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

[d21mike]

I think you are on the right track but keep in mind the following:

1. You will not generate as much as you think all the time. My 12k system (+ 2 power walls) works very well but on a cloudy day it drops a lot. Also, all panels are not at full capacity at the same times. So, best to hope your power failure happens on a sunny (solar friendly) day.

2. Once your power wall is at 100% and you are using all the solar you need then your extra solar normally goes to the grid but the grid is down. So "I believe" solar just shuts down and you are only pulling from the batteries until they need to be replenish again. Not positive exactly how this switch works.

3. Do you have a Tesla in the mix? To recharge it will take a lot of power. Recently a change was made where you can set the App to stop charging the car if the power walls drop before a certain percentages. I have mine set to 95%. Some have had their power walls drained at night during a power failure before this feature was added. Also, some people do not "backup" the car chargers so they would not pull from the batteries ever. I have everything "backed up". So this was a critical feature update for me. I went on faith that Elon would keep his word and add the feature (took long time).

I am very happy with my setup. Just to give you an idea of what I have been saying.

It is 9:20am in southern California. My peak solar generation is around noon.

My App is showing 6.8 kw of solar generation with 1.2 kw going to my home and 5.9 kw going to my power walls which are current at 88%. They will be full soon and then excess will go to the grid generating credits.

My settings are that my power walls never drop below 65% (unless power failure) because when the sun goes down I do not use more then 30-35% before by super low rates kick in at 10pm. My 2 Tesla Cars are set to charge during super low rates from the grid mostly using the credits I created during the day at a higher cost per kw. Anyway, this is my first year and have a lot of credits built up.

 

[wjgjr]

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?

The way I would think of it is that if you use backup-only mode, your PWs are full, so you will always start any outage with an extra 13.5 kWh per PW - 27 kWh for two. So, what you have available to you on day one is 27 kWh plus whatever solar you produce. (And if you want to get into precise numbers, there are some losses due to operating the PWs and round-trip losses to store energy.) If you use more than you produce, you will have less than 27 kWh in the PWs on day 2, and so on. But it is not as simple as saying 50 kWh day one and then 23 kWh the next.

Beyond that, it is important to recognize that 23 kWh is an annual average. My system is expected to produce ~8 kWh annually, but that works out to ~30 kWh average in June and ~12 kWh in December - so 2.5 times more in June (and Tesla seems to suggest for some the ratio can be even greater.) Even then, in the two months I've had my system in June/July, I've hit a high of 45 kWh and a low of 11 kWh on a very sunny day and an extremely cloudy/stormy day, respectively. So it is important to understand that there is both a predictable (time of year) and unpredictable (weather) aspect to production. And, of course, your usage likely varies throughout the year. In our case, having an electric A/C and gas furnace means our usage is much higher in the summer than the winter.

 

[Cutwolf]

I think you are on the right track but keep in mind the following:

1. You will not generate as much as you think all the time. My 12k system (+ 2 power walls) works very well but on a cloudy day it drops a lot. Also, all panels are not at full capacity at the same times. So, best to hope your power failure happens on a sunny (solar friendly) day.

Right, but all I can do for now is sort of gauge on an "average" day.

2. Once your power wall is at 100% and you are using all the solar you need then your extra solar normally goes to the grid but the grid is down. So "I believe" solar just shuts down and you are only pulling from the batteries until they need to be replenish again. Not positive exactly how this switch works.

This is part of my question. Do the panels wait until the batteries are fully depleted to begin recharging them, or does it continue charging them while they're in use during an outage/

3. Do you have a Tesla in the mix? To recharge it will take a lot of power. Recently a change was made where you can set the App to stop charging the car if the power walls drop before a certain percentages. I have mine set to 95%. Some have had their power walls drained at night during a power failure before this feature was added. Also, some people do not "backup" the car chargers so they would not pull from the batteries ever. I have everything "backed up". So this was a critical feature update for me. I went on faith that Elon would keep his word and add the feature (took long time).

I probably won't back up the charger. We have a supercharger about 15 minutes away that I can drive to if needed, and hopefully with storm watch I'll have sufficient time to get the car plugged in and charged before it hits.

Thanks for the insight!

 

[Cutwolf]

The way I would think of it is that if you use backup-only mode, your PWs are full, so you will always start any outage with an extra 13.5 kWh per PW - 27 kWh for two. So, what you have available to you on day one is 27 kWh plus whatever solar you produce. (And if you want to get into precise numbers, there are some losses due to operating the PWs and round-trip losses to store energy.) If you use more than you produce, you will have less than 27 kWh in the PWs on day 2, and so on. But it is not as simple as saying 50 kWh day one and then 23 kWh the next.

Understood. So the solar continues to charge the PWs even while the PWs are in use? You don't have to deplete one for it to begin to charge up, correct?

Maybe I should state my assumptions:

1. If it's an "average" day where I generate 23 kWh, and I have 27 kWh in the batteries, I will have 50 kWh to use that day?

