Powerwall for Planned Power Outages

[gpez]

Okay, here's my current thinking the three options.

1. Buy one powerwall $7,600
2. Buy two powerwalls $14,100
3. Upgrade to a new system 7.56kW $24,790 after tax credit

I think but am not sure that I have eliminated just one Powerwall. I would like to be able to be relaxed if and when a power outage occurs. Just one Powerwall might be enough. But two ready to go during wildfire season would be quite comfortable with charging the Tesla at the superchargers nearby.

The new solar system would be double of what I have, which is fine. I don't really mind giving away electricity for free as it helps the environment.

I got one response on donating the panels, but no follow back email yet. Sounds like it is not an easy or standard process. I may end up trying to find a friend who can use it instead or selling them for a marginal amount of money if I don't hear back in a few days. I looked through craigslist and there doesn't seem to be much selling of old solar systems - just a panel here and there for $30.

Very much on the fence still but appreciate everyone's help. You are all much more knowledgeable than the folks at Tesla. One person did give me a specialist that I could call but haven't done so yet.

Is it worth the effort to look at other solar companies? I am not a nickel and dime person.

Any strong opinions on the three options? My electrical bill annually will only be $500 max with the existing panels.

Just to double check - is storage your only consideration for the extra Powerwall? As has been alluded to here on the thread, though in somewhat muddied terms, is that each Powerwall has a 5kw inverter limit. You mentioned you're only interested in backing up relatively low-wattage appliances (fridge, freezer, router, etc) but if you have an air conditioner or electric oven, dryer, hot water heater, etc that may tip you to get a 2nd Powerwall so you have the option of running those appliances (even if for short periods of time).

Inverter size aside I did a lot of the same calculations you did as we were putting in our 8.55kw system and looking at the Powerwall for backup only. We opted for only 1 Powerwall because:

1) Our outage scenarios are relatively short but not uncommon weather related outages (<24 hours) and as I mentioned with some moderate conservation one Powerwall would last that with absolutely no input from the PV (which is pretty uncommon)
2) We don't need high wattage appliances during an outage - it doesn't get warm enough here to necessitate running the A/C but it's nice and the water heater is a tankless natural gas one that draws little power when in use (actually ran it on a computer sized UPS during an outage before our system was installed, fun story)
3) In an emergency I can charge my EV with a backed up 120v/15a outlet (though not great it will do in a pinch)
4) In an extreme situation, like an earthquake or extended outage, I have an inverter harness kit so I can use my EV to power appliances directly to reduce draw on the Powerwall + PV

And probably most importantly:
5) Until the utility starts TOU pricing the battery tech is only going to get better so we opted to get only 1 with the thinking that later down the line we may want to get a second or whatever "Powerwall 3" is.

 

[Owner]

but if you have an air conditioner or electric oven, dryer, hot water heater, etc that may tip you to get a 2nd Powerwall so you have the option of running those appliances (even if for short periods of time).

5) Until the utility starts TOU pricing the battery tech is only going to get better so we opted to get only 1 with the thinking that later down the line we may want to get a second or whatever "Powerwall 3" is.

I have two air conditioners, but use maybe one of them 3 days of the year. Not an issue.
Dryer - don't remember how it is powered. But most clothes are dried with natural air. Dryer used about once a month. Not needed during an outage.
Hot water - natural gas fueled.
Stove - gas
Oven probably electric - not important can use stove during an outage.

Definitely have TOU pricing here.

Have an ancient plasma TV that needs replacing. It is really old. It pulls most of the electricity other than the car as it is a power hog. I'm sure the energy needs will drop when this is finally swapped out. Just one of those to do things that don't happen till they break as it is not an important thing.

Maybe I'm now -- a few hours later -- leaning toward one powerwall with existing system. Then waiting for say 10 years when something dies in my solar system and then I can upgrade it with an additional powerwall.
I don't have anyone in real life that understands this, so again very helpful.

 

[ra88it]

That's interesting that you choose to undercharge your PW's on purpose. How long of an outage based on your consumption do you expect the undercharged PW's to be able to support?

Pardon me? Undercharge? They're a storage device, like your petrol tank in your standard vehicle. If the petrol station is out of fuel and only has 18 of the 25gal that you need... is your car going to make it to 100k miles before the motor explodes?

