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How long does a Powerwall support a home during a power outage?

miimura

Well-Known Member
Aug 21, 2013
6,166
5,761
Los Altos, CA
@DrGene what you're proposing may be technically more efficient, but has a couple downsides.

1. You're proposing special equipment that is not commercialized, so would be expensive, at least to start with.
2. If the car and the solar are dependent on each other, what happens when you need to drive the car somewhere?

There are several benefits to the way the current Powerwall system is designed.
A. Any brand and type of solar grid-tied inverter can be used.
B. The system is ready to go at all times and can either offset household use while on grid or power the house when the grid is down.
C. The Powerwall system is completely self contained with battery and inverter with only the Gateway box as other special equipment. Everything else in the installation is commercial electrical equipment like breakers and breaker panels.

During a prolonged outage, you can charge the car without charging from the Powerwall batteries with double AC-DC conversion losses - you can charge from the AC power generated by your solar before it even goes into the Powerwall. Sure, a DC-DC converter from solar might be slightly more efficient, but if you already have the on-board charger in the car and you already have the solar inverter for grid-tied usage, there is no additional equipment necessary. I feel like your proposal is a solution looking for a problem that doesn't really exist.
 
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Wol747

Member
Aug 26, 2017
773
310
Tea Gardens
jboy210:

>>Can you have batteries that provide power if you flip a switch to disconnect the home from the grid prior to powering the home from the batteries? <<

We don't have batteries on our solar but IIRC it's illegal to have a grid connection outbound.
 
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GenSao

Member
Aug 3, 2017
534
920
Pleasant Hill, CA
The Power Wall was designed for a normal grid that has an occasional outage. As a professional engineer I have been looking at long term outages. Even here in Austin Texas there is a type of outage that could cause the power to be off for months. So I have been thinking about the emergency aspects of using solar panels and our Teslas for backup power during emergencies. This is for the grid being down.

Suppose our Tesla wall charger which is usually 40 amps at 240 volts was replaced by a special type of inverter in which it can take power directly from solar panels (they are not wired into the home) and ties to the EV and charges the Tesla battery. It does this charging without the power wall battery. The inverter operates directly from the solar panels to the EV battery. The inverter communicates through the cable that plugs into the car to coordinate the amps charge rate. This will insure the home owner will always have power to drive their EV.

But this system can do the power wall function without the power wall. When the grid is down the power can flow from the car back through the cable into the inverter box and back into the homeowners home. The homeowner is now responsible for watching the charge on their EV and not running the car charge too low. The solar panels need to be at least 10 kW capacity to charge the car and have some energy left over to run something in the house like a refrigerator and maybe a couple of lights, but certainly not the AC for the home. Remember this is an emergency setting.

There are several advantages of this:
1) the solar panels only charge the EV so permitting is simpler.
2) power for the home only comes from the EV battery and only when the grid is down.
3) the cost of this is lower than the power wall by avoiding the need for a battery for the power wall.
4) The EV can serve up to 10 kW peak load for a while and the owner uses the charge state of the EV to know how much energy is left.
5) This system is more easily installed during the emergency than trying to wire in a power wall during the emergency since the wiring is entirely to the inverter box that replaces the wall mounted charging unit we currently own.
6) I can install this system because it gets around Austin's stringent rules for solar panels whereas the power wall installation does not.
7) this lower cost scheme would allow Tesla to sell more solar panels more quickly to help those in distress.

Gene Preston PE PhD www.egpreston.com

Gene,

Interesting idea, Vehicle-to-Grid (V2G) has it's pros and cons.

1) Tesla cars have a 120/240 volts AC to DC inverted to convert grid/solar energy. The existing wall connector and mobile connector provides pass though power and does not do any conversion. A special inverter akin to Tesla's DC supercharger is possible.

To have solar panels directly feed into a car (DC) would require a special inverter to do a DC-DC conversion that is comparable to the car voltage. There would be some minor DC-DC loss (though less than the current DC-AC-DC conversion). The only standard charging protocal that is Vehicle-to-Grid (V2G) compliant is CHAdeMO. Although Tesla cars can use a CHAdeMO adapter, their onboard inverter would need to be replaced as the car's inverter are only designed to charge a car. Currently, no power can go to the home from the car.

2) As miimura stated, backup power can be lost when the car is used as a vehicle. Furthermore, the additional wear to the car's battery may reduce or void the car's warranty. Keep in mind the vampire drain (additional energy) when the car is active.

3) A stationary Powerwall battery will always be on standby. A car can and should be used for transportation.

Tesla needs to implement coordination of the solar received and the charging of the Tesla car. Tthe Tesla Gateway can direct spare/excess solar generation (not used in home loads or Powerwall charging) to the Tesla car. The Tesla car can adjust it's charging rate to minimize any AC-DC-AC conversion with the interaction of the Powerwall battery. In the case of an emergency (backup mode), excess solar production can be stored instead of the Gateway turning off solar production when the batteries are full.

