How many powerwalls?

[djgarrett21]

I'm looking at a 17 kW system. I've got 1 EV and soon to be 2 once the CT is produced next year. I've done my own production estimates down to the hour on a yearly basis and compared that to my consumption over the past two years and projected forward a second EV. So...I've crunched a lot of numbers. The 17 kW system should produce 98% of what I consume. Essentially dead even.

What I'm having a difficult time estimating is how many powerwalls I should purchase. My goal is to maximize consumption of the solar power I generate since my utility doesn't have the most favorable net-metering terms.

It seems that the number of powerwalls would come down to a daily comparison of solar production vs consumption. The variable I'm having difficulty with is figuring out the time-based aspect: max solar output is during the afternoon while my max consumption is going to be in the late afternoon/evening. I'd like to be able to store just enough so that I'm able to draw from the powerwall in the late afternoon/evening without having to draw from the grid.

Anyone have any resources or insights into correctly sizing powerwall? I'd love to hear opinions from those who already have one/some. I've already spoken with a Tesla rep and they didn't really offer much assistance beyond "the configurator says you should get 2".

 

[GJones]

I have 2 PW with 10kW solar. It's an adequate combo for me...I produce about 16,000 kWHrs in a year and consume about 15,000 (with 1 EV). The 2 PW work just fine during high production (May-Oct) but lack enough storage and enough solar for recharge to be considered grid independent during the winter. My goal has been to survive grid failures and it has worked just dandy for that purpose even in winter. Keep in mind that 10% of your production will be lost in the round trip of battery charge/discharge. The perfect setup for me would be about 14kW solar and 4 PW to go grid free year round...but the excess production in summer is simply a gift to the utility. I am not that magnanimous.
For your example of afternoon/evening battery usage - again - my configuration works just dandy even while running AC in 100+ degree heat. The struggle would be running AC 24 hours (right now I can open the house up for cooling at about 10pm).
If indeed you intend to go with 17 kW solar - my opinion is that 2 batteries will not suffice for your projected use case. I would go with at least 3 and probably 4.

 

[jjrandorin]

A 17kW PV system, with an emphasis on self consumption (not just trying to time shift from energy from a time of use 4-9pm time frame, but self consume as much as possible) probably needs either 3 or 4 powerwalls for that.

I cant believe the configurator would say that you should have 2 powerwalls for that system size, actually.

 

[n.one.one]

I'm looking at a 17 kW system. I've got 1 EV and soon to be 2 once the CT is produced next year. I've done my own production estimates down to the hour on a yearly basis and compared that to my consumption over the past two years and projected forward a second EV. So...I've crunched a lot of numbers. The 17 kW system should produce 98% of what I consume. Essentially dead even.

What I'm having a difficult time estimating is how many powerwalls I should purchase. My goal is to maximize consumption of the solar power I generate since my utility doesn't have the most favorable net-metering terms.

It seems that the number of powerwalls would come down to a daily comparison of solar production vs consumption. The variable I'm having difficulty with is figuring out the time-based aspect: max solar output is during the afternoon while my max consumption is going to be in the late afternoon/evening. I'd like to be able to store just enough so that I'm able to draw from the powerwall in the late afternoon/evening without having to draw from the grid.

Anyone have any resources or insights into correctly sizing powerwall? I'd love to hear opinions from those who already have one/some. I've already spoken with a Tesla rep and they didn't really offer much assistance beyond "the configurator says you should get 2".

There is indeed a lot to evaluate. Allow me to add some questions for your thought process.

