Output of 2 x Powerwall 2?

[Bigtanuki]

I have two units installed and will be installing a third. The two units will supply 10 kW of load. However, I am experiencing some issues that I have sent off to Tesla support for resolution. The Powerwalls will supply 10 kW for about 30 minutes and then start backing down to about a total of 8 kW. A related issue is that, under load, I have noted that the cooling fans aren't running at the same speed. One of the units is blasting out fairly hot air while the other is putting out about a third the amount of air at a lower temp. I decided to run some experiments this weekend and shut down one unit at a time to identify if one is supplying 5 kW and the other 3 kW (after backdown) or if both units are backing down. I suspect that one unit is having an issue and I think this will prove it.
Assuming your units are working correctly, you should be able to do what you want. My goal has been to NOT sell any solar back to the power company, rather use as much as possible and as little grid power as possible. I have been discharging my batteries into the car when they near full to have capacity until sundown. It's working pretty well, but is still a bit clunky with the software.

I am looking for some info from existing owners who have multiple Powerwall 2s or perhaps an installer.

I am aware that the max output power of a Powerwall 2 unit is 5kW.
I intend to have 2 x Powerwall 2s installed, mostly to provide adequate storage capacity for my electrical needs. Can anyone tell me if I daisychain 2 x Powerwall 2s, does that give me a max output of 10kW?
I am trying to work out if I can run my 6kW electric water heater at the same time as charging my Model S at 4kW without drawing grid power for example.

I have 240v 3 phase power to my house (I live in Switzerland) as well as 3 phase coming from my solar panels. Will I be able to get 3 phase out of the Powerwall 2s or do they just output single phase 240v?
I currently charge my car using the UMC plugged into a 3 phase plug. I run that at 5A, 400v (technically 380v I think) 3 phase giving me 4kW on the screen in the car/app. I'd love to maintain 3 phase charging but am not sure if Powerwall 2 supports it.

 

[Siggy101]

Thanks Bigtanuki. I was hoping for real work confirmation and you've given me exactly what I asked for. Many thanks!
I will talk to Tesla about the 3 phase questions as that's quite specific to my location but it's great to hear that multiple units do multiply the output.
Best of luck with your troubleshooting. It sounds like you're on the right track to me.

 

[Frankman60]

Has anyone used one Powerwall as a whole house backup in the event of a grid outage? What I mean, is that all circuits have usable power in the house rather than just selected circuits? I realize the sustained output for one PW is 5 kWs and the capacity is approximately 13 kWh.

 

[SoundDaTrumpet]

Has anyone used one Powerwall as a whole house backup in the event of a grid outage? What I mean, is that all circuits have usable power in the house rather than just selected circuits? I realize the sustained output for one PW is 5 kWs and the capacity is approximately 13 kWh.

This may have been mentioned before. Another way to look at it 2 PW is 30 Amps, and 1 PW is 15 Amps. Break open your load center and you will see typically circuit breakers rated at 30A and upwards for clothes dryer, oven, stove, air conditioner, and perhaps water heater. I am mostly gas except the large loads are oven and air conditioner. One would work if you are all gas, and don't have A/C nor electric vehicle. For one thing most homes have a electric dryer circuit even if you are on natural gas. Tesla's electrical plans showed the 30A dryer circuit as being backup even though nothing is plugged in (for the next person that move into the home). The boiler plate response is whole house requires 2 PW.

From a practicality standpoint, baking cookies, charging my EV, or the A/C raging on a hot summer night will surely drain 2 PW quickly. Hopefully, I will be able to reliability shut off loads via home automation.

 

[zanary]

Has anyone used one Powerwall as a whole house backup in the event of a grid outage? What I mean, is that all circuits have usable power in the house rather than just selected circuits? I realize the sustained output for one PW is 5 kWs and the capacity is approximately 13 kWh.

On average you'll need more than one to power a full house. In my case since my electrical bill shows I used around 14.5kw per day on average, I would need two Powerwall for whole home backup (AC excluded), and that's what I did and have it running,

Total Cost of my two Powerwall project was $13800. SGIP gives me back a cash rebate of $9200, so the delta is around $4500 so that's just less than one Powerwall, so it worked out well for me.

 

[Frankman60]

I know exactly how much energy I am using each day, and producing each day so I can estimate very closely the amount of energy I need in case of a power outage. My question is whether I can control my usage inside my home in a power outage, OR is it required that two Powerwalls are required even though they may not actually be needed? For example, on a 15 amp circuit in my home I may only have only 2 amps of power needed. Is the Powerwall requirement based on the 15 amps or the actual amps that I know that the circuit is using? Other examples are AC, oven, and Tesla charger. I have them, but I don’t need to use them during a grid power outage.