4) The Model 3 can charge at 250 kW (DC), so 10 kW is doable. A slight concern is the safety for residential applications for high amount DC (if this method is used). D

5) In order for a home to have backup power, a Tesla Gateway must be installed to isolate the home from the grid. Typically all (or part) of the loads are moved to a new load panel. The amount of labor and coordination/approval of the local building permit agency could take months and not quick to install in an emergency.
 

PBBear

Member
Oct 2, 2018
60
51
Australia
Here in Australia it's not possible to use batteries during power outages!
The excuse is that it's impossible to have batteries in parallel with the mains so that they take over when the mains stop being live, and that it's dangerous because linesmen could be at risk with backflow.
So now, all you folks in the USA, you know it doesn't work! <g>.

Yeah - Nah. There's plenty of PW2 batteries installed in Australia, and many other brands of batteries, and they work fine during power outages. Its perfectly possible and legal and common practice to have batteries back up the mains, just like most other countries.
Its not dangerous, the power-backup gateway does the same thing it does everywhere else - the relay disconnects and isolates the grid, so the battery is providing power to the house while causing an open-circuit towards the grid and NOT backfeeding any dangerous voltage towards any linesman.
Not too sure where you're getting your info from Wol, but you need to find another drinking-hole mate!
 

bob_p

Active Member
Apr 5, 2012
3,664
2,790
If it's even technically feasible, Tesla would likely be able to detect using a vehicle battery for backup power - and would almost certainly cancel the warranty for the battery pack, and possibly the entire vehicle.

We have an S 100D and X 100D - if they were both fully charged, that's as much power as about 14 PowerWalls - and in an emergency could power most homes for days.

Hardware would be needed to convert the pack power so it can be used as 110 or 220 - and something like a gateway for switching between grid and the battery packs.

With the recent CA power outages, Tesla has focused on helping S/X/3 owners find locations to charge - and for PW owners to have fully charged batteries prior to an outage.

Might be interesting for Tesla to treat the vehicle batteries like PowerWalls - using the wall connectors to both charge the vehicles under normal operation, and in emergencies provide power to the home. Of course, that would require changes inside the vehicle and would make the wall connector installation considerably more complicated.

And approval would likely be needed with the local electric utility and possibly even an HOA.

Though it is an interesting idea - and because of Tesla's market leading battery technology - could create another capability that would be difficult for the competing EV manufacturers to match (who are vehicle manufacturers only - not vehicle/energy companies).
 

Thp3

Member
Aug 21, 2017
480
439
Michigan
We live in Michigan so we rarely see the sun during the winter. And we installed a gas-powered automatic whole house backup generator as backup to our electric utility.

Am I correct in assuming that because of our cloudy winters, it would not be possible to justify a solar system? We pay 10-12 cents/kWh.

Nope, solar power in Michigan is justified and economic if you plan to be alive in 15 years. If not, well, maybe part of the estate then. It’s paid for in 8-15 years depending on how much you buy. And it’s a great way to reduce your carbon footprint.

I have a whole house gas generator and solar panels. When I move to two Tesla’s, then I’ll add one or two powerwalls.

In Michigan, we pay about 19-20 cents a kWh since grid/distribution charges are also part of the cost.
 

jboy210

Supporting Member
Dec 2, 2016
4,946
3,022
Northern California
Might be interesting for Tesla to treat the vehicle batteries like PowerWalls - using the wall connectors to both charge the vehicles under normal operation, and in emergencies provide power to the home. Of course, that would require changes inside the vehicle and would make the wall connector installation considerably more complicated.

In Europe some companies are using their EVs this way in Vehicle-to-Grid (V2G) arrangements, Tesla played with it on the original Roadster. Nissan was playing with Vehicle-to-Home (V2H). V2G: What's the state of play with vehicle-to-grid, vehicle-to-home technology? | The Driven
 

liuping

Active Member
Jul 23, 2013
2,242
896
San Diego
The solar panels need to be at least 10 kW capacity
You would need a very large solar array to generate 10kW. I have a 33 panel "8.3kW" system. In summer I can generate about 7kW max, but only for about an hour.