2. If I fully drain the batteries and the 23 kWh being generated on day 1, then on day 2 I can only make use of whatever I generate that day, and day three would be whatever is leftover from day 2 + whatever I generate on day 3, correct?

 

[wjgjr]

This is part of my question. Do the panels wait until the batteries are fully depleted to begin recharging them, or does it continue charging them while they're in use during an outage/

It will continue charging them during an outage. However, once the batteries are full, solar production will stop until the batteries are no longer full. This may only be a 5-10 minute stop until the batteries drop from 98% to maybe the 95%-97% range. It definitely does not let them go to 0%. It does mean if an outage starts around noon and you have full PWs, you may lose some of your solar for that day because you have nowhere to send the load. (This might be a good time to temporarily charge your EV, or do laundry early or turn on the A/C early, for example.)

 

[Gray468]

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?

Not sure I saw you mention your daily consumption. We were out of the house for several days and I took care to shut off all non-essentials The house still consumed 11 kWhr a day. Our 5.2 kWhr solar delivers between 3.5 - 4.9 kW as a daily peak depending on time of the year, temp, wind, cleanliness etc which translates to 20-36 kWhr daily generation.... about 9,500 kWhr a year.

We have two powerwalls to protect against blackouts. Once a month I run them on “self-powered” (basically simulate “off the grid” for several days to cycle the batteries). We run the house normally (no turning off the luxuries). The two powerwalls drop to around 50% on average and are charged up by noon most days. The excess goes to the grid. That give me confidence that two with my solar should give continuous power... unless there are a couple of days of cloud cover or thick ash from wildfires.

FYI, when the grid is not available for the excess, powerwall changes the current frequency just enough to trip the solar inverters, but not so much that it hurts your appliances. But have the installer check that by truly simulating a power outage... eg trip the main breaker to the grid, turn off powerwalls if they are not full. We did that and and found the ceiling fan lights blinked on and off. The installer called tesla, they made a real time tweak to the frequency and voila... everything work perfect.

and you do not get 27 kWhr out of two. There are many threads on that topic. If memory serves, it would be closer to 21-23 depending on losses. And if you drain them by 10 kWhr, expect to have generate 11-12 to replace it.

Bottom line, a solar system that delivers 9.5 kWhr per year (over 7 yrs, it’s been 9.2 to 10, we are on track for 9.7 this year) and two powerwalls seem like a good combo for a home that consumes about 25 kWhr per day, 750 a month.

 

[BrettS]

Understood. So the solar continues to charge the PWs even while the PWs are in use? You don't have to deplete one for it to begin to charge up, correct?

Maybe I should state my assumptions:

1. If it's an "average" day where I generate 23 kWh, and I have 27 kWh in the batteries, I will have 50 kWh to use that day?

Yes, this is basically correct. However the thing to think about with solar power is the fact that it is kind of a use it or lose it thing and it’s only generated at certain times. So your house might use 23kWh, but that’s going to be spread from midnight one day to midnight the other day. Your solar might generate 23kWh, but it will only be generated from, say 7am to 8pm. If your house doesn’t use 23kWh during the time the solar is generating then that excess power is lost. The powerwalls allow you to store that excess power, but obviously they can only hold so much.

Here is an example. We’ll assume that the powerwalls are fully charged when the outage happens at 3AM. There is no solar at this time, so your house will operate on the powerwalls. When the sun comes up the powerwalls will be partially drained, say they are down to 70%. When the sun comes up the solar will go to the house first. As the sun starts rising there might not be enough solar to power the house initially, so all of the solar will go to the house and it will continue to draw some power from the powerwalls. After a few hours, as the sun continues to rise there will be more solar power available. At that point the house’s usage will be covered 100% by the solar and any excess power will go into charging the powerwalls. Since the powerwalls only got down to 70% they should charge pretty quickly. So, by say noon the powerwalls will be fully charged. At that point there will be excess solar power that can’t be used, so the system will turn off your inverters and run the house entirely on the powerwalls. This is because there is no where to send the excess power. (If the grid was up it could go back to the grid, or if the powerwalls weren’t fully charged it could to go the powerwalls, but in this case neither of those are true). The house will run on the powerwalls alone for a few minutes until their state of charge drops by a couple of percent. At that point the inverters will come back online and power the house with the excess going to charge the powerwalls. The system will continue to cycle like this until the grid comes back online (and the system goes back to normal) or the sun goes down. So when you are at this point it will be able to use enough solar power to meet your home’s immediate power demands while it keeps the powerwalls at a full charge. But some solar power will be ‘lost’ because you have no way to use it at the time it is generated. So because of that you may not get your full 23kWh of solar power that day, but you will have your house’s immediate needs met and the powerwalls will remain at a full charge and be ready to take over when the sun goes down.