I bought the max the TEG would allow due to extended outages or winter months with lack of PV, my PV system is quite large and easily fills them on a spring day.... let alone a summer one.

You need to do the correct calculations for the amount of energy you use, in order to know how much storage you wish to pay to install on your own home. For me, this was the most viable solution.

 

[Owner]

So looking more at my usage I'm going to eliminate the two powerwall option, it just doesn't make sense. Even on the best days with an empty house and good sun, there was never a net usage to the grid more than 11kW. So a little extra energy if there was a shutoff, but the second powerwall would never be topped off with my current system.

So I'm now down to two options:

1. Buy one powerwall $7,600

3. Upgrade to a new system 7.56kW $24,790 after tax credit

 

[cali8484]

I guess you could also mean ra88it's statement on 30-85% status being "undercharged" but I interpreted that to mean that during an extended outage the relatively small PV system wouldn't be able to completely keep up with the house demand so the Powerwalls will drain over time to accommodate.

Yes, this is what I was referring to and I assumed he shared the typical state of charge range for his system.

 

[cali8484]

Pardon me? Undercharge? They're a storage device, like your petrol tank in your standard vehicle. If the petrol station is out of fuel and only has 18 of the 25gal that you need... is your car going to make it to 100k miles before the motor explodes?

I bought the max the TEG would allow due to extended outages or winter months with lack of PV, my PV system is quite large and easily fills them on a spring day.... let alone a summer one.

You need to do the correct calculations for the amount of energy you use, in order to know how much storage you wish to pay to install on your own home. For me, this was the most viable solution.

Relax. Nothing is going to explode regardless. Just wondering what is the longest outage you expect to get through with the batteries.

 

[cali8484]

So looking more at my usage I'm going to eliminate the two powerwall option, it just doesn't make sense. Even on the best days with an empty house and good sun, there was never a net usage to the grid more than 11kW. So a little extra energy if there was a shutoff, but the second powerwall would never be topped off with my current system.

So I'm now down to two options:

1. Buy one powerwall $7,600

3. Upgrade to a new system 7.56kW $24,790 after tax credit

Based your expectation of only running critical loads for short outages #1 should meet your needs.

 

[gpez]

So looking more at my usage I'm going to eliminate the two powerwall option, it just doesn't make sense. Even on the best days with an empty house and good sun, there was never a net usage to the grid more than 11kW. So a little extra energy if there was a shutoff, but the second powerwall would never be topped off with my current system.

@Owner I'm still really confused by this statement.

What is it you mean by the second Powerwall would never be topped off?

I believe you mean "net usage to the grid more than 11kWh" (the hours being important) but that doesn't matter if you have 1 or 10 Powerwalls. If your net consumption when the grid is down and the PV is not producing is lower than the storage capacity of all of your Powerwalls then you're good. If you expect that your PV can't keep up with your consumption or the sun is unreliable (as it is here by me in the pacific northwest) then you'd want more Powerwalls to help ride you through grid outages.

I think there's a disconnect in how you're thinking about this and I'm trying to understand

 

[ra88it]

Relax. Nothing is going to explode regardless. Just wondering what is the longest outage you expect to get through with the batteries.

alright duder... no one wasn’t relaxed. You asked a question and were provided an answer, you didn’t like it or looked entirely too far into it and I wasn’t the only one to call you out on your reaction.

 

[Owner]

@Owner I'm still really confused by this statement.

What is it you mean by the second Powerwall would never be topped off?

I believe you mean "net usage to the grid more than 11kWh" (the hours being important) but that doesn't matter if you have 1 or 10 Powerwalls. If your net consumption when the grid is down and the PV is not producing is lower than the storage capacity of all of your Powerwalls then you're good. If you expect that your PV can't keep up with your consumption or the sun is unreliable (as it is here by me in the pacific northwest) then you'd want more Powerwalls to help ride you through grid outages.

I think there's a disconnect in how you're thinking about this and I'm trying to understand

I was thinking that if say the power was out for several days. With either 1 or 2 powerwalls, I would run out of the initial charge and need the PV system to keep refilling them.

My scenario assumes it is in late summer through fall. The skies are clear. If it had rained already there likely wouldn't be a power outage because PG&E wouldn't be worried about wildfires. We just need one significant rain and the fire department stops worrying.

So in this scenario, the second Powerwall only gives me one more day in the sequence of days.

