 100 amp circuit (depending on what kind of circuit it is connected to – it can also provide less, all the way down to 12 amps on a 15 amp circuit).

It is important to understand that your EV is virtually guaranteed to be the #1 power draw in your house in terms of total power draw and also in many cases it will be the #1 peak draw user as well. This is not something to take lightly or be careless with. While Teslas have advanced safety features built in to reduce the stress they place on your electrical system and to detect potential issues before they result in a fire, these are not foolproof.

Please note that cold weather effects EV’s dramatically. Energy use in the winter is much higher due to needing to heat the passenger space and the battery. You will have less range in the winter and your charging demand will also increase for the same number of miles driven. Charging will also be less efficient as it *must* heat up and keep the battery warm before charging. In some cold temperatures you might not be able to add much if any range at slower charging speeds. It is also extremely helpful to have high current 208/240v charging power available so you can pre-warm the car and the battery from “shore power” allowing you to leave in the morning with a “full tank”.

Should I install a NEMA receptacle or a Wall Connector?

This is the crux of the decision most folks are looking for an answer on. Should they install a NEMA receptacle (or use an existing one), or should they install a Wall Connector. Then the second phase of the question is what amperage receptacle (or circuit for the Wall Connector) should they install?

The short answer, is that nearly everyone will be perfectly happy with either a NEMA 14-50 receptacle, OR a Wall Connector. The longer answer is a lot more complicated and comes down to how you plan to use your car and has a lot more nuance to find the optimal solution.

I ask the following questions as inputs to the decision:

  • Do you park your vehicle indoors or outdoors? The Wall Connector is a superior technical solution for outdoor installs, but it is more expensive in some scenarios.
  • Do you commonly go stay overnight at other peoples homes where there is not permanent charging infrastructure (i.e. going to a friends vacation home for the weekend)? If you are often needing to unplug your UMC and take it with you (or you are prone to forgetting to do so), you may want to install a Wall Connector at home so your UMC can stay in the car at all times or buy a second UMC to always have in the car.
  • Do you have the ability to charge at work? For free? For a fee? Do you need to bring your own cable to plug into a wall receptacle? If you need to take a UMC to work with you every day I would recommend a Wall Connector or a second UMC at home as you don’t want to be putting that many plug/unplug cycles on a NEMA receptacle (they were not designed for that – like the connector end into your car was).
  • Does your utility offer “Time of Use” or other specialized billing plans where they make it financially advantageous to schedule your charging within certain limited hours of the day? Depending on how these hours overlap with the schedule of your car being at home and the amount of range you need to recover each day, this might push you to install faster charging solutions so you can “fit” within the heavily discounted charging window.
  • How many miles a day do you drive on average? At peak? Do you ever come home after a very long trip (where you may be near empty) and then need to go on a long drive the next day again? Even a relatively slow charging solution at home is good enough for most people since their average charge need may not be that high, however, you need to plan your charging solution for your peak needs, not your average needs. My peak need is coming home from work on a Friday afternoon and then wanting to “top off” while I pack to go on a weekend trip.
  • Does your schedule ever unexpectedly change during the day requiring you to drive somewhere unexpected and spend the night where you then may need to charge off a regular wall outlet? Or asked another way, would you feel uncomfortable not keeping a cable in the car with you at all times just in case you needed to charge somewhere unexpected? (for most folks driving patterns this is almost never necessary, but for many it is critical to have piece of mind) The question here is do you need/want to keep a UMC in the car with you at all times (I have never needed to use mine “unexpectedly”, but I am the type of person that always wants one on hand regardless).
  • Do you rent or own your current home? If you rent you may not want or be able to make modifications to the electrical system (especially if you can not easily take them with you).
  • How long do you intend to stay in your current home? If not very long, it may make sense to implement a minimal cost charging setup. One huge cost driver is if your house needs a main electrical service upgrade (like to go from 100a to 200a). Sometimes living with a slower charging solution is worth it for a short period of time.
  • What is your level of cost sensitivity? Are you a starving recent college graduate with a bunch of student loans to pay off? Or are you independently wealthy and just want the “best” solution regardless of cost?
  • Do you own any other EV’s or do you plan to in the future? If so, what brands? Typically most people either go with a NEMA receptacle or a Tesla proprietary Wall Connector (since it is an incredible device at an amazing price point), but there are a minority of folks that will go with an “industry standard” style J1772 EVSE instead of a Tesla one and then just use the J1772 adapter that is included with the Tesla to charge. These units are typically more expensive and less capable than the Tesla Wall Connector, and it is annoying to have to use the J1772 adapter all the time on your Tesla (I often recommend buying a second adapter to keep at home in this case), but they serve their purpose.
  • I then ask a ton of questions about age of the home, age of the electrical panel, what size main electrical service it is, whether they have natural gas for their furnace and water heater, etc… Commonly this requires folks providing pictures so I can determine how modern their electrical panel is, how large a capacity it has, and if it has physical space and available capacity to add EV charging.

