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Whole house backup possible with one Powerwall?

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?
 
I think of it as a three layer analysis;
a) peak draw
b) total capacity / the house load = hours of backup w/o sun
c) Is the expected solar output - house loads more than what your powerwalls can accept (5kW/Powerwall)

I think you have covered #1 really well, #2 is your call, but if #3 isn't right, the powerwalls will shutdown the solar and will keep your system from charging much at all.

Just my way of looking at it. I think you need to think about your typical draws, especially during sunlight hours.

All the best,

BG
 
I think of it as a three layer analysis;
a) peak draw
b) total capacity / the house load = hours of backup w/o sun
c) Is the expected solar output - house loads more than what your powerwalls can accept (5kW/Powerwall)

I think you have covered #1 really well, #2 is your call, but if #3 isn't right, the powerwalls will shutdown the solar and will keep your system from charging much at all.

Just my way of looking at it. I think you need to think about your typical draws, especially during sunlight hours.

All the best,

BG
So is the third one only during off-grid operations, such as during an outage? I somehow assumed that during normal grid-tie, that any excess power beyond what the Powerwalls and house loads need was just pushed out to the grid.

Nominally I'd like both the existing 4kw system (peak I've ever seen is about 3.3kw) and the additional 4.8 kw to both be able to feed the Powerwall. But if that's too much for one Powerwall, then maybe only the new Tesla 4.8kw would tie into the Powerwall? That seems to be fairly common design to exclude the existing solar, whether the owners want to or not.
 
So is the third one only during off-grid operations, such as during an outage? I somehow assumed that during normal grid-tie, that any excess power beyond what the Powerwalls and house loads need was just pushed out to the grid.

Nominally I'd like both the existing 4kw system (peak I've ever seen is about 3.3kw) and the additional 4.8 kw to both be able to feed the Powerwall. But if that's too much for one Powerwall, then maybe only the new Tesla 4.8kw would tie into the Powerwall? That seems to be fairly common design to exclude the existing solar, whether the owners want to or not.
Yes, you are right, #3, the 5kW/Powerwall limit, is only true for off grid. Living within a couple miles of the San Andreas fault line, I thought that offgrid operation might be important. If it isn't, skip #3, as it won't apply. If you think that #3 is uncommon there would be ways to throttle the production (through the breaker for one of your PV systems) if you ever needed to.

Al the best,

BG
 

miimura

Well-Known Member
Aug 21, 2013
7,314
7,292
Los Altos, CA
I would think that a Powerwall+ and a Backup Switch (the one that goes behind the utility meter) could work well in the OP's situation. The Tesla solar could be curtailed as necessary by the integrated solar inverter leaving the Enphase solar to run while off-grid until the battery is full. However, it's really up to the Tesla design folks to decide.
 
Yes, you are right, #3, the 5kW/Powerwall limit, is only true for off grid. Living within a couple miles of the San Andreas fault line, I thought that offgrid operation might be important. If it isn't, skip #3, as it won't apply. If you think that #3 is uncommon there would be ways to throttle the production (through the breaker for one of your PV systems) if you ever needed to.

Al the best,

BG
Thanks for clarifying. I guess there are really two ways the Powerwall benefits off-grid. One is just drawing down the power stored in the Powerwall - that would be most important. The second is allowing the solar to keep generating - not as important for us as the random outages during the day tend to be < 1 hour, and just drawing from the Powerwall is fine to get through that. I suppose the latter would have great benefit during multi-day outages, but we don't have PSPS or other storm-based multi-day outages - it's mostly falling tree branches on the power lines, and they tend to get those fixed quickly. And if the BIG ONE hits, I think we'll have other bigger issues to worry about...