You can extrapolate from there other examples. If the power failure happened at 9PM, then your house would run all night on the powerwalls and they would be more drained in the morning, so more solar power would be used to recharge them and you might then be able to use the full 23kWh that day. If the power failure happened at 10AM, then the house would already be fully solar powered, so you wouldn’t need to rely on the powerwalls at all until the sun went down. In that case the inverters would start cycling immediately so you could use as much solar power as possible while keeping the powerwalls fully charged. But because you can’t use all of the solar power as it’s generated you might generate considerably less than 23kWh that day.

 

[Cutwolf]

Thanks all! Extremely helpful.

 

[jrweiss98020]

What is your daily consumption?

The solar panels will first power the house, then direct excess production to the Powerwalls. If you use less than what the panels produce each day, you will be able to power your house indefinitely, recharge the Powerwalls, and have the Powerwalls still in reserve.

Any consumption in excess of solar production will be drawn from the Powerwalls.

 

[hurjio]

If I only get 1 Powerwall, high load appliances (like A/C) would have to be put on a separate panel, right? Who determines what things 1 PW won't be able to support? Is this just figured out during install, or beforehand?

On the same vein, does that mean whenever I use the A/C, it will always be using electricity from the grid (never the Powerwall)? Or can the Powerwall still "support" some of the energy requirement of the A/C.

My situation is I'm considering (trying to justify) 1 Powerwall (with solar) for TOU energy arbitrage. Not really concerned about prolonged outages, but having basic electricity during short outages is a luxury bonus. So not concerned about running A/C or charging car during outages. Average daily usage probably 20 kWh, but 30-35 kWh on high A/C usage days.

 

[miimura]

If I only get 1 Powerwall, high load appliances (like A/C) would have to be put on a separate panel, right? Who determines what things 1 PW won't be able to support? Is this just figured out during install, or beforehand?

On the same vein, does that mean whenever I use the A/C, it will always be using electricity from the grid (never the Powerwall)? Or can the Powerwall still "support" some of the energy requirement of the A/C.

My situation is I'm considering (trying to justify) 1 Powerwall (with solar) for TOU energy arbitrage. Not really concerned about prolonged outages, but having basic electricity during short outages is a luxury bonus. So not concerned about running A/C or charging car during outages. Average daily usage probably 20 kWh, but 30-35 kWh on high A/C usage days.

When the grid is up, the PW can offset the consumption of the A/C for TOU even if the A/C will lose power during a grid outage.
One rule of thumb is that any load with a breaker greater than 30A cannot be backed up by a single PW. 60A for two, etc. For loads with a high starting current like a A/C compressor, there may be additional considerations.

For your buying decision, you should look at your household consumption during the Peak hours. If you use 20kWh during Peak, you won't be happy with a single Powerwall because it only stores 13kWh.

 

[jboy210]

If I only get 1 Powerwall, high load appliances (like A/C) would have to be put on a separate panel, right? Who determines what things 1 PW won't be able to support? Is this just figured out during install, or beforehand?

On the same vein, does that mean whenever I use the A/C, it will always be using electricity from the grid (never the Powerwall)? Or can the Powerwall still "support" some of the energy requirement of the A/C.

My situation is I'm considering (trying to justify) 1 Powerwall (with solar) for TOU energy arbitrage. Not really concerned about prolonged outages, but having basic electricity during short outages is a luxury bonus. So not concerned about running A/C or charging car during outages. Average daily usage probably 20 kWh, but 30-35 kWh on high A/C usage days.

With 1 powerwall you are correct that most likely you will only be able to backup critical loads (120 v circuits), and not high draw items like A/C, Dryers, pool pumps, etc. The items that will get backed up are determined before install, and any rewiring required to achieve this splitting of the loads is priced into the install. However, whether you backup critical loads only or not, a lot of the basic equipment and wiring is the same. In addition to the Powerwall(s) you need a Tesla Energy Gateway which is a very smart switch that can blend power from the grid, powerwall(s), and solar at the same time. And also switch in milliseconds from the grid if it fails. You also need the associated switch boxes. If you only go with one Powerwall you will also need an additional critical loads panel.

Pricewise you end up somewhere around $9,500 or so for one Powerwall. The cost of a second (third, ...) powerwall is an additional $6,500 each plus install.

 

[hurjio]

For your buying decision, you should look at your household consumption during the Peak hours. If you use 20kWh during Peak, you won't be happy with a single Powerwall because it only stores 13kWh.

Thanks. To clarify, my 20kWh daily usage is for 24hrs

 

[jrweiss98020]

If I only get 1 Powerwall, high load appliances (like A/C) would have to be put on a separate panel, right? Who determines what things 1 PW won't be able to support? Is this just figured out during install, or beforehand?

YOU decide which circuits are on the critical loads panel. The lower the load, the longer it will last. OTOH, the lower the load, the lower your flexibility.

YOU have to decide how well you can decide which loads you can manually shut down. otherwise, only put your refrigerator, critical lights, and [gas] furnace blower on the critical circuits.