# Potential for a Tesla Powerwall?



## RASelkirk (Sep 18, 2020)

Hi All,

Doing preliminary groundwork for a whole-house system. I've looked at the Tesla Powerwall as a stop-gap that would allow me to temporarily shut down the generator for maintenance (one of these PWs would allegedly run my home's normal load for 6 hrs). The PW's factory ATS does the switchover in < 100ms. so power is never even blipped. In order to plan for this as an eventuality, how would one stub in the wiring? Both ATS' in series? Is there a 3-way ATS?

Ideally on a utility fail, flow would switch to the PW while the gen comes online, then totally switch over to the gen and recharge the PW. When the gen is manually s/d, the PW wall comes in until either the gen comes back up or the PW is depleted.

Russ


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## drmerdp (Apr 10, 2018)

I feel like you need to choose one or the other. Power wall or standby generator.

Ive never heard of a single phase ATS that will switch between 3 power sources. Maybe there is something in the 3 phase world that can be adopted to residential use.

The power wall is pretty cool, but it’s still just a battery and needs to be recharged. I wonder if a small charging Generator can be used in conjunction with the power wall during operation.


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## iowagold (Jan 22, 2020)

the best way is to use an inverter setup to power the house..
those can be stacked for more power.
then use grid or the generator or solar to charge the battery.

that way you do not have a switching time issue.
look at the larger solar systems, the inverters for those can get super large as they use them for farms here in Iowa.

and any power source will charge the battery of choice with the proper charger for that battery.

the key is affording enough battery...

solar, wind is one of the good solutions for the mid west..
now if you are in a snow pack area or an area with natural springs.
hydro electric is also and option.

any time you convert energy to another state you have loss of energy..
keep that in mind.


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## RASelkirk (Sep 18, 2020)

Was able to find a PDF on the PW install. The PW appears to come into the service panel thru a standard breaker and thus back-feed the mains. I'm assuming it "floats" on the utility when available for charging purposes. The stock diagram had the utility power entering the top of their ATS (called backup gateway), I took the liberty of editing it to show a possible solution for using both.

When the utility goes down, the PW would instantly supply (back-feed) power to the mains. That loss of the utility would also trigger the generator to spool up and supply the PW's backup gateway once it's fully online, which would put everything back to "normal" as far as the PW is concerned. Then it's just a matter of the utility coming back online and triggering the gen to shut down. I think...

Russ

EDIT: Had to edit the pic to add the gen!


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## drmerdp (Apr 10, 2018)

The backup gateway looks like a Teslatized ATS. As an unqualified source on the matter... If it is legal and acceptable to have 2 ATS back to back, then that looks and sounds feasible.


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## exmar (Jun 29, 2015)

Seems "doable" but looking at the cost of the PW, estimated at $7,000 from the Tesla site, seems like an expensive and problematic UPS? Also, as I understand the sequence, you lose power, PW kicks in, when PW is depleted, then the generator would go through it's normal starting sequence so you'd be without power for "X" amount of time. PW recharges, you decide to shut down the generator for maintenance, and this cycle continues. Presumably there's no syncing issue when the PW kicks in as it's been monitoring the utility or generator power. What I'm having trouble getting my head around is how you would get the generator connected after maintenance without the normal starting sequence and power interruption. e.g. The generator could be drastically out of phase from the PW if you just switched. Not good. I'm probably over thinking it as I remember being in control rooms when we were syncing a unit prior to putting it back online (Utility).

If the PW is rated for constant duty, perhaps the way to approach it is to have the ATS and generator just feed the battery charging portion of the PW? Shutting down the genset for maintenance would be unseen by your house power. Might be a question for Tesla. I think I'd just look at a few UPS units for critical items and go with a conventional whole house generator. Lot less imagineering and warranty issues that way.


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## exmar (Jun 29, 2015)

Combining Systems | Powerwall Support


Adding energy storage via Powerwall to your solar system is the best way to maximize your system’s value – allowing you to use solar power day and night.




www.tesla.com


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## iowagold (Jan 22, 2020)

the big thing is to look at the cost per watt on a system.
and the real time run time.

a few real questions;
so how many kwh is each power wall?
and how much are the extra battery banks?
how many extra battery banks will the power wall system support?

and what is the battery charge rate on the power wall controller?


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## exmar (Jun 29, 2015)

Looked into this a little further, could find nothing on power required for charging a PW, did find that the charger for a Tesla EV, they recommend a 50breaker, at 240V that's 12KW. Perhaps these charge at a lower rate. Also one PW will provide 3KW of power which is considerably less than the average house would use so load shedding circuitry would be required dedicated to the PW being operational. Unless you plan on multiple PW's which they recommend. Where I was headed on this was trying to determine the impact on your generator size by adding a PW. I didn't find anything definitive, but there will be an impact as this would be a critical load, perhaps multiples if you find you need multiple PW's for your application. So, if you go with this unit, factor in it's load(s) in sizing your generator.


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## drmerdp (Apr 10, 2018)

Personally, The power wall is a slick battery pack for an off grid dwelling with solar but just too expensive and impractical for home backup.


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## RASelkirk (Sep 18, 2020)

With the placement of the gen's ATS, a failed utility would always make the gen run until the grid comes back. Right? The PW, thru it's "ATS", would supply the breaker panel until it the panel saw power back at it's main input. With the 100 ms speed of the change-over, phasing might not be an issue. 

Anyway, it was just a thought. I looked at my electric bill from August, average usage was 3.2 kW per hour so one PW _might_ last 2 hrs with my A/C off. Even one, let alone stacking a couple, would not really be cost-effective as a means for comfort while "routining" the gen...

Russ


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## iowagold (Jan 22, 2020)

i would look at scalable inverters..
and then battery packs..
then you could add on when you have the extra cash.

green energy is not free...
the cost per kwh is way more than 5 times..
at least until you have paid for the gear.

but as a back up or for an area where you are fuel challenged it is way cheaper than pulling grid power miles to some remote area.

power wall is a cool idea. super pricey.
also do the solar shingles and a solar farm. and add on wind farm too.
add the extra batteries, and make sure they wall setup is setup where you can add on extra inverters.

they make smart power strips now..
kinda cool as you can put things to full sleep mode to save lots of power.


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