But it sounds like there should be multiple ways to make sure the off-grid solar stays below the 5kw Powerwall limit. The way I understand it, the gateway is supposed to curtail by getting the inverters to track to a less-productive MPPT point, all the way down to zero production. If it can do it for both the new Tesla inverter AND the old Enphase micro's, everything good. If it can only do it for the Tesla inverter, most of the time it only needs to curtail the new array down slightly to 2kw or so, assuming it is aware of the production of the microinverters. And worst case as you say, cut the breaker on the Enphase array, so that it's only controlling the Tesla inverter - less production, but still getting something in off-grid situation.
 
yeah, the 40A electric oven is likely going to be a problem for getting whole house backup with one Powerwall. However, the guidelines may be different for a Powerwall+.
The electric oven is I guess what I'm worried about the most. I just hope that's not the ONE thing that prevents whole-house backup, for something that is rarely used and certainly not during a grid outage. But I suppose they're worried about that one-in-a-million chance where I just happen to be pre-heating the oven AND using a hair dryer at the same time, and suddenly the power goes out, or something like that.
 

ucmndd

Well-Known Member
Mar 10, 2016
8,521
16,613
California
The electric oven is I guess what I'm worried about the most. I just hope that's not the ONE thing that prevents whole-house backup, for something that is rarely used and certainly not during a grid outage. But I suppose they're worried about that one-in-a-million chance where I just happen to be pre-heating the oven AND using a hair dryer at the same time, and suddenly the power goes out, or something like that.
Powerwall 2s are connected to the rest of your loads by a 30 amp breaker, so your 40 amp oven could overload it all by itself, no need for other loads. I think the Powerwall+ is 40 amp - might be worth looking into.

The v2 gateway makes it pretty easy to exclude a few loads using the internal breaker panel. I have a “whole home” backup with the exception of my hot tub and one of our two EVSEs, which are landed on the gateway.

EFF5C464-4387-4A12-9C3A-84F4F1C07EBF.jpeg
 
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miimura

Well-Known Member
Aug 21, 2013
7,314
7,292
Los Altos, CA
My house is wired in a partial backup configuration. I have a bunch of large 240V breakers remaining in the main panel, some of which aren't actually used. These include:
50A NEMA 6-50 outlet for car charging (Leviton 40A EVSE)
50A NEMA 14-50 outlet for car charging (Tesla Mobile Connector)
50A Range plug (not used, gas range installed)
40A circuit for A/C (not used)
40A circuit for A/C (not used)
120V 20A circuit for tile floor heater (master bath)
120V 20A circuit for tile floor heater (2nd bath)
120V 20A circuit for tile floor heater (laundry room)

Everything else that was in the main panel was re-routed to a sub-panel. Most of my household loads were in a sub-panel already. The Gateway 1 is fed by a 125A circuit breaker in my main panel that used to feed the household loads sub-panel. We generally don't notice when those loads that are not backed up are down.
 
The electric oven is I guess what I'm worried about the most. I just hope that's not the ONE thing that prevents whole-house backup, for something that is rarely used and certainly not during a grid outage. But I suppose they're worried about that one-in-a-million chance where I just happen to be pre-heating the oven AND using a hair dryer at the same time, and suddenly the power goes out, or something like that.
Sunrun set us up for a 90% back-up, only the Micro, stove, washer and dryer are not on the backup. Everything else is. What's funny is that in normal conditions they draw from the solar and PVs
 
My house is wired in a partial backup configuration. I have a bunch of large 240V breakers remaining in the main panel, some of which aren't actually used. These include:
50A NEMA 6-50 outlet for car charging (Leviton 40A EVSE)
50A NEMA 14-50 outlet for car charging (Tesla Mobile Connector)
50A Range plug (not used, gas range installed)
40A circuit for A/C (not used)
40A circuit for A/C (not used)
120V 20A circuit for tile floor heater (master bath)
120V 20A circuit for tile floor heater (2nd bath)
120V 20A circuit for tile floor heater (laundry room)

Everything else that was in the main panel was re-routed to a sub-panel. Most of my household loads were in a sub-panel already. The Gateway 1 is fed by a 125A circuit breaker in my main panel that used to feed the household loads sub-panel. We generally don't notice when those loads that are not backed up are down.

