Charging via Back-up Generator

[rgedad]

I have a 2018 Model X. I am in the process of upgrading my house to add a back-up generator. I live in the hot state of Texas and am looking to get a 20KW unit so I can run both of my power hungry AC units in the middle of Texas summer.

On the worst days in July/August I am pulling about 9KW for the AC units. On those days the AC units run pretty constant until about midnight or so.

I currently have a Tesla Wall Charger with 90 Amp breaker (200 Amp house service)

I currently have it set for charging at 72 Amp, but given our somewhat limited (50 miles) daily around town drive, can certainly reduce the charge rate to fit within the back-up generator constraints. I have it start charging at about 2 AM, and we seldom use the car before 8 AM. In short - I have over capacity, under utilization of the curent charging capability.

In my discussions with one potential generator sales / installer, he mentioned there might be an issue with EV charging on a stand-by generator and that I should look into that. I believe the issue may be in the harmonic distortion / frequency stability performance requirement.

Does anybody have any information of charging via a stand--by whole house generator, or know the power input performance requirement for the Model X on-board charger??

 

[brkaus]

There are a few threads here on generators. Portable and whole house.

On a portable generator, the majority of the problems are related to floating ground. It has to be bonded to neutral and many generators do not do that.

 

[rgedad]

I did one sweep looking for articles on charging via generators , but I will look again.

I am looking at a permanent installed (non-portable) unit, powered via permanent (black pipe) connection to the house Natural Gas line. The units at this size will have an automatic transfer switch operation. I assume (but will certainly verify) that it will be fully grounded by code.

Given the city inspector came out to inspect the Wall Charger install, and went so far as to have the installer remove all panels to verify the wiring, and Wall Charger settings, I feel confident they will make sure the install is to code.

Thank you for that the item to check though - It is always the "assumptions" that get you.

 

[DCGOO]

The car will consume 100% of your 20 kw generator capacity if you let it. 240V @72 amps = over 17 KW.

Given the cost of natural gas (or whatever you backup fuel source is) is at least 5 to 10 times your normal electricity cost, it makes no sense to charge your car from a backup generator. You would need to at least double the capacity of your generator to make it viable. It may not even be possible. Generators in the 40-50kw size typically require more NG pressure than is available on residential service.

My standby generator is not quite as large as yours, but I left the car off the generator. If I really get jammed up, There are a couple of superchargers within 20 miles of me, which I never visit. But if I really got jammed up, I could go there. You also could reduce the charge rate down to 4 or 5 KW. This would not affect your cost to charge, but might allow some energy to feed your car. You will have to monitor it carefully.

Have you considered 4 Powerwall units? (they're good for about 5Kw each). I have a discount code you can use, LOL.

 

[ewoodrick]

Take a look at the pricing for running the generator. I believe that you may find that the cost of fuel to be significantly more than you pay for electricity. And above the basic fuel cost, don't forget that additional costs of oil and maintenance on the generator, I believe that they are often rated for about 100 hours of operation.

 

[Keiki]

Last time we lost power in my area (hurricane Sandy) we also got caught out with NO gasoline. Even if it's much more expensive (in the short run) it's nice to be able to get around. Gasoline shortages at that time lasted for days and days (as did the power outage)

 

[rgedad]

This is not meant as a permanent charging option, but as a back-up when/if the power drops off. If I am in a no power situation - I really do not care the cost of the limited number of re-charges of the car (or at least I would hope a limited number situation excepting like hurricane Sandy or Houston floods).

Yes, I realize I would need to reduce the charge rate of the car when operating off the back-up generator, but that should not be an issue given our normal usage patterns with limited miles per day, and typically full nights available for charging.
The 20KW back-up generator should be ample (with margin) for the AC units along with the car charging at say 5KW.

This unit will run on Natural Gas, not on gasoline. I"assume" that will keep on even with an electrical power fail situation. FYI: The typical 20KW rated unit is typically derated to ~18KW when running on Natural Gas vs Propane.

FYI: I am looking at about $10K-$12K purchase /install cost. The cost of the installation is just about equal to the cost of the actual unit. Installation includes about $800 cost to run the Natural Gas line along the side of the house direct from the gas meter to the generator location by the Garage / AC Main Panel (and away from any windows, doors and the like).

When purchased the Tesla, and started to look at a solar approach, I was informed that putting on solar panels was not an option from an house roof line Point of View. And the Powerwall units cannot currently be configured as anything but a back-up power when connected to the main line power.

What I am looking for is any information on required power quality for charging, or impact on a back-up generator (ie power factor issues or harmonic generation and the such).

 

[brkaus]

I think you will be fine. You will probably have to back down the charge current, as you already know. Most larger generators put out pretty clean power, but frequency may sag a bit when new loads come online

The one thing I would be a bit concerned about is what happens if you are charging at max rate, there is a power outage, and the generator automatically switches on. Pretty big draw on the generator instantly, and the car may not be happy with the switch over. Might consider putting a 14-50 on the generator and leaving the HPWC off of it? Then you can still charge, but it would be manual and you could monitor it. Just thinking out loud.