• Does your utility offer a EV or night rate when you can schedule charging your 2 EVs
  • GA Pwr is mine and the overnight EV rate is about 1.3 cents/kWh
• Does that special rate apply if you have solar
  • GA Pwr has confirmed I can have the EV rate with solar
• Do you have frequent power outages
  • I do and part of what I was solving for was how long could I run if the grid was down during cloudiest days during the worst solar months (Dec/Jan) which is when we sometimes have ice storms and the grid can be down for multiple days
  • Since that was my priority I went for 4 PWs (I also hope that the PWs will have a longer lifetime because other than storms I won't charge them as high and won't routinely deplete them as low)

 

[jjrandorin]

Since that was my priority I went for 4 PWs (I also hope that the PWs will have a longer lifetime because other than storms I won't charge them as high and won't routinely deplete them as low)

This is a completely different discussion, but powerwalls do not have a charger slider like the vehicles do, so you wont be able to prevent your PV system from charging them to "full". You can set your reserve at different levels to prevent draining them to "empty" but you wont be preventing them from charging to full.

There are no regular settings I am aware of to make the PV stop charging the powerwalls before they get full. You can certainly set your reserve to a high level to prevent draining them if you want, but if you set your reserve to 80% for example, it will charge to full and drain to 80%. There is no "set to 90", and I doubt tesla will enable anything like that for powerwalls.

 

[BGbreeder]

Given your projected energy use, I would round up closer to 20kW (or more) as the marginal cost now for additional solar is low, and much higher later.

To keep the Powerwalls from throttling the solar down/off during an off grid (power outage) scenario, you want a maximum of 5kW of solar per Powerwall. (That is a not to exceed number, unless you are using inverter that can throttle power down gracefully as frequency increases, e.g. microinverters.) So at 17kW of solar, having four or more will make off grid charging more stable. If power outages aren't an issue, ignore the comment. Household demand can increase the solar/Powerwall ratio, but, and it is a big but, that load will have to be there for the Powerwalls not to have trouble.

I would observe that homes seem to be increasingly electric, so demand is likely to increase.

So, I would recommend more solar, and an increased number of Powerwalls.

All the best,

BG

 

[jrweiss98020]

Rule of thumb per my installer is 1 Powerwall for each 7 kW of rated solar output. That will allow the Powerwalls to absorb all the solar production if necessary. With 17 kW of solar, you should install at least 3 Powerwalls.

HOWEVER, depending on what you want to use the Powerwalls for, you may want more than 3. If you want a whole-house backup, and have heavy electric demands (air conditioning, electric water heater...), AND you can afford them, you may want 4 or 6 Powerwalls to carry the load for a significant time. You should figure out the minimum essential load you do not want to turn off during a power failure (lights, electronics, refrigerator,...) and the minimum amount of time you want them to run when there is no grid or solar production.

 

[n.one.one]

This is a completely different discussion, but powerwalls do not have a charger slider like the vehicles do, so you wont be able to prevent your PV system from charging them to "full". You can set your reserve at different levels to prevent draining them to "empty" but you wont be preventing them from charging to full.

There are no regular settings I am aware of to make the PV stop charging the powerwalls before they get full. You can certainly set your reserve to a high level to prevent draining them if you want, but if you set your reserve to 80% for example, it will charge to full and drain to 80%. There is no "set to 90", and I doubt tesla will enable anything like that for powerwalls.

Thanks for that info. I should have pointed out that I'm still researching and learning. I don't have a system yet because Tesla has not started installing in the Atlanta area (but my order has been in since February of this year, system designed, and a permit has been issued).

 

[h2ofun]

Rule of thumb per my installer is 1 Powerwall for each 7 kW of rated solar output. That will allow the Powerwalls to absorb all the solar production if necessary. With 17 kW of solar, you should install at least 3 Powerwalls.

HOWEVER, depending on what you want to use the Powerwalls for, you may want more than 3. If you want a whole-house backup, and have heavy electric demands (air conditioning, electric water heater...), AND you can afford them, you may want 4 or 6 Powerwalls to carry the load for a significant time. You should figure out the minimum essential load you do not want to turn off during a power failure (lights, electronics, refrigerator,...) and the minimum amount of time you want them to run when there is no grid or solar production.

I know with PGE, if you go over 30kw, different rules apply. Since my batteries were 25kw, and solar 29kw, I did not have to find out what that meant.

