Charging battery using a home stand-by generators in an emergency situation

Charging battery using a home stand-by generators in an emergency situation

I'd like to start a conversation around charging the battery when the power from the grid in not available.

In terms of KW capacity of the generator, how much more than what the house needs would you get to ensure proper

In terms of charging while home appliances also need the current, should one plug the car into a 110v so that it does not suck in all the Amps provided by the generator (assuming you don't have a monster generator).

In terms of class of generator, should one get an industrial grade? After all, this is the insurance against bricking the battery.

In terms of fuel type, what is the best in an emergency? Gasoline, diesel, NG/LP? Do I want to wait in line to Gasoline if the diesel line is shorter? (it could happen)

What else should one be prepared for?

VincentA | 5 luglio 2012

1.5 kW of capacity dedicated to the vehicle should be enough for 120V charging, but recall that this will only charge a few miles of range per hour of running the generator. Power would have to be off for many days for this to be necessary or sensible.

We live in a place with frequent power outages in winter (BC, Canada), and so have a 15 kW propane-fired generator supplying the house and garage. However, in two years of owning our Roadster we have never needed to charge it from the generator. Since Tesla's have such huge range in relation to normal daily driving, there is a lot of margin available before this would become an issue.

jerry3 | 5 luglio 2012

The important thing is to have a back up generator that produces true sine waves (some produce square waves which won't work at all, and some produce modified square waves which may work but will stress the electronics). Many backup and portable generators do not produce true sine waves, and Roadster owners will tell you that those won't charge a Roadster.

Getting equipment that is safe for data centre computing equipment (since it's very unlikely that generator suppliers know about EVs) should be fine.

From all I've read, LPG is best because in a real emergency situation, it's the fuel that is likely to be most available. The NG lines are convenient, but likely to be cut in a disaster. Diesel fuel and gas will be in short supply during a disaster. Some generators work with both NG and LPG, so you can use NG for normal power outages and LPG for a disaster situation.

The Model S is touted to be good for at least 12 months. If your disaster lasts 12 months or longer, you'll have a lot bigger problems than bricking the Model S.

It is not efficient to charge at 110V. You'll actually use more power. 40 amp charging has been found to be the most efficient in the Roadster. The Model S will likely be similar (of course, no one has actually measured this yet).

A 20,000 kWh to 30,000 kWh should be enough for all but the largest houses.

Sudre_ | 5 luglio 2012

Also think of it this way. Most cars get about 300 miles per fill up. I have to fill my ICE car about once every two weeks. That means my power would have to be off for at least a week before there would be any concern about charging. (I am getting the 60kw battery)

I would be more interested in the reverse. Running my house from the car.

jerry3 | 5 luglio 2012


Running the house from the car only works with hybrids. There have been any number of people who have done this with their Prius.

nolngr-grsing-s... | 5 luglio 2012

Great insight guys, please keep them coming. Thanks for sharing your actual roadster experience.

My concern is about coming home from a long trip and then getting hit with a storm. I am in a wooded neighborhood and trees fall on lines frequently. This week in the MD-DV-VA area, some people have been without power for almost a week now.

The true sign wave requirement is a great one I had not thought of. I certainly cannot go for a cheap generator.

nolngr-grsing-s... | 5 luglio 2012

Its MD-DC-VA area. Sorry for typo.

Teoatawki | 6 luglio 2012

'Bricking' should be a total non-issue for any reasonable length power outage.

From the Plug it in blog entry:
For example, a Model S battery parked with 50 percent charge would approach full discharge only after about 12 months. Model S batteries also have the ability to protect themselves as they approach very low charge levels by going into a “deep sleep” mode that lowers the loss even further. A Model S will not allow its battery to fall below about 5 percent charge. At that point the car can still sit for many months. Of course you can drive a Model S to 0 percent charge, but even in that circumstance, if you plug it in within 30 days, the battery will recover normally.

So, if you arrive home with a flat battery, you don't need to worry until the power outage exceeds 30 days. With at least 5%, you've got months before you need to worry.

rbmartin | 6 luglio 2012

All engine-driven mechanical generators produce sine waves. I think you are confusing generators with inverter systems which produce AC electronically from DC sources like batteries or solar panels.

Timo | 6 luglio 2012

@jerry3 A 20,000 kWh to 30,000 kWh should be enough for all but the largest houses.

