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Whole house backup possible with one Powerwall?

Tile heat is one of the most silly and favorite things about our bathroom remodel. Stepping on a cold tile floor from a warm shower is not long acceptable. They use Schluter DITRA underlayment to separate the tiles from floor and reduced the possibility of a tile cracking.
It's all about the creature comforts, isn't it? We did choose to put the tile heaters in, in the kitchen and the master bath, after all. But you know what, after putting in double-pane windows and new insulation with the remodel, the bathroom and all the other rooms are no longer drafty, despite the thermostat being set a degree or two lower than before, and stepping out of the shower doesn't feel cold at all anymore. So the incremental comfort of having warm tiles underfoot didn't really add much, so we just turned them off.

But you know what I do love though, is the master bath floor register ended up right underneath our towel bar. So during the cold months, if I time my morning shower right, the bath towels are nice and toasty from the furnace just finished heating up the whole house in the morning. Toasty warm bath towel, now that's COMFORT...
 
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I have an existing 4kw micro-inverter system, but just put in to Tesla for a 4.8kw system plus one Powerwall (one is all I really need for either load-shifting or backup).

I know the general parameters in this forum are that one needs at least two Powerwalls, even three, for whole-house backup, but most of those threads are regarding homes with air-conditioning and the compressor startup loads. But I don't have central air-conditioning. Is the two Powerwall rule an absolute, or does it depend on the actual house panel and breakers, or even more specifically on actual loads?

I have a fairly new 200A panel (one of those California ones with the meter socket in half of the panel. The panel is largely full, but there are only three 240V breakers:
-40A for air-conditioning compressor. However, my air compressor from the 60's has not worked since I remodeled, so this circuit was re-purposed for my 1st gen Tesla HPWC. Tesla's whole-home backup page says TWC are compatible with whole-home backup, and I could swap the HPWC for a TWC if the communications are needed to shed the EV charging load. Even if I do get a new compressor, I'm planning to get an inverter compressor in the 2-2.5 ton range, so it shouldn't have a heavy startup load.
-30A for electric dryer. However, we have a gas dryer, so this circuit is largely unused.
-40A for electric oven. I do know the oven draws about 5 kw continuous when pre-heating, and then intermittently to maintain . So certainly the oven alone would hit the max output of one Powerwall - but I also know I would not plan to do any baking during a grid outage.
-There's also the 240V breaker for the existing solar - but that's generation, not load.

Aside from the 240V loads, there are 28 20A breakers for 120V. But in practice, 1-2 kw is really the max ever being used at one time. Certainly if all the circuits were running at max, the load would exceed one Powerwall - but then again it would exceed 8 Powerwalls. So there's a huge difference between actual expected loads and theoretical max loads.

Is it possible to get a load evaluation to determine if I could have whole-house backup with just one Powerwall? Or with my type of integrated panel, would they have to move all of the backup loads off the main panel anyways, so it really doesn't matter much between whole-house and partial backup?
Is there a reason to not buy two Powerwalls - other than it's twice the price?

The peace of mind from the additional capacity (and the additional margin of error that comes with it) would seem to be well worth it.
 
Is there a reason to not buy two Powerwalls - other than it's twice the price?

The peace of mind from the additional capacity (and the additional margin of error that comes with it) would seem to be well worth it.

Just that the additional capacity is not needed for either outages or load-shifting, so mainly peace of mind. But one of the reasons I'm asking about the whole-house backup, is that if two Powerwalls does simplify the layout significantly and reduce the number of additional boxes (e.g. alleviates a backup subpanel), the 2nd Powerwall is a strong consideration given the limited space near the meter.

OTOH, the preferred location inside the garage for the Powerwall, I think would not require bollards for just one. But a 2nd one, if it can't be stacked, might start encroaching to where our AHJ might ask for bollards, which would be deal killers as it might prevent us from parking two cars in our smallish garage.

We'll see what Tesla comes back with for initial design, so far nothing but it hasn't been quite two weeks yet, and I'm sure they're slammed with new order requests. I did get an automated e-mail saying it's taking them longer than expected.
 
Depends on where you live (latitude and climate), how much energy you use, and other energy sources.

For our house, yes, as long was we only run the pool pump during the summer only. Since we have gas for all appliances, use very little electricity (about 10-15 kWh per day in the winter), live in a mild climate (very little need for heat which is a gas source anyway), live in the southern part of the country (longer winter days and higher sun angle than northern parts), and have a lot of sunny days, one PW is plenty enough to not only get us through the night, but for a whole day with no sun at all.

For most people, they would probably need at leas two, most likely 3 or more
 

miimura

Well-Known Member
Aug 21, 2013
7,314
7,292
Los Altos, CA
Is there a reason to not buy two Powerwalls - other than it's twice the price?

The peace of mind from the additional capacity (and the additional margin of error that comes with it) would seem to be well worth it.
I strongly recommend two Powerwalls to anyone that can afford it. I have a small solar system but two Powerwalls. It allows me to cycle the Powerwalls to completely avoid higher priced energy usage for 8 months of the year, while retaining backup reserve at all times. Basically, one PW worth of capacity is cycled and one is saved for backup. This also allows them to last longer with shallower cycling.