Interesting, so even with two Powerwalls, things like the floor heaters are not backed up. We have two tile floor heaters as well, the larger one on its own 120V 20A breaker - but we literally never use them, since they're an energy hog. Who makes the judgment about whether things stay in the main panel, vs moving to the backup panel but not used during outages?

There are pros and cons to whole house backup in our situation, aside from the actual outage convenience.

Pros:
-If PG&E and our local AHJ (Santa Clara county, not city) allow for the meter ring Backup thingy, makes for a cleaner install since we have the integrated main panel
-Very little room on the narrow outside wall where the main panel is located, to land a large backup subpanel, if it's going to house 20+ breakers. It's towards the street, but at 90deg angle. Below is conduit going down underground to the street, above is >8 feet high. The two adjacent walls both face the street (unsightly), but more importantly one is mostly window at ground level, not stucco; the other has the gas meter 4' away.

Cons:
-Our main panel is already chock full, and a bunch of the breakers are already the half-size ones. If they move the current solar to the gateway so it can feed the Powerwalls, I guess that breaker moves off the main panel and frees up that space for the gateway? Otherwise not sure they have room.

Really the nearest possible place to land a large backup panel at eye level would be at least 10' away, the nearest desirable place would be the garage 30' away. Would they really re-run 20+ backup circuits in conduit 10-30+ feet away? Or would I be forced to choose like just a small practical handful of backup loads?
 
Last edited:

miimura

Well-Known Member
Aug 21, 2013
7,314
7,292
Los Altos, CA
Interesting, so even with two Powerwalls, things like the floor heaters are not backed up. We have two tile floor heaters as well, the larger one on its own 120V 20A breaker - but we literally never use them, since they're an energy hog. Who makes the judgment about whether things stay in the main panel, vs moving to the backup panel but not used during outages?
In my case, they just did what was easy. I only insisted that crucial circuits be relocated to the backup side. That ended up being the following:

2x 20A 240V solar circuits (existing Enphase installation)
1x 20A 120V bathroom GFI for both downstairs bathrooms
1x 20A 120V AFCI bedroom circuit for the room directly behind the main panel.

The tile heaters are really a luxury item that just provide a warm feeling under your feet. We rarely use them and only installed them on the tile floors that were on concrete slab. I had to convince the tile installer to put a foam sheet on the slab as a thermal break, otherwise I would just be sinking the heat into the slab.

The key point in this kind of installation is that you have to have the Grid CTs extended to the main panel so that they can measure ALL your grid usage, not just what goes through the Gateway. Some main panels are constructed in a way that is incompatible with CT installation. That is a strong argument for using the Backup Switch because it performs the Grid measurement. However, I suppose that if my electric ovens were the only thing remaining in the main panel, then I may not care if the Powerwalls were blind to that usage.
 
Interesting, so even with two Powerwalls, things like the floor heaters are not backed up. We have two tile floor heaters as well, the larger one on its own 120V 20A breaker - but we literally never use them, since they're an energy hog. Who makes the judgment about whether things stay in the main panel, vs moving to the backup panel but not used during outages?

There are pros and cons to whole house backup in our situation, aside from the actual outage convenience.

Pros:
-If PG&E and our local AHJ (Santa Clara county, not city) allow for the meter ring Backup thingy, makes for a cleaner install since we have the integrated main panel
-Very little room on the narrow outside wall where the main panel is located, to land a large backup subpanel, if it's going to house 20+ breakers. It's towards the street, but at 90deg angle. Below is conduit going down underground to the street, above is >8 feet high. The two adjacent walls both face the street (unsightly), but more importantly one is mostly window at ground level, not stucco; the other has the gas meter 4' away.

Cons:
-Our main panel is already chock full, and a bunch of the breakers are already the half-size ones. If they move the current solar to the gateway so it can feed the Powerwalls, I guess that breaker moves off the main panel and frees up that space for the gateway? Otherwise not sure they have room.