Day 1 - Use juice from Powerwall 1
Day 2 - Use juice from Powerwall 2
Day 3 - Use solar power that recharged Powerwall 1
Day 4 - Use solar power that recharged Powerwall 1 again
etc...

So the only advantage of the second powerwall would be that cushion of Day #2. Because the solar when the power is off is not strong enough to support more than one powerwall.

This is all kind of theoretical, but the advantage of the second Powerwall is not that significant. With one powerwall the scenario is almost the same:

Day 1 - Use juice from Powerwall 1
Day 2 - Use solar power that recharged Powerwall 1
Day 3 - Use solar power that recharged Powerwall 1 again
Day 4 - Use solar power that recharged Powerwall 1 again

So the extra powerwall just softens the impact a little bit. But not enough without new solar panels.

It could be tight if the solar situation was down to say 5kWh but would be okay to keep the fridge going and electronics. Perhaps not perfect but completely livable.

Another powerwall seems too pricey for what it would offer.

Last year I think there were three big outages. Fortunately I am not affected by the PG&E outage other than internet goes out after about 5 hours.

But there were I think there were some glitch outages that lasted less than 1 day.

I don't know what will happen in 2020. PG&E is going to do some more power outages.

 

[gpez]

So in this scenario, the second Powerwall only gives me one more day in the sequence of days.

Day 1 - Use juice from Powerwall 1
Day 2 - Use juice from Powerwall 2
Day 3 - Use solar power that recharged Powerwall 1
Day 4 - Use solar power that recharged Powerwall 1 again
etc...

So the only advantage of the second powerwall would be that cushion of Day #2. Because the solar when the power is off is not strong enough to support more than one powerwall.

This is all kind of theoretical, but the advantage of the second Powerwall is not that significant. With one powerwall the scenario is almost the same:

Day 1 - Use juice from Powerwall 1
Day 2 - Use solar power that recharged Powerwall 1
Day 3 - Use solar power that recharged Powerwall 1 again
Day 4 - Use solar power that recharged Powerwall 1 again

I think you're trying to simplify thinking how it works but it's not quite like that - Powerwalls drain together (or at least you have no control over it) so adding more of them simply increases your capacity. Only the difference between what your PV is producing and what your home is using is taken or stored in the Powerwall - power produced by the PV is immediately consumed by your home first. You need to do the actual math about how much power your PV produces and how much your home consumes which will then tell you how long each Powerwall will last.

The best way I've found to think about it to draw a water analogy. The Powerwall is a bucket of water and the PV system is like a pipe that supplies the bucket with various rates of water over the day and your home is a pipe that takes water from the bucket. During an outage if the amount of water that your home is consuming exceeds the PV production then the excess is taken from the bucket. Conversely if more water is flowing from the PV than the house is using then the bucket will fill back up, until it is full of course. If you had a massive bucket and only a tiny bit from the PV your house would run for a while. If your PV produced more than your house all of the time (even at night) it wouldn't matter how big your bucket is because you'd never need it. Of course reality is you're somewhere in between. But if you have two Powerwalls it's not like you have two buckets that you're alternating filling and emptying. The PV flows to the Powerwall flows to your home so it's possible (and happens often) that the power produced by your PV is immediately used by your home and not stored in the Powerwall at all.

Could we try the math together? For the scenario you outlined during an outage in the late summer/early fall:
- On a good, bad, and average day how much does your PV produce, in kwh?
- How much power does your home use when the sun is up, in kwh? (during an outage, remember)
- How much power does your home use when the sun is down, in kwh? (during an outage again)

I looked at my production history and last September my 8.55kw system generated from 7.8kwh to 39.4kwh with an average of 22.2kwh. Let's look at the worst possible scenario: only minimum solar power days, power goes out right as the sun sets, and I reduced our power consumption to 7.2kwh during the day (12 hours @ 600w) and 4.8kwh at night (12 hours @ 400w) then here is how my 1 Powerwall with 13kwh stored would fare looking at 12 hour chunks when the sun is up and down:

In my scenario the 1 Powerwall will last just under 2 days.

How long would 2 Powerwalls last with no change in solar production? Should be twice that, right? Let's see:

Wait! That's actually closer to 3x as long! How is that right? Well it's because production happens only during part of the day and your consumption changes at night as well. Once the Powerwall reaches 0 it's over even if the sun is about to come up.

All of this is SUPER dependent on your actual numbers, of course, and who can predict the weather?