Ideally, in your set up, Tesla could monitor the home usage, and adjust it's charging rate to just use the left over solar.
 

d21mike

Active Member
Aug 28, 2017
1,097
722
Torrance, CA 90503
Tesla would install slow start devices on your AC to help with the initial draw.
My recent installation they provided a SureStart device but they so have not been able to get it working. Still waiting for a resolution. The city did not approve one or more of the breakers so waiting for that to be resolved as well.
 

d21mike

Active Member
Aug 28, 2017
1,097
722
Torrance, CA 90503
In often cases, the A/C is not compatible with soft start devices. In the sales phase, they are speaking generalization without know more about the particulars of your AC. 2 powerwalls can easily offset your A/C loads during peak periods. I have two BASIC A/C units that peak 8-9kW, and a single one does 4.8kW. In theory if you have one powerwall you can offset one A/C unit. Keep in mind both A/C are not backed but ARE monitored by the powerwall system... in other words, they don't operate when grid is out, but when grid is on... between 4-9pm, nothing draws from grid (until powerwalls reach the powerwall user's specified-reserve limit). In fact when the sun is out, solar is sent to grid while A/C + home loads are powered by Powerwall. (while in Time Base Control: either flavors). I am biased towards 3 powerwalls after coming out of 6mo of ownership during the summer, but see that 2 powerwalls are a happy medium, while 1 powerwall is essential for fire season power shutoffs in NorCal.
As I just stated in another post they could not get the SureStart device to work (2 visits). They are planing to review it and get back to me. City did not approve the installation because of one or breakers. They are going to get back to me on that as well.

But regarding your comment about "are not backed but ARE monitored by the power wall system". My system seems to be different. ALL of my breakers were moved into the new control panel and are backed up. Honestly I thought they would not have done that for my A/C and Wall Chargers but they did. I.E. they could have put those in a separate panel like others have done (I think) or simply left them in the main power but all were moved.
 

SpartyTM3

Member
Mar 5, 2019
16
17
Michigan
Nope, solar power in Michigan is justified and economic if you plan to be alive in 15 years. If not, well, maybe part of the estate then. It’s paid for in 8-15 years depending on how much you buy. And it’s a great way to reduce your carbon footprint.

I have a whole house gas generator and solar panels. When I move to two Tesla’s, then I’ll add one or two powerwalls.

In Michigan, we pay about 19-20 cents a kWh since grid/distribution charges are also part of the cost.
Thanks. The reason I bought my TM3 was to reduce my carbon footprint, the rest is icing on the cake. But I was wondering how much drop in electricity generation we should expect from April-Sept vs. October-March.

I’m interested in finding a good company to work with, in case you can recommend anyone. We live on Gun Lake, south of GR.

We live in Consumer Energy territory and use their time of day plan that they recommend for EVs. Unless I’m missing something our off peak cost is 13.5 cents per kWh....around 9 cents for electricity and another 4.5 cents for distribution.
 

Hugh-SG

Member
Jun 3, 2019
89
96
Vancovuer, BC
Thanks for the replies. Looks like people are getting some great backup support with their powerwalls.

I ran across this backup time estimator, How long will Powerwall last in an outage? | Tesla . Looks cool. But, could anyone comment on how accurate it is?

Hi JBoy,

It is a not bad estimator, but as everyone's daily consumption is different what would be 3 days of power for one home would be 18 hours of power for another.

It is obviously set up for sunny temperate places as it does offer offer AC consumption...

.... but has no button for the round the clock consumption of the fan's of a forced air Natural Gas Furnace heating system working in near or below Freezing Conditions.

It does need a button for a Deep Chest food freezer running around the clock. (no one wants to lose several $100 dollars of deep frozen food).

Also the ability to add a 2nd fridge would be nice, for those of us with a 2nd Veggie-Perishables Fridge in the basement.

As my install Tech from TerraTek told me, you stop ALL heavy draw uses Immediately to get the most from a Powerwall.

I mean you go around and turn off or UnPlug what is not essential.

Yes it means you have to unplug a Beer-Wine Fridge, it just became a Luxuary, not an essential. Warm beer or wine never hurt anyone.

He also told me Never use a Stove's Oven, the Washing Machine or Cloths Dryer or Charge your Car unless your Solar can pick up the load at Mid-Day, After the Powerwall is recharged.

... and the kids can learn to Towel their hair dry. Put the Hair Dryer out of reach and turn off all the 24 hour a day running DeskTop computers.

My home currently averages 15 to 18kw a day.

In summary it is important to preserve battery for the kitchen fridge with perishable foods in it and to run a Nat Gas Furnace so you do not freeze your home and cause a flood when your water pipes freeze up and split open.

You now have me curious to see how much we would consume if cut everything back to the bare survival minimum.

Thank you for your question, it was a good one.

Food for thought,
Cheers! Hugh-SG
 
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Thp3

Member
Aug 21, 2017
480
439
Michigan
Thanks. The reason I bought my TM3 was to reduce my carbon footprint, the rest is icing on the cake. But I was wondering how much drop in electricity generation we should expect from April-Sept vs. October-March.

I’m interested in finding a good company to work with, in case you can recommend anyone. We live on Gun Lake, south of GR.

We live in Consumer Energy territory and use their time of day plan that they recommend for EVs. Unless I’m missing something our off peak cost is 13.5 cents per kWh....around 9 cents for electricity and another 4.5 cents for distribution.