 

[DCGOO]

Yes, I realize I would need to reduce the charge rate of the car when operating off the back-up generator, but that should not be an issue given our normal usage patterns with limited miles per day, and typically full nights available for charging.
The 20KW back-up generator should be ample (with margin) for the AC units along with the car charging at say 5KW.

A 20KW generator will never be "ample" for your setup. It may be "marginally acceptable," which might be good enough under the right conditions. To be ample, you would need 50KW (for 200 amp service). Worst case though, you may pop the main breakers from the generator, but that is what they are for. IOW, properly installed it should not be dangerous. It just might not work as you expect. Especially if you happen to not be home when a failure occurs.

For an extended outage, you are probably looking at $3,000 to $5,000 per month hit to your gas bill. The conversion efficiency of NG to electricity is pretty awful.

 

[rgedad]

Yes - I concur with that over-power / start-up current draw concern. In discussing that potential issue, one solution discussed with the representative would be to put in a prioritized transfer switch where if the generator is running it would prioritize the usage and effectively disable the Wall Charger if it might draw too much current.
The transfer switches are getting pretty smart these days. They can be much more that just transfer either Main or back-up power to the main breaker. We did not get into the complexity / cost of that approach, but it is a possibility and it is standard set-up they have available.

A simpler, and probably less expensive/complex approach, would be to just reduce my normal Tesla charging rate to a sustainable rate and be done with the problem. Should not affect my normal daily routine given my typical daily milage, and available charge time. In the unlikely/seldom needed high charge rate was needed, and I know I have main power, I could just increase the charge rate for that particular session.

 

[rgedad]

Yes - I concur with that over-power / start-up current draw concern. In discussing that potential issue, one solution discussed with the representative would be to put in a prioritized transfer switch where if the generator is running it would prioritize the usage and effectively disable the Wall Charger if it might draw too much current.
The transfer switches are getting pretty smart these days. They can be much more that just transfer either Main or back-up power to the main breaker. We did not get into the complexity / cost of that approach, but it is a possibility but it is standard set-up they have available.

A simpler, and probably less expensive/complex approach, would be to just reduce my normal Tesla charging rate to a sustainable rate and be done with the problem. Should not affect my normal daily routine given my typical daily milage, and available charge time. In the unlikely/seldom needed high charge rate was needed, and I know I have main power, I could just increase the charge rate for that particular session.

 

[brkaus]

Yes - I concur with that over-power / start-up current draw concern. In discussing that potential issue, one solution discussed with the representative would be to put in a prioritized transfer switch where if the generator is running it would prioritize the usage and effectively disable the Wall Charger if it might draw too much current.
The transfer switches are getting pretty smart these days. They can be much more that just transfer either Main or back-up power to the main breaker. We did not get into the complexity / cost of that approach, but it is a possibility but it is standard set-up they have available.

A simpler, and probably less expensive/complex approach, would be to just reduce my normal Tesla charging rate to a sustainable rate and be done with the problem. Should not affect my normal daily routine given my typical daily milage, and available charge time. In the unlikely/seldom needed high charge rate was needed, and I know I have main power, I could just increase the charge rate for that particular session.

I’m sure a fancy load shedding transfer switch would handle it. The beauty of adding a 14-50 (or 14-30) is that there are no fancy electronics. In the unlikely event of an outage, you get out your UMC. No what if’s, etc.

 

[Only Trons]

After a four day loss of electrical power here in my community in Northern Virginia several years ago, followed by outages of 10 - 12 hours a few other times, I threw in the towel and purchased a stand alone installed generator similar to what you are discussing. 20 kW hooked via permanent connection to the natural gas line. Two ATS installed, but neither is "smart" enough to do fancy load shedding or prioritizing. My charging equipment consists of a NEMA 14-50 plug in the garage with the early version of the UMC (e.g. 40 amp capable).

After I had the generator installed, I did a test in which I tripped the utility power, let the generator automatically start and pick up the load, and then I plugged in the Model S and watched it for awhile. No problems experienced. I agree that having the car start charging immediately after the generator kicks in could cause start-up current concerns, but I've not tested for that specific condition. If I were to be running both house HVACs at the same time, upon a power out condition they go to a five minute "wait" period before the compressors restart, so the HVACs shouldn't cause a problem.

I've had this installed for several years now and am happy with it.

 

[mwesterheide]

I have a Generac 20KW generator. What I had done is to put a Generac Smart Module in the line supplying power to charge my Model X. The smart module will detect when the Generator is under strain and drop the load if necessary. The smart module is rated at 50 amps. I thought this was the best solution as I have 2 a/c units, a pond (with pump which gets dropped if there is a power failure and the generator kicks in) and electric dryer. I only charge at 40 amps but I found that to be enough. I also have a TED (the energy detective) so I can monitor how much power my house draws even when on generator. So far the only power outages I have experienced have been when the car was already charged up.