Really the nearest possible place to land a large backup panel at eye level would be at least 10' away, the nearest desirable place would be the garage 30' away. Would they really re-run 20+ backup circuits in conduit 10-30+ feet away? Or would I be forced to choose like just a small practical handful of backup loads?
If you want to have whole home backup, and I would recommend it, then the installer can run heavy gauge from the (new) meter panel over to a new main panel, and thence to the gateway/powerwalls, and then a single set of wires back to your existing panel to feed it as is. However, if you plan to add more loads, (or are even possibly thinking about adding more loads), I would recommend that you use the opportunity to install a new service panel at the meter, and a new, upgraded panel running the house loads in addition to the gateway panels, and ditch your existing, older breakers, while you are at it.

At some level, if you don't have the wall space, you don't, and you need to make lemonade. Would anyone set out to design a home with all those extra panels and cabling? No..., but it may be the best solution for what you have, especially compared with retrenching a new entry point, or re-wiring your home.

All the best,

BG
 

jboy210

Well-Known Member
Supporting Member
Dec 2, 2016
6,705
4,584
Northern California
In my case, they just did what was easy. I only insisted that crucial circuits be relocated to the backup side. That ended up being the following:

2x 20A 240V solar circuits (existing Enphase installation)
1x 20A 120V bathroom GFI for both downstairs bathrooms
1x 20A 120V AFCI bedroom circuit for the room directly behind the main panel.

The tile heaters are really a luxury item that just provide a warm feeling under your feet. We rarely use them and only installed them on the tile floors that were on concrete slab. I had to convince the tile installer to put a foam sheet on the slab as a thermal break, otherwise I would just be sinking the heat into the slab.

The key point in this kind of installation is that you have to have the Grid CTs extended to the main panel so that they can measure ALL your grid usage, not just what goes through the Gateway. Some main panels are constructed in a way that is incompatible with CT installation. That is a strong argument for using the Backup Switch because it performs the Grid measurement. However, I suppose that if my electric ovens were the only thing remaining in the main panel, then I may not care if the Powerwalls were blind to that usage.
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.
 
If you want to have whole home backup, and I would recommend it, then the installer can run heavy gauge from the (new) meter panel over to a new main panel, and thence to the gateway/powerwalls, and then a single set of wires back to your existing panel to feed it as is. However, if you plan to add more loads, (or are even possibly thinking about adding more loads), I would recommend that you use the opportunity to install a new service panel at the meter, and a new, upgraded panel running the house loads in addition to the gateway panels, and ditch your existing, older breakers, while you are at it.

At some level, if you don't have the wall space, you don't, and you need to make lemonade. Would anyone set out to design a home with all those extra panels and cabling? No..., but it may be the best solution for what you have, especially compared with retrenching a new entry point, or re-wiring your home.

All the best,

BG
Well that's the thing, our 200A panel is 10 years new and already "upgraded". When we did our "60% of house" remodel, we crossed that threshold where ALL of the 50-year-old wiring in the house essentially had to be upgraded to current code. So pretty much all the walls had to be opened up to re-wire, turning it into a 90% remodel. We upgraded from 100A to 200A service, and despite expanding the house only about 25%, the new panel filled up pretty quickly - because every bedroom now had its own ARC breaker (taking up two slots no less), every bathroom its own GFCI, every appliance it's own breaker (including ones we don't even own like electric dryer or AC compressor). The Enphase solar then took up 4 of the spare slots, I did discover I do have a single 120V 20A breaker labeled "spare" remaining near the bottom of my panel.

I somehow thought that you can't bypass the meter side of these integrated panels, as that's the PoCo's side of the demarcation, so the only option would be to have a new main panel fed from the existing panel. But I guess we paid for the integrated panel and our contractor installed it, so no reason a new separate meter socket couldn't be placed ahead of it?
 

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