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Is Powerwall an reasonable option for me?

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Hello everyone. We are in the process of building a large home (9000 sq ft) in a PGE area with poor power reliability. That means high cost (esp in band 5 usage which a home like this one would definitely hit), and dealing with reliability means a generator (since I travel a lot and I want my wife and kids to not have to be improvising if I am not here when the power fails). I have been looking at these forums and talking to solar contractors, but still can't quite understand how the Powerwall wiring is supposed to work and what the control mechanism is. I am an engineer so this is rather frustrating for me.

So far, I think I can get about 21 kW of solar array capacity put into place on the roof. Hooking that into a few grid tie inverters and doing net metering is pretty straight forward. There is the question about microinveters, vs SolarEdge, vs string inverters, and I am trying to understand what makes the most sense for maximum kWH production, but there are lots of choices that should work and get me out of the most expensive usage bands from PGE. Installing a solar roof is an option, but I am about 6 months away from having to make that call, though right now it doesn't look like Tesla will be ready by then.

The house will have a 400A panel, and for backup I plan to install a generator that can handle the whole house load with a conventional transfer switch. I am also using a Mitusbushi VRF type system for HVAC, so that actually avoids the high startup current that many normal HVAC systems create on the power system, as it starts up slow and ramps up with demand.

Now, I'd rather avoid generating the pollution the generators use, and if for some reason the power were to go out for an extended period of time, having the ability to operate on solar alone would be nice. Also, if I could do some TOU shifting, I could probably zero out my power bill completely.

If the Powerwall were still being made in a DC version, the path forward would be easy. There many solutions (Fronius, SMA, SolarEdge) that have hybrid systems, including generator support my configuration. But an AC Powerwall is not supported officially by any of them as far as I can tell. And I'd rather not go down the lead-acid battery path, since it's much more expensive and I don't like the environmentals of those batteries.

So setting aside how the genset support would work for now, is there a Powerwall 2 configuration that would work with some set of multiple inverters that would work in my my situation? No matter which inverter type and vendor I would use, I'd have to have multiple of them. Does the Powerwall work with only some of them, or does it use it's frequency management to enable any old inverter to island properly?

I am confused about the "gateway" and what it does. It appears you have to have one to shut off the AC feed to prevent feeding the grid in an outage, but I can't find any info about what exactly the specs for these are in terms of amperage and how they are controlled. I assume it doesn't talk via the cloud because if there was an internet outage at the same time nothing would happen.

If the purpose of the gateway was just isolating the house, a generator automatic transfer switch would seem to be a better choice, as the controls could be set to isolate the house from the grid, but not turn the genset on unless there was a power deficit that needed making up, presumably only at night when the panels were not generating any power. A 3 pole transfer switch could also support disconnection from the grid as well as the genset.

Because of the lack of any reasonable documentation from Tesla, solar contractors and electricians can't tell me if a powerwall 2 could or couldn't work in this setup. They seem as clueless as I am. :) Even if the powerwall 2 for backup couldn't work now, but could in the future, I'd like to wire things in a way that would allow then to be retrofitted in, but again, we don't really know how to do that now.

Do you all have any documentation that would help with these decisions (types of inverters, transfer switch choices or wiring, etc...) that would help at least with being able to use one in the future if not when the house is completed (likely end of next summer)? I'd sure be obliged.

Thanks!
mike
 
A few points that may help:

1) I totally agree that the publicly available documentation from Tesla is lacking. Moreover, the documentation that is available is not so easy to find. If you haven't found it already, see Powerwall Support and the links there labeled "Powerwall Specifications" and "Backup Gateway Specifications". Some of the other links are of interest, too.

2) I don't know if Tesla has any plans to support generator use with a Powerwall. I'm sure they'd prefer you just get more Powerwalls and ditch the generator. If the power disruptions aren't long term, or if your long term solar production will supply your needs, that would be a viable solution.

3) The Backup Gateway is the same thing as an Automatic Transfer Switch, just for the Powerwall rather than a conventional generator.

Cheers, Wayne
 
No specialist at all but curious to what your HVAC load is on a house like that. Trying to power AC units with power walls doesn’t look very easy unless you get a whole row of them. Did you look at you max load?
 