This is a typical spring day for my system. The big usage spikes are EV charging during Off-Peak. The Powerwalls start discharging at 3pm when Off-Peak ends. 9pm-12mid is Part-Peak but the price is not that different than 4-9pm Peak until the Summer rate season starts in June.

Chart 2022-05-19_Home.jpg
Chart 2022-05-19_PW.jpg
 
I strongly recommend two Powerwalls to anyone that can afford it. I have a small solar system but two Powerwalls. It allows me to cycle the Powerwalls to completely avoid higher priced energy usage for 8 months of the year, while retaining backup reserve at all times. Basically, one PW worth of capacity is cycled and one is saved for backup. This also allows them to last longer with shallower cycling.

This is a typical spring day for my system. The big usage spikes are EV charging during Off-Peak. The Powerwalls start discharging at 3pm when Off-Peak ends. 9pm-12mid is Part-Peak but the price is not that different than 4-9pm Peak until the Summer rate season starts in June.

View attachment 808214 View attachment 808215

Cool, and just to clarify since I'm not yet familiar with the Energy screens in the app, is that all on Time-Based Control, none of the new Grid Charging or Export Everything options that some have and some don't? So looks like 100% of the home load is being fed by the Powerwalls after 3pm? But it looks like from noon-3 pm, everything is being powered by solar - so I presume they're still generating after 3 pm, and TBC allows you to push 100% of solar generation after 3 pm out to the grid?

So if you did this with just one Powerwall, you'd have load-shifting, but you'd be at a critically low reserve level, instead of looks like 50% reserve, overnight in case of outages. I see what you're saying.
 

miimura

Well-Known Member
Aug 21, 2013
7,314
7,292
Los Altos, CA
Cool, and just to clarify since I'm not yet familiar with the Energy screens in the app, is that all on Time-Based Control, none of the new Grid Charging or Export Everything options that some have and some don't? So looks like 100% of the home load is being fed by the Powerwalls after 3pm? But it looks like from noon-3 pm, everything is being powered by solar - so I presume they're still generating after 3 pm, and TBC allows you to push 100% of solar generation after 3 pm out to the grid?

So if you did this with just one Powerwall, you'd have load-shifting, but you'd be at a critically low reserve level, instead of looks like 50% reserve, overnight in case of outages. I see what you're saying.
Yes, those charts show normal Time-Based Control without anything special added. I am PG&E NEM1, so Buy and Sell is the same price. When the Powerwalls get to 100% the solar naturally goes to the home loads and surplus goes to the grid. On this particular day, they got full at about 1:00. Since the price goes up substantially at 3pm, it basically treats the whole 3pm-12mid period as Peak and powers the house from battery and lets the solar go to the grid during those hours.

Yes, that was my point. Since I use about 9-12kWh every day between 3pm-12mid, having only one Powerwall would leave it critically low and it could not power through an overnight power outage to last until the solar started up again. As it is, I set the Reserve to 40%, but it only gets down to about 50% due to my Part-Peak and Peak usage. It always stops discharging at midnight when the Off-Peak period starts.

The behavior shown is almost ideal for my particular situation. The only further optimization could be to slow down the charging from about 11:00 to 3:00 to allow more solar self-consumption and less export during Off-Peak. There's really not much point to charging with All Solar during this period when they know how much time remains to the end of the Off-Peak period and how much energy they need to charge the batteries to 100%. However, the existing behavior is conservative and you never know when some clouds may blow in and reduce the solar production which could prevent filling the batteries if they tried this optimization. If you have different buy and sell prices during Off-Peak, it may try to do it to reduce your imports.
 
Additional Powerwalls are usually not twice the cost of one. The cost of each one goes down as you go to 2, 3, 4, etc.
OK - "approximately" twice the price.

But this even further supports my argument - why not buy two Powerwalls? Yes, I know budget and cost constraints matter, but one has to look at the big picture here - it's an investment in your home, AND an investment in your personal safety, security & comfort.

I can think of many, many worse things to spend your money on than Powerwalls for your home.
 
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Just that the additional capacity is not needed for either outages or load-shifting, so mainly peace of mind. But one of the reasons I'm asking about the whole-house backup, is that if two Powerwalls does simplify the layout significantly and reduce the number of additional boxes (e.g. alleviates a backup subpanel), the 2nd Powerwall is a strong consideration given the limited space near the meter.

OTOH, the preferred location inside the garage for the Powerwall, I think would not require bollards for just one. But a 2nd one, if it can't be stacked, might start encroaching to where our AHJ might ask for bollards, which would be deal killers as it might prevent us from parking two cars in our smallish garage.

We'll see what Tesla comes back with for initial design, so far nothing but it hasn't been quite two weeks yet, and I'm sure they're slammed with new order requests. I did get an automated e-mail saying it's taking them longer than expected.
All that, plus you need to account for possible failures. If you have two PWs and one fails, at least you're only down to 50% capacity. If you only have one unit and it fails, now you're down hard.

