Residential Powerwall Application Question

[Joelgjr]

Hi Everyone, I currently have what will be 27kw of solar between my roof and back yard. It was EXTREMELY challenging getting San Diego to approve this much solar, because you have to "prove" that you will use it and aren't building solar for the sole purpose of selling it back to the grid. My long term plan is to convert everything to electrical - house heat converted to heat pump, pool heater already converted to heat pump, gas stove converted to electric, etc.

When you remove the high draw devices I use about 20-30 kwh / day. My pool pump alone is an additional 13.6 kwh/day, plus an additional 43 kwh/day for the pool heater (6 out of 12 months per year), 10 kwh for home AC 6 months out of year, anyway I can go on but if I cut out high draw devices I had pretty reasonable draw, but if I want to, I go easily go up to >200kwh/day.

I completely understand that running the highdraws off batteries would be prohibitively expensive. However, I want to be able to use my high draw devices direct from solar in the event of a power failure, as I want to eventually transition to off grid. What I cannot seem to determine is, if I have lets say 2 powerwalls, and the power goes out, am I limited to the 10kw max output of the two powerwalls, or will I still have access to the full 26kw of power my solar array is producing during daylight hours? Ideally I'd like to run the heat pumps and pool pumps directly off the solar inverters from 10am-3pm, and then off hours switch them off and run from powerwalls/lingering solar the rest of the day. Obviously when the grid is up this is irrelevant because the full output from the solar goes into your electrical panel.

It was suggested that I simply make these high draw devices on their own off grid circuit, but that means It would be impossible for me to use grid power with them in the winter months.

Thanks!

 

[miimura]

Technically speaking, you could run a lot of your loads when the grid is down and the sun is shining. The problem is that there are no standard ways to control everything so that the system would run properly without overloading the solar + battery draw or try to pump too much solar into the batteries when the loads are not running. The basic installation rules-of-thumb would prevent you from placing most of your heavy loads on the backup side of the Tesla Backup Gateway. It also has a 200A limit and it sounds like you may run into that limitation as well.

 

[neroden]

That's disappointing. Seems like a major design flaw. It can't be complicated electrical engineering to allow the solar to "bypass" the Powerwalls when there's high house draw.

 

[cwied]

The solar power does not go through the Powerwalls. In fact during the day you can use the sum of your solar and Powerwall output while the grid is out. The limitation is what can be placed behind the backup gateway. I believe with the amount of solar you have, they won't want all of it behind the backup gateway with only two Powerwalls. Likewise, they won't allow any loads with breakers greater than 60 amps behind the backup gateway with two Powerwalls. They don't want to put the Powerwalls in a situation where changing house load could overload them.

 

[miimura]

Just to be clear, when the grid is up there is no problem. Relative to your house, it is an infinite sink and source. The problem comes when the grid is down and your house is isolated from the grid. Something has to balance the micro-grid. 26kW of solar, 30+kW of loads and only 10kW of Powerwalls is not easy to balance. If you increased it to 4 Powerwalls, the number of times you would have to manually or programmatically intervene to balance the micro-grid would be drastically reduced.

 

[m3-pw2]

Each powerwall has a limit charge of 5kw. So your system is too big for two powerwalls. You'll need to get more powerwalls, or split the system so that you have about 10kw max.

During grid outage, you can't export any power on the grid lines. So the powerwall controller will shutdown the solar system when powerwalls are fully charged, and then waits until the powerwall is drained a bit before reactivating the solar system to recharge.

At home, I have two powerwall as well matched to a 10kw solar inverter. Both AC and car charger were not added to Powerwall.

 

[Dan123]

I completely understand that running the highdraws off batteries would be prohibitively expensive. However, I want to be able to use my high draw devices direct from solar in the event of a power failure, as I want to eventually transition to off grid. What I cannot seem to determine is, if I have lets say 2 powerwalls, and the power goes out, am I limited to the 10kw max output of the two powerwalls, or will I still have access to the full 26kw of power my solar array is producing during daylight hours? Ideally I'd like to run the heat pumps and pool pumps directly off the solar inverters from 10am-3pm, and then off hours switch them off and run from powerwalls/lingering solar the rest of the day. Obviously when the grid is up this is irrelevant because the full output from the solar goes into your electrical panel.



Thanks!

The answer to your question is complicated. In theory, you will have access to 10kw (14kw peak) from the batteries, and 26KW from the solar. However, there may be some complications.