Some things not to do

First off, please do not use extension cords for your home charging solution. It just is not worth it. Extension cords are the cause of a huge percentage of electrical fires. Electric vehicles have the potential to be especially dangerous since they can suck the full capacity of a circuit for many hours on end (usually while you are sleeping!).

Using additional adaptors or extension cords defeats some safety mechanisms built into the Tesla UMC Gen 2 units. The UMC Gen 2 has thermal probes built into the plug end of the Tesla adaptors. These are intended to monitor the temperature of the receptacle they are plugged into so that if it overheats, the car can discontinue charging before it melts or causes a fire. When you use an additional adaptor or an extension cord you add another point of connection that does not have this thermal monitoring.

Spend the money to have a proper dedicated circuit receptacle or hardwired EVSE installed near your vehicle.

While I have used extension cords when traveling when there are not other options I am exceedingly careful with them. I only use high quality oversized cords and I check them regularly for heat buildup during charging.

I do *not* recommend regularly charging at home on circuits that are shared with anything else. If charging on a 120v circuit the car is very likely to take 100% of the capacity of that circuit, leaving no capacity for any other devices. These general purpose 120v circuits are typically daisy chained from one receptacle to another creating lots of potential failure points for a loose connection where arcing can occur.

If your situation allows, try to avoid charging at home on 120v in general (especially on 15a circuits where you can only actually draw 12a) since a decent amount of that power is burned by overhead during charging (heating the battery, running the battery coolant pump, running the car computers, etc…). If you can get any form of 208/240v charging (even if as low as 15a) it is vastly better than 120v. If 208/240v is not an option, then at least explore if the circuit you have access to is a 20a circuit (20a breaker and at least 12 gauge wire). If so, with the 5-20 adapter for your UMC you can draw 16a instead of 12a (you also need the right wall receptacle to support this – in the US the National Electrical Code allows 15a receptacles on 20a circuits and it is extremely common – you may need to swap the 15a receptacle for a 15/20a version). While that sounds like only a 33% improvement, it is in reality more than a 50% improvement in effective charging speed due to a lot of the base 12a being eaten up by “overhead”.

It is also a bad idea to rely on a human being to manually set the current draw down in the car to a lower rate than the EVSE signals it that it is allowed to charge at, or to rely on a human to always set the car to only charge at times when other major appliances are not running (stoves/ovens/dryers, etc…). Humans make mistakes and so it is important that the charging solution auto limits you to safe charging levels. The example where this most commonly comes into play is where you buy a 3rd party adapter that allows you to connect to some smaller capacity receptacle (like a NEMA 5-20) but using the 14-50 adapter on your UMC Gen 2. This means the car thinks it is allowed to charge at up to 32 amps, but the circuit is only capable of 16 amps continuous. Yes, in theory a breaker will blow if you forget and draw too much, but a reasonable percentage of breakers out there are defective and won’t function properly. Again, I have utilized some of these 3rd party adapters when traveling, but it requires specialized knowledge and training and fastidiousness to operate them safely.

So what *do* you recommend?

I am a big fan of the Wall Connector for the following reasons:

  • It supports the widest range of amperages with the most granular steppings. One common solution I recommend is to wire it up with larger wire and a corresponding large breaker, but then set it down to a max current that is lower than the wire supports in order to fit within your “load calculations”. Then later if you upgrade your panel/service or remove an electrical appliance (like a water heater or dryer) then you can turn the Wall Connector up higher (taking over that freed up load capacity).
  • It supports the fastest charging speeds that any Tesla is capable of.
  • It is less expensive and more capable than any similar EVSE units from other manufacturers.
  • It can be twinned with up to three other Wall Connectors to share the same circuit and dynamically balance the current to connected cars so that it does not overload the circuit.
  • It comes with a built in cable manager (you coil the cable over it and it has a holster for the handle). This saves you from needing to buy something to hold your cable and connector (if you use the UMC).
  • It is more vandal resistant than a UMC – it has a bunch of high security Torx screws holding the cover on and it can’t be easily disconnected since it is hard wired.
  • It does not require a GFCI circuit breaker (about a $100 savings).
  • You do not have to buy a heavy duty 14-50 receptacle since it does not need one.
  • Now that Tesla no longer includes the 14-50 adapter for the UMC, if you go the UMC route you will need to buy one for $35.
  • It only requires two “hot” conductors and a ground. You don’t need a neutral wire which saves you wire cost and conduit space (if running in conduit). The wire is easier to run as it is more flexible and smaller if using NM cable (Romex). (Note that there are receptacles for the UMC that also do not require the neutral – it is never used by any EV charger unless you are charging at 120v)
  • It comes in both an 8.5 foot and a 24 foot version (the UMC is only 20 feet). The 8.5 foot version is great if you can mount the unit exactly where it needs to be. Less cable clutter.
  • Being hard wired reduces points of failure – the most common EV failure point is the receptacle you are plugged into. This eliminates that possibility.
  • It is much more waterproof than the UMC (so great for outdoor installations).