 

[jeremymc7]

In most any case anyone wanting to have 220 backup for HVAC and the like needs at least 2 powerwalls. The rest depends on how much downtime you think you might have. I'm never down more than a few hours; esp one time 20+ years ago we were down a whole day. But for people that could be down for days in summer heat and you want to have HVAC all day than that number goes way up.

 

[Southpasfan]

If your goal is to maximize the use of the solar power you generate, with the emphasis on the words "you generate" I would say three or four.

I have three, and a 16.32 system, will probably add one as four would have been better.

We have not had an extended outage yet, and certainly at 13.5kwh per PW four (53 Kwh reserve) is significantly better than three (40kwh reserve).

But what happens is this. You mostly figure out what your big energy draws are.

Cars can be charged during the day. You an adjust a Tesla to charge at an appropriate rate so that it does not exceed solar production at the time you charge it. For example, on our 60 amp circuit it can charge at 48, but I set it to 20.

Anyway, you also pick a reserve setting on the PWs. With 2 or 3 you might end up hitting your reserve and going on grid in the middle of the night many nights if AC is needed at night.

Obviously, if there is a power outage in the middle of the night that's bad.

But mostly it comes down to powering your house from the sunlight you gather. When the PWs reach their limit the grid, barring wind power, is probably mostly fossil fuel burning.

Just get 4 and don't worry, nobody every complains about too big a system or too many PWs.

 

[roblab]

I have 3 power walls and 11 kw of solar. I sometimes wish I had 4 PWs, but they're too expensive for just a wish.

I will say, with all the PG&E power outages every week here in northern CA, the power walls have been very useful. My wife and I sit and watch TV, read, basically live our lives in comfort while the neighbors sit in the dark, sometimes for days. The local school turns on generators so locals can go charge phones and ipads in the evenings. And we never think about it, other than the blip when it switches from grid to batteries. Power walls were made for this.

And then every day the batteries charge up for free.

 

[h2ofun]

I have 3 power walls and 11 kw of solar. I sometimes wish I had 4 PWs, but they're too expensive for just a wish.

I will say, with all the PG&E power outages every week here in northern CA, the power walls have been very useful. My wife and I sit and watch TV, read, basically live our lives in comfort while the neighbors sit in the dark, sometimes for days. The local school turns on generators so locals can go charge phones and ipads in the evenings. And we never think about it, other than the blip when it switches from grid to batteries. Power walls were made for this.

And then every day the batteries charge up for free.

I keep waiting for a nice long power outage

 

[h2ofun]

I'm looking at a 17 kW system. I've got 1 EV and soon to be 2 once the CT is produced next year. I've done my own production estimates down to the hour on a yearly basis and compared that to my consumption over the past two years and projected forward a second EV. So...I've crunched a lot of numbers. The 17 kW system should produce 98% of what I consume. Essentially dead even.

What I'm having a difficult time estimating is how many powerwalls I should purchase. My goal is to maximize consumption of the solar power I generate since my utility doesn't have the most favorable net-metering terms.

It seems that the number of powerwalls would come down to a daily comparison of solar production vs consumption. The variable I'm having difficulty with is figuring out the time-based aspect: max solar output is during the afternoon while my max consumption is going to be in the late afternoon/evening. I'd like to be able to store just enough so that I'm able to draw from the powerwall in the late afternoon/evening without having to draw from the grid.

Anyone have any resources or insights into correctly sizing powerwall? I'd love to hear opinions from those who already have one/some. I've already spoken with a Tesla rep and they didn't really offer much assistance beyond "the configurator says you should get 2".

I have 5 powerwalls, just about have 7. Started with 15kw of solar, then added 5 batteries.. After using for many months, my input to folks is forget getting tons of batteries, unless you really have lots of money to spend, get as MUCH solar as you can!! I took that battery money and used it towards another 15K of solar, so just about 30K now. What I found is was easy to use the batteries, but if I was not able to recharge, ESPECIALLY during winter since I have heat pumps, the number of batteries meant nothing.