(Should note too that I simplified a lot here: the Powerwall has a 90% charge/discharge efficiency and technically has up to 13.5kwh usable)

 

[Mokuzai]

The number of powerwalls you can get permission to operate may be dependent on your total generation capabilities. We have a 12.5kw solar system and are being told that SCE will only provide PTO for three powerwall units based on that size. Also being told that backing up our AC will require four total powerwalls to cover the total continuous draw for the house. So now I’m trying to find a way to see if SCE will allow an exception or consider expanding the solar system size to back up the whole house.

 

[Owner]

I think you're trying to simplify thinking how it works but it's not quite like that - Powerwalls drain together (or at least you have no control over it) so adding more of them simply increases your capacity.

All of this is SUPER dependent on your actual numbers, of course, and who can predict the weather?

I unfortunately can't get this level of detail. I only know how the net values on the grid. I have no idea how much power per hour the solar is producing or the power per hour the house is using, only the net. The system is old and there is no meter to give me the details.

But since I have an hourly report of the net, I did the calculations as best as I could. That is where I came up with that even on the best sunniest day with only the fridge running in the house as it is empty, the system can only fill up one powerwall, it just isn't strong enough to fill up two if it was off grid.

I see your point about production based upon sun, weather and time of day. Would be very valuable to calculate this PG&E turned off power here intentionally. I live very close to where they do, and the calculation would be entirely different if the house were 5 blocks away, then I would definitely buy 2 powerwalls and probably upgrade the solar.

From what I can gather and guestimate is that the refrigerator and all the other plugged in devices that pull power when not in active use are using 4.8kw per day.

Yes perhaps a second Powerwall would extend a long term outage by more than just one day. But I don't think I have ever experienced in my life an outage of more than a day and a half at the most at any location. I've never lived way out in the boonies or anywhere with wicked storms.

So one powerwall might be a bit challenging but should be enough to power the essentials: fridge, router, computers etc... No need for A/C.

I can't see the justification for spending the $6500 -- unless the PV system was being replaced.

I've gotten a few tepid responses from non-profits to take the old panels. But no one is jumping on it and does not feel like a promising way to go. Kind of makes sense as they would need someone with a specialty to do the install.

 

[gpez]

But since I have an hourly report of the net, I did the calculations as best as I could. That is where I came up with that even on the best sunniest day with only the fridge running in the house as it is empty, the system can only fill up one powerwall, it just isn't strong enough to fill up two if it was off grid.

I feel like I'm not getting through

"Strong enough to fill up two" or "the system can only fill up one powerwall" are not things if your PV produces more than your consumption at any time. You need to stop thinking about it in this way. How many Powerwalls to get is almost exclusively a function of what your consumption is, not what your PV system produces.

If you have 2 empty Powerwalls and your PV system produces 17.8kwh/day and your house consumes 4.8kwh/day then your 2 empty Powerwalls will fill to 100% after 2 days.

If you have 2 empty Powerwalls and your PV system produces 5.8kwh/day and your house consumes 4.8kwh/day then your 2 empty Powerwalls will fill to 100% after 26 days.

In both cases they eventually "fill up", the second scenario just takes more time. You can say "Strong enough to fill up two in a day" or "the system can only fill up one powerwall in a day", but in a day is an arbitrary amount of time as I was trying to illustrate in my example above.

Not trying to convince you to go with more than one (clearly I only got one!) but want to help clarify some of the technicalities of how the system works to help you make a more informed decision.

 

[Owner]

I feel like I'm not getting through You need to stop thinking about it in this way. How many Powerwalls to get is almost exclusively a function of what your consumption is, not what your PV system produces.

Sorry, I don't mean to frustrate you! I much appreciate your input.

I do understand what you are trying to say as it is based upon consumption.

If there is zero consumption, all the powerwalls can stay filled and overflowing as time goes on regardless of the size of the solar panels.

so if there are two powerwalls, there is more of a running start as there is 26kW to be used not just 13kW, so in most scenarios draining 2 and topping off 2 will last quite a bit longer.

And also with some usage models depending upon when electricity is required - say at a business or something when morning electricity is needed but little solar is being produced, the second powerwall could be really handy. And there I completely see your point.

Even with modest consumption, one powerwall may not have any juice left for a few hours say in the early morning.