For what it is worth, Tesla is moving in the direction of recommending the Wall Connector for just about everyone I think. – this is why they are removing the 14-50 adapter as an included item with the vehicle.

Now that is not to say that using the UMC with a NEMA outlet is not a completely valid option. A *ton* of people do this and are extremely happy with the solution.

There are some down sides to the Wall Connector over the UMC:

  • When compared to just using the included UMC it costs more (though if you are going to buy a second UMC to keep in your car at all times the cost is about a wash).
  • It requires a knowledgeable person to mount and hard wire it in.
  • It is more difficult to take with you when you move.
  • It is Tesla proprietary, so if your next EV is not a Tesla it will need to be replaced.
  • If it fails you can’t just easily plug in a replacement, it requires a knowledgeable person.
  • You can not “take it with you” in order to secure it. Someone could come cut the cord off of it or vandalize it (though you should not be plugging/unplugging a NEMA cord every day anyway).

Beyond the Wall Connector vs. NEMA receptacle debate, my general recommendation is to max out the charge speed of your onboard charger if you can do so cost effectively. If you bought a Model 3 SR or MR you can do this with the included UMC (32 amps) and a NEMA 14-50 or 6-50 receptacle. Other Tesla variants will require a Wall Connector or 3rd party J1772 EVSE to max their speeds. Many houses have a 200a electrical service with the panel in the garage, so maxing the charge rate is often not very expensive at all if you have the spare capacity and the wiring run is not very far.

One critical gating factor we have not discussed yet is “how much capacity do you have available”? In your house you have an electrical “service” from the utility. Typically they are 100a, 125a, 200a, or 320/400a, but other variants are possible. The NEC has a whole section (Article 220) on how to calculate what capacity you need based on the “load” (i.e. devices) in your dwelling. If you then have subpanels off your main panel the load on the wires between the main and the subpanel is also calculated using these same formulas.

I am not going to dive deeply into Load Calculations here as there are multiple ways to calculate them (as allowed/outlined in the code), but it is critical that these calculations are done to ensure the solution you come up with does not push your load over the limit. Unfortunately, the code is not very crisply defined and so frequently different people (including the local inspectors) interpret the requirements differently.

If your service will not support the charging level you optimally want and you are using a Wall Connector, you can simply crank down the rotary dial selector in the Wall Connector to a lower setting that IS within your load limits. If you are using a UMC, this is a bit more complicated, but you still have options. If you can’t support a full 32a continuous (40a calculated) using a NEMA 14-50 or 6-50 adapter, then your other options are to install a different receptacle with a different Tesla UMC adapter that signals a lower maximum charging current to the car:

  • 14-30 (new dryer receptacle) / 10-30 (old dryer receptacle) – 208/240v 24a (30a circuit)
  • 6-20 – 208/240v 16a (20a circuit)
  • 6-15 – 208/240v 12a (15a circuit)
  • 5-20 – 120v 16a (20a circuit)
  • 5-15 – 120v 12a (15a circuit)

In all this discussion I should also call out that beyond making sure your load calculations support the new circuit, you also need enough physical space in your panel to add a new breaker. Adding a new 208v/240v circuit will require a free “double wide” breaker position in your panel or a dedicated 120v circuit will require a free single wide breaker position.

Do not get too concerned however if you look and do not immediately see free breaker positions in your panel. Many panels support “tandem” breakers that allow you to basically double up and put two breakers in the same slot. Sometimes you need some creativity to rearrange things in the panel to make it all fit, but most of the time there is a solution available. The worst case scenario is that you need to add a sub-panel next to the main panel to provide more space.

The Final Say

Most peoples situations are unique and so there is no one size fits all solution. Hopefully this article has armed you with enough knowledge to make a more informed decision.

I encourage everyone to have a solid charging solution at home as it is by far the easiest way to charge without worry. I feel strongly that owning an EV should be effortless and should not require you to modify how you live your life in order to accommodate charging needs. (note that I am a strong proponent of plugging your vehicle in every day when feasible per Tesla’s recommendations)

Even if your home charging capabilities are somewhat limited, *any* form of home charging is better than nothing. Even 120v @ 12 amps will give you 30-40 miles of range a night which is sufficient for many peoples driving needs.

Remember that you are planning for the “edge cases” and not the average cases. Once you get over the install cost, you will never be unhappy you have *more* charging capacity than you absolutely need on a daily basis. Running copper wire and upgrading electrical services to increase capacity have lifespans measured in decades. The costs are trivial when amortized over 20 years (you might switch your EVSE out in that time, but the copper in the wall is typically reused).

If your time horizon is short you may choose to make concessions on your charging setup – otherwise spend the money to “do it right”.

Last but not least, ENJOY YOUR CAR! As we are still early on in the EV adoption cycle, figuring out your charging solution may seem daunting, but for the vast majority of folks it is not that big of a deal. Once surmounted you should never have to worry about it again (unless you move).

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