Solar City says I can't buy one powerwall, have to buy two

[Andygt02]

Hi all,

I'm trying to add a Tesla Powerwall to my existing solar setup. The site assessment was last week and although I put down a deposit for one unit, they're saying I can only move forward with two.

The reason given is:

"The powerwall can handle a maximum of 30A of backfeed. Currently, your solar is putting more than 30A of backfeed into the main panel (it's 50A)."

Two units is pricey and more than I'm looking to spend. Is this absolutely necessary? I'm not an EE and would appreciate some background or an alternate solution I can suggest to Solar City.

[SunEagle]

Unfortunately the formula for solar panels wattage and battery size needs to be balanced. Too little wattage is not good for the battery as well as too much wattage can be no good.

Sounds like Tesla is trying to make sure you do not charge that one battery too fast so they recommend going with 2. The only other option I see would be to reduce your panels wattage charging the battery. I don't know if that equipment can do that without a wiring change.

[ButchDeal]

So you currently have 24 Enphase M250 micros with 24 LG 300w PV modules?
And you are looking to add a powerwall 2 to the system for backup capabilities?

You have at least two strings of the micro inverters so they should be able to connect just one to the powerwall, though the powerwall should be able to control them individually limiting their output as needed.

[jflorey2]

Hi all,

I'm trying to add a Tesla Powerwall to my existing solar setup. The site assessment was last week and although I put down a deposit for one unit, they're saying I can only move forward with two.
The reason given is:

"The powerwall can handle a maximum of 30A of backfeed. Currently, your solar is putting more than 30A of backfeed into the main panel (it's 50A)."

Two units is pricey and way more than I'm looking to spend. Is this absolutely necessary? I'm not an EE and would appreciate some background or an alternate solution I can suggest to Solar City.

Short answer - no.

Longer answer - the Powerwall uses a bidirectional inverter and a battery to store energy when it's available (like during the day) and release it back to either your home or the grid during times of high demand. Since it is AC coupled to the solar system, it effectively doesn't know what the solar system is doing - it can accept energy from the grid or from solar; it's all the same to the Powerwall.

The Powerwall has a 5kW inverter, so it can accept 5kW of power. If you are generating 4kW and the Powerwall is set to maximum rate charge (5kW) then 4kW will come from solar and 1kW will come from the grid. If you are generating 6kW and the Powerwall is set to maximum rate charge (5kW) then 5kW will come from solar, and the solar array will send 1kW back to the grid. It won't overload anything; you just won't be storing all the energy you receive.

The only reason you would have to worry is if you are in a place like Hawaii that is not allowing new solar customers, and you are trying to use a Powerwall to achieve "net zero" (i.e. no power back to the grid, ever) - in that case your Powerwall rating would have to be the same as, or greater than, your solar array rating - both in power (KW) and energy storage (KWHR.)

And BTW if you are concerned about money there are FAR more cost-effective solutions than the Powerwall.

[Andygt02]

Couldn't they put the battery on a 30 amp breaker? Excess would just spill out into the grid.

[Andygt02]

So you currently have 24 Enphase M250 micros with 24 LG 300w PV modules?
And you are looking to add a powerwall 2 to the system for backup capabilities?

You have at least two strings of the micro inverters so they should be able to connect just one to the powerwall, though the powerwall should be able to control them individually limiting their output as needed.

That's my solar setup, yes.

I'm looking to store some of the solar energy produced during the day to help offset usage at night (mainly from charging an EV). I live in an area that compensates excess energy at the wholesale rate and doesn't have any TOU rates, so what happens is I'm pushing all that solar energy to the grid while I'm at work, then drawing it back at night. I'm fine with still drawing some energy from the grid as needed.

[ButchDeal]

That's my solar setup, yes.

I'm looking to store some of the solar energy produced during the day to help offset usage at night (mainly from charging an EV). I live in an area that compensates excess energy at the wholesale rate and doesn't have any TOU rates, so what happens is I'm pushing all that solar energy to the grid while I'm at work, then drawing it back at night. I'm fine with still drawing some energy from the grid as needed.

It would never matter unless you are going to get a backup solution where your solar and the powerwall would be the only thing. But even in that case they could easily just use part of your solar.

If it is not going to have backup capabilities it doesn't matter a bit and they are giving you a line of crap, likely due to poor training.