1. If you solar is producing 26KW (20kw to be more realistic), and if your house is not consuming 20kw at the moment, then the energy will need to go somewhere. Powerwalls can consumer 10kw (if they are not already full). What about the rest? If your PV array is producing more power than can be consumed, then the Powerwall will shut down the array, and you will drop to 10KW available from the Powerwall. This will make the system with 2 Powerwalls and 26KW solar - unstable. The solar will be going on and off, and you will not be able to reliably run more than 10KW.

2. Do you have separate breakers for parts of your array? One solution would be to manually shut down part of your array, so you will have 10KW from the Powerwall, and up to 10KW from solar. That will work, as long as the powerwall is not full. If it is full, then solar will be shut down again, and you will end up with 10KW.

3. You can also split your solar system into two parts. One will be connected to your house loads, and will work all the time. The other part will be grid tried (outside of the Powerwall gateway) and will works only when the grid is on.

4. I think there are devices that are used to soak up excess power production. Like a smart water heater for example. You can install one and heat water, when you have too much electricity.



Frankly, based on your usage, 26KW of solar seems oversized.

 

[Joelgjr]

First of all, thank you everyone for the replies. This is significantly more complicated than I anticipated.

The answer to your question is complicated. In theory, you will have access to 10kw (14kw peak) from the batteries, and 26KW from the solar. However, there may be some complications.

1. If you solar is producing 26KW (20kw to be more realistic), and if your house is not consuming 20kw at the moment, then the energy will need to go somewhere. Powerwalls can consumer 10kw (if they are not already full). What about the rest? If your PV array is producing more power than can be consumed, then the Powerwall will shut down the array, and you will drop to 10KW available from the Powerwall. This will make the system with 2 Powerwalls and 26KW solar - unstable. The solar will be going on and off, and you will not be able to reliably run more than 10KW.

2. Do you have separate breakers for parts of your array? One solution would be to manually shut down part of your array, so you will have 10KW from the Powerwall, and up to 10KW from solar. That will work, as long as the powerwall is not full. If it is full, then solar will be shut down again, and you will end up with 10KW.

3. You can also split your solar system into two parts. One will be connected to your house loads, and will work all the time. The other part will be grid tried (outside of the Powerwall gateway) and will works only when the grid is on.

4. I think there are devices that are used to soak up excess power production. Like a smart water heater for example. You can install one and heat water, when you have too much electricity.

To answer your concerns:

1 and 4 - it seems like the concern is excess solar production while off grid, and in item 4 you mention there may be "smart" devices that can automatically manage this. I will definitely look into this because I've seen power be dumped to electric heaters in instances where wind turbines are over producing.

2 - the system isn't built yet, but the current plan is to put the full 26kw into a 400amp panel (that receives 200 amp service from the city). the power will be split up among three breakers corresponding to 3 inverters: 2x 10kw inverters for the ground arrays, and 1x 6kw inverter for the existing solar on the roof.

3 - This is an option which we are considering.

Frankly, based on your usage, 26KW of solar seems oversized

We used 1,300kw in March (which resulted in a $489 bill AFTER SOLAR) and 1,775 KW in April. This is without running my heat pump or A/C at all. I also don't commute to work so very little EV charging right now, but that could change if my job changes.

26kw will result in around 2,808 kw per month (26kw * 0.80 Correction Factor * 4.5 hrs sun/day * 30 days), averaged across the whole year, so higher in summer and lower in winter. I'm already at 1775 without running heat or air conditioning, before I convert my hot water to electric, and with basically no EV charging. Its very plausible that I can hit 2,808.

The panels I got extremely cheaply through a contact I have in china that is a solar supplier. Essentially $0.35/watt for 350 watt panels, plus the 25% Trump duty fee. The downside is the minimum purchase was a pallet, which is 60 panels. The other option is to sell some of the panels and not mount all 60. Not sure what the interest would be on that.

Anyway, sounds like for this to work the issue will be excess solar generation during the day while off grid, and the potential existence of a smart device to "soak up" excess power production. Thanks again!

 

[miimura]

The smart device I was thinking of was a controllable breaker that could be turned off when there is too much solar generation while off-grid. When the load is too much, you could do the same, forcing some loads off by switching off their breakers.

Leviton’s new smart Load Center brings app control to your circuit breakers

Of course, there are other ways to do this for moderately sized loads.

This Insteon device can switch up to 3,600W of 240V loads.
Insteon Remote Control DIN Rail On/Off Switch

You could also use a low power 120V smart on/off switch to control a contactor (big relay) for larger loads.