My whole year is just 2000kWh, I take that you mean 20 to 30 kWh, not 20 to 30 MWh.

nolngr-grsing-s... | 6 luglio 2012

Question for VincentA or anyone living near a mountain.

If you drove your Roadster to the top of the mountain, to Whistler perhaps, and then coasted back down, how much of the what you spent in Wattage would you generate back.

What if you loaded the trunk up on the way down, such as with rocks, would you generate more, perhaps get back the energy you burned on the way up.

I have this dream of one day being able to drive an electric car for a long time up and down a mountain using only the energy from the initial charge.

In an ideal world, where Tesla would build something for our benefit as well as their shareholders, we would be able to charge as much as physically possible via regenerative breaking as well as maybe other reasonable means. So, it is conceivable that, if i drove alone up a mountain, and spent X Watts to get there, i could transport a couple more people (or jugs of water) on the way down and regenerate more than X in watts.

MandL | 6 luglio 2012

Running the house from the car only works with hybrids.
Not true. You can (or will be able to soon) with a Leaf

And of course, while bricking might not be a serious concern from extended power outages, potentially being stranded with a discharged car would at least be a hassle. Hopefully there will be power at a mall or somewhere where you can plug in for a while.

GoTeslaChicago | 6 luglio 2012

"if i drove alone up a mountain, and spent X Watts to get there, i could transport a couple more people (or jugs of water) on the way down and regenerate more than X in watts."

Assuming for the sake of argument that regenerative braking is 50% efficient, then your vehicle would have to weigh twice as much on the way down as the way up to break even. Thats a lot of people (or water jugs) to fit in the trunk (and frunk) if is the Model S!!

Brian H | 6 luglio 2012

Bringing down rocks will eventually reduce the mountain to a molehill. The skiers will be displeased!

jerry3 | 6 luglio 2012


You are correct. I typed too many zeros.

jerry3 | 6 luglio 2012

Brian H,

- Bringing down rocks will eventually reduce the mountain to a molehill. The skiers will be displeased!

That's why you only bring down snow, not rocks.

jerry3 | 6 luglio 2012


I should have said "charging for an extended period is only possible with a hybrid". I don't think I'd use any EV to do that in a power outage situation. I guess the idea is to power the house during the peak period to reduce electric costs and then recharge the car at night.

EdG | 6 luglio 2012

The idea would be to plug in the fridge now and then to keep the food from wasting away, while waiting for the power to come back on.

stevenmaifert | 7 luglio 2012

nolngr-grsing-s... - One of these will work for you: They connect to your home power panel, feature auto start, run on either NG or LP, and are certified for sensitive electronics. Unless you have a lot of heavy draw appliances you need to run all at once, you could probably get by with the 8Kw version.

nolngr-grsing-s... | 7 luglio 2012

stevenmaifert: tanks for the link. This is exactly what i was looking at but, i need to get a larger tank than 100 Gallons. I happen to have 500 gallon oil tank that i would like to use but diesel generators that are larger than 6.5 KW are expensive.

Brian H | 7 luglio 2012

Daisy-chain a couple! ;)

TonyF | 9 luglio 2012

This year I purchased a 20KW Generac whole house (propane) generator for my home. When I did this, I had the electrician install Generac's DLM Load Control Modules for each of my high power appliances (dryer, Hot water heater, AC, and - you guessed it - my Tesla). These load control modules will 'balance' the distribution of power from the generator to these appliances on a priority basis because the generator can't power all four 220V appliances all at once while keeping other circuits in the house powered. But you can prioritize these modules so that the generator looks to power one appliance before another as draw lessens and power becomes available. I had the Tesla placed as highest priority, but if it doesn't need to be charged, I simply won't plug it in to my 220 outlet I had installed in my garage so the next highest priority will be powered.

Since I don't have my model S yet, I have not tried the generator to see how this all works. But Generac is a leader in generators and I have no reason to believe that it will not charge my car. I ran the numbers to charge the car using propane (at the time it cost $3.86/gal) and it turns out to be (if I remember) about 4 times the cost of using utility electricity. The generator I purchased supplies 20KW at full power and will consume 2.9 gal of propane per hour at that output. So 85KWh of energy will cost me 12.3 gallons of propane. Almost as bad as going back to the pump. But not too bad in an emergency.

EdG | 9 luglio 2012

If you want the car to take low priority even when plugged in, you should be able to tell the car not to charge until a time that suits you better. At that time, because it would start to draw it would take the priority juice.