Everyone seems to think of Tesla energy as bulletproof, but it isn't. I've had two major service outages in two years, not a great track record. And the summer thunderstorms and fall hurricanes aren't going to sit idly by and wait for Tesla's service to get their *sugar* together and fix my impaired systems before tearing through my neighborhood. So I would say it pays to think about reliability and resiliency where you can.
 
Two weeks after a major quake while still being able to charge cars.
Also Tesla fuxxored my solar system and took me from 10kw to 6. I don't trust them for anything and need to get it repaired.
Not owning an EV (Tesla or otherwise) I guess I can't advise any further.

Although at least with the cars, you have the option to take them to a nearby Tesla supercharger (assuming there is one near you, and assuming they would also be working after a major quake - two big assumptions, I grant you).

The little old lady across the street from me bought a Tesla, and she has no solar, no PWs, no 220 in her garage. When she needs to charge her Tesla, she takes it to the mall and lets it supercharge while she shops.
 
Hey all, after seeing a few threads bumped with people receiving design or re-design after a month of wait, I got a bit optimistic that some backlog might clear up a bit. Indeed, I just got a text today that my system design for 4.8kw + 1 Powerwall has been completed.

Given this is an add-on, the best parts of the south-facing roof already have panels. Instead of choosing to put all 4.8kw in one contiguous block on the east-facing roof, they split it into 8 there, and 4 on a remaining patch of south roof that gets shaded around 1-2 pm. They're probably about equal compromises in terms of amount of morning and afternoon shade; but their shading timing can be offset by an hour or so from the other roof plane.

Questions for the wise folks here:
-Given the different orientation and shading times, can I be sure they'll put the 4 south panels on its own string? Anyone know what is the min string size with the Hanwha panels? I think it would be bad in terms in MPPT tracking to have south and east panels combined. I feel like I've read 5 is the min, but when I read how it's calculated, it's supposed to be the inverter min voltage (60V for Tesla inverter) divided by min panel voltage (45V), so that would say 2 is the min?
-Second, I read here everyone mentioning details of their system design including location of inverter and Powerwalls, type of equipment, whole-house vs partial backup, wiring diagrams, etc., and sometimes asking for changes before accepting. I have nothing in my account except the one drawing of the roof panel layout. It is asking me to accept (and then deposit becomes non-refundable). How are people getting all these details of their design before accepting?
 
Hey all, after seeing a few threads bumped with people receiving design or re-design after a month of wait, I got a bit optimistic that some backlog might clear up a bit. Indeed, I just got a text today that my system design for 4.8kw + 1 Powerwall has been completed.

Given this is an add-on, the best parts of the south-facing roof already have panels. Instead of choosing to put all 4.8kw in one contiguous block on the east-facing roof, they split it into 8 there, and 4 on a remaining patch of south roof that gets shaded around 1-2 pm. They're probably about equal compromises in terms of amount of morning and afternoon shade; but their shading timing can be offset by an hour or so from the other roof plane.

Questions for the wise folks here:
-Given the different orientation and shading times, can I be sure they'll put the 4 south panels on its own string? Anyone know what is the min string size with the Hanwha panels? I think it would be bad in terms in MPPT tracking to have south and east panels combined. I feel like I've read 5 is the min, but when I read how it's calculated, it's supposed to be the inverter min voltage (60V for Tesla inverter) divided by min panel voltage (45V), so that would say 2 is the min?
-Second, I read here everyone mentioning details of their system design including location of inverter and Powerwalls, type of equipment, whole-house vs partial backup, wiring diagrams, etc., and sometimes asking for changes before accepting. I have nothing in my account except the one drawing of the roof panel layout. It is asking me to accept (and then deposit becomes non-refundable). How are people getting all these details of their design before accepting?

You will need to call support and request the full design package to get that level of detail. I do recommend doing that. The package should give you all the detail on how they intend to wire the strings and the general location on where the equipment will go. They shouldn't combine the panels into a single string from two different roof planes. I have a 3 panel string and it is the older panels that are only 32V. Admittedly I have not looked at how well that string is producing relative to the others since I would have to power cycle the inverter every time I would want to check.
 
You will need to call support and request the full design package to get that level of detail. I do recommend doing that. The package should give you all the detail on how they intend to wire the strings and the general location on where the equipment will go. They shouldn't combine the panels into a single string from two different roof planes. I have a 3 panel string and it is the older panels that are only 32V. Admittedly I have not looked at how well that string is producing relative to the others since I would have to power cycle the inverter every time I would want to check.
Thanks, anyone know the number for support, is it specific to Solar division? They don't quite make that obvious in the online pages...
 
So the only support number I could locate led to a menu where all options queues are no longer active... but I did find on the website where I could schedule a 30-min call with an adviser for next week, which I did. Meanwhile though, I did get an e-mail response saying that there are no design documents created until I move to the next phase.

So it sounds like I have to "accept" the "system design" with just a roof layout, and also commit my payment details and $250 deposit to be nonrefundable, before they do site survey and actual design? Am I "accepting" just the roof layout, or everything they come up with going forward? The e-mail said I would be able to request changes to the designs later, but the prevailing advice I see here is to not "accept" the design until I'm fully satisfied with all the details.
 

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