[Andygt02]

Tesla's response:

"Understood, but the full/partial vs the amount of backfeed are two different things. Meaning, with 1 battery you could back up part of your house no problem. However the 1 PW would not be able to be charged by the solar, because of its large energy flow that would exceed the amount 1 PW could handle (30A max, solar gives 50A). Thus in order for the solar to work in conjunction with the batteries we need 2 (and then you get everything in your house backed up as a side benefit)."

[ButchDeal]

Tesla's response:

"Understood, but the full/partial vs the amount of backfeed are two different things. Meaning, with 1 battery you could back up part of your house no problem. However the 1 PW would not be able to be charged by the solar, because of its large energy flow that would exceed the amount 1 PW could handle (30A max, solar gives 50A). Thus in order for the solar to work in conjunction with the batteries we need 2 (and then you get everything in your house backed up as a side benefit)."

It is simply a matter of SolarCity being a cookie cutter or one size fits all type company. They are not capable of comprehending that you have two strings of micros and one could be associated with the powerwall by being on the powerwall side of the disconnect and the other on the grid side of the disconnect. Such logic simply evades them.

[SunEagle]

It is simply a matter of SolarCity being a cookie cutter or one size fits all type company. They are not capable of comprehending that you have two strings of micros and one could be associated with the powerwall by being on the powerwall side of the disconnect and the other on the grid side of the disconnect. Such logic simply evades them.

Or they are smart like a fox and try to convince people to buy more then really needed.

[adoublee]

Tesla's response:

"Understood, but the full/partial vs the amount of backfeed are two different things. Meaning, with 1 battery you could back up part of your house no problem. However the 1 PW would not be able to be charged by the solar, because of its large energy flow that would exceed the amount 1 PW could handle (30A max, solar gives 50A). Thus in order for the solar to work in conjunction with the batteries we need 2 (and then you get everything in your house backed up as a side benefit)."

This may be accurate in the case of use in back-up (off-grid) mode if the Tesla can't limit the Enphase with control over the powerline.

There may be something more here to related to tax credits. Is this to be a leased system? They may be wanting to take a tax credit on the battery as part of the PV system, but have to be able to justify that the battery is charged by solar.

[jflorey2]

This may be accurate in the case of use in back-up (off-grid) mode if the Tesla can't limit the Enphase with control over the powerline.

I don't even think it's accurate in that case. "the 1 PW would not be able to be charged by the solar, because of its large energy flow that would exceed the amount 1 PW could handle (30A max, solar gives 50A)." - it could still charge at some maximum rate even if that doesn't capture 100% of the solar generation.

[adoublee]

I don't even think it's accurate in that case. "the 1 PW would not be able to be charged by the solar, because of its large energy flow that would exceed the amount 1 PW could handle (30A max, solar gives 50A)." - it could still charge at some maximum rate even if that doesn't capture 100% of the solar generation.

If the power is produced by the microinverter it has to go somewhere. If loads are not in place to take the generation and the battery regulates the current it will take, then the voltage would have to rise. So you are looking at overcurrent or overvoltage depending how the Tesla inverter works, neither of which are acceptable and why the best AC coupled systems communicate so the solar inverters are not trying to continually produce maximum power.

[jflorey2]

If the power is produced by the microinverter it has to go somewhere. If loads are not in place to take the generation and the battery regulates the current it will take, then the voltage would have to rise. So you are looking at overcurrent or overvoltage depending how the Tesla inverter works, neither of which are acceptable and why the best AC coupled systems communicate so the solar inverters are not trying to continually produce maximum power.

All of that might be true - during an outage. During normal operation the one bank will accept charge at its maximum rate. And they certainly seem to be talking about normal operation, since they mention backup as a "side benefit."

During an outage there are a couple of possibilities.

1) He makes no changes - then he can't run solar during outages.
2) He moves one of his two arrays to the protected side of the system - then he can run his system until the battery is close to full, at which point you have the same problem you mention above ("it has to go somewhere.") At that point either the entire system shuts down or the voltage goes out of range and the inverters drop off.
3) He integrates with a system that provides for explicit control of microinverters via frequency control. Then he can run off grid with any number of PW's.

[Sunking]

I would not worry to much about it. Seriously doubt Tesla will ever make a Powerwall available to you. They just took your deposit to invest it and make dividends and interest off your money. At some point they will have refund your money after they make some cash off of it. They are cooking the books. They are just trying to get more cash out of you for a second unit and stalling.It is called Plucking the Pigeon twice. They will keep stalling until you demand a refund. The longer they keep your cash, the more money they make off it.