I have a 4.5 kWh set of PV panels now. I’ve had them for 7 years. Before the Tesla, I got 40% of my electricity bill from them. I’m upgrading to ~9 kWh real soon now (depending on weather).

I live with Consumers too, but don’t use the split billing that they recommend, just regular electrical rate. Most of my usage is during the day, so the std rate is probably a better choice.

In April-October, pre-Tesla I build up a credit, in October-March, not very much. After the Tesla I pay a bill in the Summer, so it’s time to add PV panels. I’ll likely add a powerwall in the future if my electrical demand increases with a second Tesla. But now it’s just fun to think about a Model Y vs Model X and plan.
 

Wol747

Member
Aug 26, 2017
773
310
Tea Gardens
Yeah - Nah. There's plenty of PW2 batteries installed in Australia, and many other brands of batteries, and they work fine during power outages. Its perfectly possible and legal and common practice to have batteries back up the mains, just like most other countries.
Its not dangerous, the power-backup gateway does the same thing it does everywhere else - the relay disconnects and isolates the grid, so the battery is providing power to the house while causing an open-circuit towards the grid and NOT backfeeding any dangerous voltage towards any linesman.
Not too sure where you're getting your info from Wol, but you need to find another drinking-hole mate!

I am pleased to be corrected!

My sources were twofold: my stepson has an array of IIRC48 panels and a roomful of industrial lead/acid batteries in the Blue Mountains. After several years of the installer trying to connect "outbound" this is what he was told - that the energy company forbade this on the grounds I posted above.

I myself have a 12 panel non battery array which sends power outbound but of course the reverse current from a battery issue doesn't arise with me. However the energy man did tell me the same tale.

I was always leery about the installer's explanation: the control electronics are not exactly cutting edge! I would suppose however that the cut-out system must be at least duplicated to avoid the reverse current problem if there's a single failure?
 

Thp3

Member
Aug 21, 2017
480
439
Michigan
I myself have a 12 panel non battery array which sends power outbound but of course the reverse current from a battery issue doesn't arise with me. However the energy man did tell me the same tale.

I was always leery about the installer's explanation: the control electronics are not exactly cutting edge! I would suppose however that the cut-out system must be at least duplicated to avoid the reverse current problem if there's a single failure?

I expect you have a hand cutoff breaker for the power company to throw if they have to work near your PV panels. I had to sign a legal agreement that the power company could walk up to the breaker outside and throw it when they wanted to, that is likely required for a non-battery PV system.

With whole house generators there is an automatic cutoff switch between the house and the power lines. If power stops over the power lines, the cutoff switch throws, and the generator starts. I suspect that a battery setup has a similar install. As for DIY stuff with lead acid batteries, well those setups are unique and perhaps less safe.
 

MorrisonHiker

S 100D 2021.4.15
Mar 8, 2015
9,568
8,753
Colorado
With whole house generators there is an automatic cutoff switch between the house and the power lines. If power stops over the power lines, the cutoff switch throws, and the generator starts. I suspect that a battery setup has a similar install. As for DIY stuff with lead acid batteries, well those setups are unique and perhaps less safe.
Yes, the Powerwalls and gateway automatically isolate the house from the grid when the grid goes down.
 

Wol747

Member
Aug 26, 2017
773
310
Tea Gardens
I would surmise that if it's just, panels with no batteries, the argument is that if the grid fails there might not be enough power (say at night) to make the disconnection happen?

It does seem a trifle risky to allow thousands of home installations to cut in and out, matching frequencies etc, using probably Chinese made cutouts costing a few dollars and made partly in sweatshop Ma and Pa subcontractors' shacks!

>>As for DIY stuff with lead acid batteries, well those setups are unique and perhaps less safe. <<

His installation was certainly not DIY! It had to be done by a licenced electrician who should have been familiar with the regulations and three phase 240v. It's a neat job, there's a battery room with a PC monitoring it all - it's just that he doesn't seem to be allowed to connect. In fact he gave up last year and just uses the system to power the home!
 

Cutwolf

Member
Aug 8, 2013
58
7
USA
Sorry to bump this old thread, but I'm looking into getting solar + powerwall primarily for backup since zoning does not allow me anywhere to place a standby generator.

I'm looking at an 8.16 kw system (8547 kWh estimated over the year) plus 2 powerwalls.

Will the solar continuously replenish the batteries while running on backup power during an outage? For example, with two powerwalls, that's 27 KwH. My daily generation estimate is 23.4 kWh. Does this mean functionally speaking, on the first day of the outage (assuming ordinary usage), I'll have 50.4 kWh of energy to use (27 from fully charged powerwalls plus the 23.4 being generated from the panel)? And then each day thereafter I'd be limited to the 23.4 kWh?
 

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