And even now for me, with so much smoke, solar only putting out maybe 50% of what it should. With extra panels I can still charge batteries. But if I had just more batteries, they would have quickly become useless. And this is with the power on.

 

[Patrick66]

Without better data, it is just a WAG, but I would likely go for 4 units. 16kW/4PW = maximum 4kW charging each, which seems reasonable.

What I did was install an IoTaWatt meter on my main and sub-panels to get the information I wanted. What that made me realize was that my PV had a lot of cloud dips in kW output (from 7kW down to 2kW); they might only last a few minutes, but it messed with my utility consumption. It also gave me a good indicator of my steady-state load, daytime average, and night time averages. From that information, I estimate I could 90% self-consume with one PW, or 99% with two (I have 8kW (DC) of PV that meets ~110% of my annual net consumption). Being able to ride through the worst week without utility power would require 4-5 units (5th covering a 2-ton mini-split used only during sleeping hours).

 

[Patrick66]

And even now for me, with so much smoke, solar only putting out maybe 50% of what it should. With extra panels I can still charge batteries. But if I had just more batteries, they would have quickly become useless. And this is with the power on.

Very good point; once you hit a reasonable capacity for the Powerwalls your primary need shifts to generation on your worst day. You just need to figure out what the reasonable battery capacity is first…

 

[jboy210]

What is your projected daily power consumption for vehicles?

Can you charge your vehicles when your solar would be outputting a lot? Weekends, etc.

FWIW, We have just two PWs but hardly use the power in them for EV charging since we can move our charging to late-morning/early afternoon when we have excess solar power to put in the vehicle. This cuts into the power we send back to the utility, so when possible we charge on the weekend when we don't receive peak rate credits.

 

[h2ofun]

Very good point; once you hit a reasonable capacity for the Powerwalls your primary need shifts to generation on your worst day. You just need to figure out what the reasonable battery capacity is first…

Reasonable, thats the big question. I still think from what I have read from most, it seems to always start with much money one is able to spend. Then one justifies from there. IMO, having batteries is an extra pain in the butt. Having just the solar would be fine. I just do not get that many power outages to justify the cost of batteries, and they really do not save much money. Meaning with just solar and PGE, one can stay on TOU-C NEM, which the differences between peak and non peak is not enough to worry about. BUT, once you get to EV2-A, different story. And if they change NEM to only whole sale rates sending back, then again, a whole different game. SO many assumptions one has to make. But if one starts with how much money one is willing to spend, the more the better for bragging rights.

 

[holeydonut]

Meaning with just solar and PGE, one can stay on TOU-C NEM, which the differences between peak and non peak is not enough to worry about.

You forgot to add "yet" at the end of your sentence. You can be damn sure that by the time 2025 rolls around, the CPUC will have approved some draconian new NEM ruleset and also authorized PG&E some way to recoup the $30 Bn or whatever they claim to need to harden their infrastructure.

And for sure, the gap between peak and off-peak for normal rate payers (ignoring CARES or other low income programs) is going to significantly widen.

And I don't know about ya'll, but the pain of adding an extra Battery to prevent getting hosed by TOU rates is going to be astronomically painful. Probably need to add bollards to protect the bollards at that point.

 

[h2ofun]

You forgot to add "yet" at the end of your sentence. You can be damn sure that by the time 2025 rolls around, the CPUC will have approved some draconian new NEM ruleset and also authorized PG&E some way to recoup the $30 Bn or whatever they claim to need to harden their infrastructure.

And for sure, the gap between peak and off-peak for normal rate payers (ignoring CARES or other low income programs) is going to significantly widen.

And I don't know about ya'll, but the pain of adding an extra Battery to prevent getting hosed by TOU rates is going to be astronomically painful. Probably need to add bollards to protect the bollards at that point.

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True, but that is why there is no answer to how many. Still starts with how much money one wants to put into this stuff. Most have limited funds. Batteries are EXPENSIVE!!!