Thanks for the replies! The Powerwall gateway specs indicate it's a 200A switch, which clearly won't work for me. Maybe one could disembowel the switch and remove the network interface part and glue that to a bigger transfer switch, but it's too bad that they didn't make their gateway and switch more standard so the gateway could control other transfer switches, including an ATS.

I think from the generator thread they are trying to find a way to support them. This is in my opinion essential from a marketing POV, because since they don't do TOU shifting now, backup poweris the main selling point, and in areas where power is seriously unreliable, people already have generators to support deal with the problem already. Maybe they don't make enough PW's to supply the demand of the market, so they can pick and choose their customer configurations, but that isn't going to generate a business that is that profitable or scalable. So I think they need to fix this problem if PW is going to be a viable product going forward.

On the HVAC side, the manual J says I have about 50,000 BTU of load needed to support. Now you have overhead from ducts and such, but 2 4 Ton outdoor units should be all that's needed as the house is very tight, and the climate I am in is quite mild. I was worried about the total load needed, and you'd probably need 3 PW's to be able to support the home, though if max cooling is really needed, that would be needed when the solar arrays are generating max power anyway, so you'd have the primary inverters running and generating power as well. In an area where you are much more heating dominated, this convenient alignment of supply and demand is inverted, so that's a different issue.

Thanks,
Mike
 
A couple comments:

You're going to have a single 400A panel fed from a 400A (Class 320) meter? It is fairly common to have (2) 200A panels fed from the 400A meter, in which case you could backup each panel separately with Powerwalls. Or you could segregate your critical loads to a single 200A panel.

If the manual J calculation gives you 50,000 BTUs/hr of heat loss/gain, then that's 4.2 tons. I don't see why you'd need two 4 ton units.

Cheers, Wayne
 
A couple comments:

You're going to have a single 400A panel fed from a 400A (Class 320) meter? It is fairly common to have (2) 200A panels fed from the 400A meter, in which case you could backup each panel separately with Powerwalls. Or you could segregate your critical loads to a single 200A panel.

If the manual J calculation gives you 50,000 BTUs/hr of heat loss/gain, then that's 4.2 tons. I don't see why you'd need two 4 ton units.

Cheers, Wayne

Yes, one main 400A panel, and then subpanels for each floor (it's 2 stories plus a basement). I guess I could do a separate panel for critical loads, and then wire the transfer switch in for that, but it doesn't make a lot of sense if I want whole house backup from a genset. We are still planning the electrical, so we have time to adjust things.

I had heard something about Tesla working on a 400A transfer switch, but it's hard to know what exactly they are working on. At some point it would be nice if they didn't have to have homeowners redo their normal configs to use all their stuff.

On the HVAC issue, this is part of the discussion I having with the HVAC guys, on how much is really needed. They tend to push overkill. I'm applying a lot of pushback. I am waiting on a manufacturer design of the Mits system now. I mentioned 2 4 ton units because that is sort of worst case. But it's not clear a single 5 Ton unit would work. And beyond 5 ton in a single unit requires 3 phase power, and I am trying to avoid getting gouged by PGE for that, and that makes the electrical work even more complicated. So it'll probably be 2 M series outdoor units with the indoor loads split across. We're in active discussions on this topic.

PS The core issue is how much derating is needed from the manufacturer specs. We live in a reasonably dry climate, so the you have to derate the tonnage by a bit to compensate. My HVAC guy is telling me that a 30% derating is needed, but I don't exactly buy that.


thx
mike
 
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@fresnoboy, Just a comment on choice of Solar panels.

Installed 6.5 kW of micro-inverter based panels (SolarBridge before they were absorbed :)) and would not choose anything other. As far as convenience (can add panels on the fly), production, reliability and longevity goes I don't believe you could do better. Any panel failure or other aspect (shading) only affects the panel in question.

From a diagnostic and monitoring perspective one has complete visibility of every panel so failures are easy to identify (haven't had any in 3 years) but one can see every panels performance statistics.

We added a PW-2 AC about 2 months ago (ordered ~7 months earlier) and the integration and operation is completely seamless.

Extremely happy and satisfied with the whole system!

(now just need to get a larger genset to allow for all contingencies :)


Cheers,

Harry E.
===
 
@fresnoboy, From the electrical wiring perspective:

If you are in the fortuitous position of building a home from scratch I'd recommend separating your wiring into 2 or 3 specific areas.