But for my scenario, I think this is manageable. Or at least I think its worth trying. If one isn't enough, I can buy a second one later if I have a lot of heavy morning usage with no juice during an outage.

I am thinking this was your point?

So if I assume the minimum the house needs is 5kW / day, 2 powerwalls would really make sense for extended outages or if there was a need to recharge the car.

The worst case scenario would be that the power goes off at 9:59pm, the car is empty and the powerwall is used to refill the car and then there is no charge left till the sun comes up.

Is there a setting somewhere to say "don't charge the car from the powerwall?"

 

[MorrisonHiker]

The worst case scenario would be that the power goes off at 9:59pm, the car is empty and the powerwall is used to refill the car and then there is no charge left till the sun comes up.

Is there a setting somewhere to say "don't charge the car from the powerwall?"

We tweeted Elon about that issue last year and last month they released an update to address it. Currently, it only works for the Model 3 and Model Y but eventually it will work with the Model S and X as well. Basically, you can set a reserve on the Powerwalls and if the grid is down, the cars won't charge if the Powerwalls are below that reserve.

Vehicle Charging During Power Outage | Tesla Powerwall

 

[gpez]

Sorry, I don't mean to frustrate you! I much appreciate your input.

I do understand what you are trying to say as it is based upon consumption.

If there is zero consumption, all the powerwalls can stay filled and overflowing as time goes on regardless of the size of the solar panels.

so if there are two powerwalls, there is more of a running start as there is 26kW to be used not just 13kW, so in most scenarios draining 2 and topping off 2 will last quite a bit longer.

And also with some usage models depending upon when electricity is required - say at a business or something when morning electricity is needed but little solar is being produced, the second powerwall could be really handy. And there I completely see your point.

Even with modest consumption, one powerwall may not have any juice left for a few hours say in the early morning.

But for my scenario, I think this is manageable. Or at least I think its worth trying. If one isn't enough, I can buy a second one later if I have a lot of heavy morning usage with no juice during an outage.

I am thinking this was your point?​

No problem, not frustrated just trying to be clearer!

I agree that your scenario doesn't really warrant a second Powerwall but I was trying to show how a second Powerwall for outage scenarios is not related to your PV size and you don't need to add more panels to "support" a second Powerwall. If your house consumption increases then you would want to consider more solar panels.

Totally agreed on simply adding another later - should be much easier (especially if you are planful as you install the first) and cheaper just to add another especially if you don't need to add/move circuits to the backed up loads.

Is there a setting somewhere to say "don't charge the car from the powerwall?"

There is! Its a new feature just released last month. Powerwall charging Tesla's during outage
​

 

[Owner]

We tweeted Elon about that issue last year and last month they released an update to address it. Currently, it only works for the Model 3 and Model Y but eventually it will work with the Model S and X as well.

View attachment 543026

Perfect, except I do have a Model S and have no plans on replacing it anytime soon. Hopefully not too far away for this feature to be added to the S.

does the app tell you that the grid is down?

PG&E sends alerts sometimes - yes sometimes.

 

[GenSao]

Okay, here's my current thinking the three options.

1. Buy one powerwall $7,600
2. Buy two powerwalls $14,100
3. Upgrade to a new system 7.56kW $24,790 after tax credit

Your pricing seems to be a bit off. See below table for Tesla pricing for California which include installation cost, permit fees, CA sales taxes at 9%, and credits. The actual price difference to go from one to two Powerwalls is only $4.9 K.

If you have roof space, consider keeping your existing system and augmenting with a separate one. That is what I did. I initially have 5.8 kW of solar. Later added two Powerwalls in 2017. And recently added 3.8 kW of solar (small) with Tesla. The Tesla energy gateway is able to control the output of the older mirco-inverters and the new inverter.

 

[MorrisonHiker]

Perfect, except I do have a Model S and have no plans on replacing it anytime soon. Hopefully not too far away for this feature to be added to the S.

does the app tell you that the grid is down?

PG&E sends alerts sometimes - yes sometimes.

Yes, it sends an app notification. That's great if you are awake but you might not notice it if you are asleep.

Some UPS devices will also beep if the Powerwalls are outputting too high of a frequency. My Powerwalls used to output 65 Hz during an outage. We contacted Tesla and they lowered the frequency so we no longer get the UPS alerts. They actually woke me up more than once.

We get the notifications from our power company as well but usually they are wrong or hours late.