 

[wwhitney]

What Tesla told me is that if you have only 1 Powerwall, they don't want any breakers larger than 30A on the Powerwall side of the Backup Gateway. The Powerwall is only good for 5 kW continuous power, while a 30A breaker is potentially good for a 5.76 kW continuous power load (like an EVSE). So this rule of thumb is a simple way for them to reduce the chance of the Powerwall having to shut down because of overload. If you say to them "hey, I'll manage my loads to make sure that the total load stays under 5 kW during a blackout even though I have some 40A circuits" I think they'll decline that solution.

Cheers, Wayne

 

[Frankman60]

Thanks Wayne. So it doesn’t seem that the requirement is because of code, it has more to do with preventing homeowners from putting too much load on the Powerwall during a grid outage. Even though the Powerwall will shut down when that happens, and can be reset by flipping the switch, I guess Tesla wants to prevent this. But by not allowing me to self manage the power load during the seldom grid power outage, it is requiring that I double my investment by purchasing as second Powerwall.

What Tesla told me is that if you have only 1 Powerwall, they don't want any breakers larger than 30A on the Powerwall side of the Backup Gateway. The Powerwall is only good for 5 kW continuous power, while a 30A breaker is potentially good for a 5.76 kW continuous power load (like an EVSE). So this rule of thumb is a simple way for them to reduce the chance of the Powerwall having to shut down because of overload. If you say to them "hey, I'll manage my loads to make sure that the total load stays under 5 kW during a blackout even though I have some 40A circuits" I think they'll decline that solution.

Cheers, Wayne

 

[wwhitney]

Thanks Wayne. So it doesn’t seem that the requirement is because of code, it has more to do with preventing homeowners from putting too much load on the Powerwall during a grid outage.

Well, there is 2014 NEC section 702.4(B)(2) which says:

(2) Automatic Transfer Equipment. Where automatic transfer equipment is used, an optional standby system shall comply with (2)(a) or (2)(b).
(a) Full Load. The standby source shall be capable of supplying the full load that is transferred by the automatic transfer equipment.
(b) Load Management. Where a system is employed that will automatically manage the connected load, the standby source shall have a capacity sufficient to supply the maximum load that will be connected by the load management system.

But 702.4(B)(2)(a) is usually addressed by doing a load calculation, which Tesla doesn't seem to be doing. This rule about 30 amp breakers is a pretty weak substitute.

Cheers, Wayne

 

[wwhitney]

10kW of powerwall (30A backfeed)

10 kW of backfeed is 40A and change (10 kW / 240V = 41.7A). Each 5 kW continuous power, 7 kW peak power Powerwall gets connected via a 30A breaker (which would allow up to 7.2 kW peak, 5.76 kW continuous).

Cheers, Wayne

 

[Kbra]

10kw. This showing ~0.6 kw house load and tesla charging at 48amps.

 

[Ulmo]

10kw. This showing ~0.6 kw house load and tesla charging at 48amps. View attachment 335084 View attachment 335085

That's fantastic! I love that example data point.

Here's my ruminations about charge rates per night, to save the longevity of both the PowerWalls and the Tesla battery pack:

When charging from battery to battery, I wonder what the optimal transfer power is during, say, an overnight charge. Assuming 300Wh/mile and assuming you are done driving by 10PM and leave the house no earlier than 4AM and drive no further than 150 miles per day, that gives a 6 hour window to charge, which is about 45kWh or so, which is 7,500Wh per hour, with conversion losses about 8,333Wh per hour, or about a 8.3kW charge rate. Half the distance traveled per day, and that would become 4.2kW charge rate, but if you live that close to work, you're likely to be able to leave a lot later and get done with your day a lot earlier, so add two hours to charge time, and it could be 3,125Wh per hour charging, so 3.2kW charge rate. 3.2kW at 240VAC is 13.3 amps, 14 amps to be safe. If you get voltage drops during charging (unlikely if coming from battery), then let's say worst case around 220VAC, amps would need to be 14.55 amps, 15 amps to be safe. 15 amps at 250VAC (upper voltage range?) would be 3,750 Watts, and 15 amps at a usual 240VAC is 3,600 Watts, enough to charge 22.5kWh in 7 hours, enough for 300Wh/mile for 75 miles per day.

Since you're already charging at 10kW, I'll assume you're in an upper band. Let's say you go 150 miles per day at 300Wh/mile:

45,000Wh. At 90% AC->DC Tesla charger conversion, requires 50kWh to charge daily. Assuming 7 hours time to charge leaves 7,142Wh needed per hour, which at a reduced voltage of 230VAC would be 31 amps. 220VAC would require 32 amps. 32 amps at 250VAC would be 8kW.

To me, it seems like on nights where you don't get back late after longer than usual errands, you could charge your car at some rate between 15 amps and 32 amps and handle most moderate to heavy driving range commutes and errands. If your commute is much less, you could even do far fewer amps than 15 amps, perhaps dropping to as low as 10 amps or 5 amps.