1. Essential loads
2. General purpose loads
3. Luxury loads

Or whatever is appropriate for your needs. Doing this allows a great deal of flexibility and choice. For example, Essential loads can be easily integrated with Powerwall type devices, others only as necessary. Even if you run a few parallel circuits to the same location the extra cost of the wire and sockets (with different colours for UPS) would be worthwhile over the life of the home (and a great reselling feature too!).


Cheers,

Harry E.
===
 
It would be strange in an expensive new house in 2017 to physically separate conventional panels by load type. I suggest you look at older generator load shedding panels and well as newer power management modules. This is the type of control that will be available for powerwall in the next couple of years. Today I would concentrate on sizing panels and battery correctly with the intent of adding more refined control as the hardware becomes available.

Personally I would not do a generator with 20kw of solar available, especially if gas is not piped to the property. Some of the money saved by not installing a generator could be put towards an extra powerwall and then limit depth of discharge when mains power is available.

If you list critical equipment it would be easier to discuss backup.
 
It would be strange in an expensive new house in 2017 to physically separate conventional panels by load type. I suggest you look at older generator load shedding panels and well as newer power management modules. This is the type of control that will be available for powerwall in the next couple of years. Today I would concentrate on sizing panels and battery correctly with the intent of adding more refined control as the hardware becomes available.

Personally I would not do a generator with 20kw of solar available, especially if gas is not piped to the property. Some of the money saved by not installing a generator could be put towards an extra powerwall and then limit depth of discharge when mains power is available.

If you list critical equipment it would be easier to discuss backup.

This has me intrigued. What exactly do you mean by more refined control? You mean through home automation? I know the old separate panels don't seem to be very common anymore - whole home backup makes sense if you can afford it. The house does have gas piped to it, so even a larger genset is not that bad. But again, this is PGE land, where they don't take care of gas lines well, and it seems silly to be running on gas via a genset if solar is available.

The problem with Solar is you can't be certain it'll be there, and a set of storms that pass through can take out poles and keep the sky grey for days at a time. It just seems too risky to depend on. Maybe if I lived in Arizona.

As for critical loads, I would say the refrigerator and freezer, the HVAC (both indoor and outdoor units), or at least part of that, DHW heating is gas, computer equipment, lighting, the range (gas cooktop and oven), and security system. Also sump and sewer pumps, since we don't live on a hill. Maybe the Washer/Dryer in an extended outage. Pretty much the normal stuff, though I have more server equipment on the computer side that what a typical family would have. Everything else can be pretty much sacrificed if needed.

Since everything is new, lighting will be all LED, HVAC will be VRF style, so very efficient, gas boiler for DHW, etc...

In the ideal world, everything is connected to the backup system. Again, my worry is where I am not home and my wife and kids have to manage things in an extended power outage. It would be great if they just didn't have to worry about what works and what didn't.

Thanks,
Mike
 
Whichever system you do, for sump and ejector you will want pumps with PSC motors in the lowest HP your plumber/builder finds acceptable. This will reduce both starting and running amps.

While these controls may not work with the powerwall, it is worth looking how generac handles power sharing between HVAC units
Generac Power Systems | Transfer Switches | Home Backup | Generac Power Systems

If you do the genset with solar, the generac guardian or ecogen should be fine. In Chicago the installed cost is about $4000 plus the equipment cost you see on the generac website. So $8-$10K before G.C. markup. the 22kw guardian with HVAC switching is probably what most generac dealers would quote. Although 4kw AC units are on the large side.

If you don't do a powerwall and will rely on a generator for multiple days per year the automotive engine generac is probably a better choice - $20K?
 
The problem with Solar is you can't be certain it'll be there, and a set of storms that pass through can take out poles and keep the sky grey for days at a time. It just seems too risky to depend on. Maybe if I lived in Arizona.
That's a fair point, as cloudy skies will obviously have a big impact on solar production. But they won't eliminate it entirely. Our solar panels are in shade for much of the afternoon, and I've actually found that overcast skies boost our solar production during those hours because they diffuse the solar radiation. By adding an oversized number of Powerwalls, you may well be able to get through those cloudy periods just fine. At least that's my expectation for our home, and we also have to be prepared for heavy snow covering our solar panels on occasion.

Also consider that events such as large earthquakes and wildfires that may cause extended power outages may also interrupt the distribution of natural gas.