Powerwall Charging

[prash]

I am in the process of installing a grid tied solar system. Also plan on adding a Powerwall 2 at a later date.
I have 2 questions
1. Those who have powerwall 2 installed on a grid tied solar- Is the setting so that the powerwall can charge both from solar and grid?
2. Those who have added powerwall 2 onto an existing grid tied solar- can one claim federal tax credit of 30% for installation of Powerwall(battery backup). My research showed that one can claim 30%tax credit for battery backups added to an existing solar system as long as it is 100% charged from solar.

 

[prash]

Found this document useful regarding tax credit and battery backups

 

[he1957]

@prash, Can't answer question 2 sorry.

For question 1, currently the PW-2 operates in one of 2 modes.

Mode 1 is "Backup-Only" whereby 100% of available battery power is reserved for grid outages. In this mode it will charge from the grid (and/or Solar if you have available). The general idea is to provide backup power when there is no grid. As far as I know if you also have Solar, then in this mode, Solar would also be used to recharge the battery if the capacity is below 100%. I have Solar but do not use this mode because it does not suit my needs - there is a small delay in switching between grid and battery in this mode.

Mode 2 is "Self-Powered". This mode is designed to maximise your own consumption/usage of Solar power generated. With this mode you can set a "Reserve" value for battery between 0 and 99%. This reserve power is only used in the event of a grid outage.

When the battery level is above the Reserve, only Solar is used for charging if there is excess Solar compared to your site load.

If you start draining the battery (ie: no sun or higher load than solar produced) it will discharge down to the Reserve level and then switch to the grid for your load.

The Reserve level stays available and will only be used if the grid fails.

If the level drops below the reserve value (because of a grid outage) when the grid is available again, the battery will charge up to the reserve level and then only charge from solar as above.

Determining the "best" Reserve value depends on your site, the amount of solar, consumption and sun hours etc.

The above is from my own observations.


Cheers,

Harry E.
===

 

[FlyRealFast]

@prash, Can't answer question 2 sorry.

For question 1, currently the PW-2 operates in one of 2 modes.


The above is from my own observations.


Cheers,

Harry E.
===

Great explanation of the current operation modes - thank you.

I'm a newcomer to the forums here but have been enjoying a 15KW grid-tied system in Florida for almost two years now and closely following PW2 innovations, use cases, availability, etc. for much of 2017. Have a Model 3 and solar roof on order and will likely be installing PW2s on all our new homes and development projects going forward.

Trying to make sure I'm clear on one key piece of the latest AC-coupled PW2/gateway operations - auto grid disconnect upon grid failure. From what I can tell so far the latest PW2/gateway system automatically and quickly (sub-second transfer switching) disconnects the PV/PW2 home system from the grid whenever the public grid fails and then continues to power the home via solar and battery much like a UPS - is this correct?

Pat

 

[bkp_duke]

Great explanation of the current operation modes - thank you.

I'm a newcomer to the forums here but have been enjoying a 15KW grid-tied system in Florida for almost two years now and closely following PW2 innovations, use cases, availability, etc. for much of 2017. Have a Model 3 and solar roof on order and will likely be installing PW2s on all our new homes and development projects going forward.

Trying to make sure I'm clear on one key piece of the latest AC-coupled PW2/gateway operations - auto grid disconnect upon grid failure. From what I can tell so far the latest PW2/gateway system automatically and quickly (sub-second transfer switching) disconnects the PV/PW2 home system from the grid whenever the public grid fails and then continues to power the home via solar and battery much like a UPS - is this correct?

Pat

That is correct. It's the major reason I'm installing 3 PWs in our home. Preparation for "the big one".

 

[he1957]

@FlyReaFast, To expand on operations and features, there is a "backup" "option" (it was ordered and supplied fitted as part of my original order for my region [Australia]) for the PW-2 that provides a sub-second switch-over much like or more exactly like a UPS.

In "Self-Powered" mode:

A. Grid Functioning:

1. If the battery is at the Reserve level and there is no sun, power is from the grid; if the grid fails, power comes from battery
2. As the sun comes up, solar power is supplied to the load and the grid use drops
3. As solar increases, grid power drops further and when solar exceeds load, the battery starts charging
4. This continues until the battery is full then excess solar is exported to the grid
5. If solar production is more than the load plus the battery max charge of 5 kW, excess is exported to the grid
6. If the battery is above reserve and load exceeds solar production, battery provides the difference and if needed so too will the grid
6a. If the battery is full, excess solar goes to the grid

The net result of this is that if battery is above reserve, and as the solar power generation changes, what is not taken as a load from solar goes to the battery and is taken from the battery as clouds pass by thereby maximising use of your own solar generation. It's rather cool I must say!

7. With sun and grid fail, PW-2 disconnects from grid and provides power from solar and/or battery - limit of 5 kW plus Solar
8. With no sun and grid fail, PW-2 provides power like a UPS limited to 5 kW and a sub-second switch-over

My understanding of a PW-2 overload as per items A7 or more likely A8 of 5 kW is shutdown/auto-start but I have not had this happen yet.


B. Grid Fail

1. Much like the above under grid fail conditions (A7 and A8)
2. With no grid, the PW-2 will also be able to use the Reserve value down to zero

What I don't yet know but has been asked elsewhere in this Forum (with no answer yet) is what happens if the grid is down, the battery is flat and the sun comes up? Because the Solar system needs to detect the grid (or the PW-2 as a fake grid) in order to enable power output.

It may be the PW-2 is "smart" enough to see the solar power, no load and figure it should start charging the battery until solar exceeds load but...

I also have a generator and changeover switch on my grid input (ie switch grid input to genset) which I can use to power my home/load so I figure I can use this to pretend the grid is alive. In theory I see no reason this won't work but I need to disconnect the Solar in case the battery gets full or Solar exceeds charge rate so as to avoid trying to back-feed the genset. As long as this is remembered, I could start the genset to get the system running (disconnect Solar, start genset to start PW-2, disconnect genset and connect Solar) then run from Solar/Battery but have not had to do this yet

Finally, the other "problem" I have is the genset is only rated at 3 kW so the PW-2 would most likely cause it to shutdown (overcurrent) but I figure it would run long enough to kick-start things. Next investment is a 15 kW diesel genset


Cheers,

Harry E.
===

 

[BobPaul]

What I don't yet know but has been asked elsewhere in this Forum (with no answer yet) is what happens if the grid is down, the battery is flat and the sun comes up? Because the Solar system needs to detect the grid (or the PW-2 as a fake grid) in order to enable power output.

Is this the problem that has to be resolved for those customers who have no grid connection and are waiting for a PW2?

 

[he1957]

@BobPaul, I would see this as a completely different issue. If you are referring to an off-grid system then your site is a stand-alone installation so you would not be using grid-connected inverters etc. In the installation I describe above this is grid connected so the most important factor [safety] is not providing power to the grid when the grid is down - don't want to shock any line workers

I "speculate" if I set my site to "no grid" [trip the main grid breaker] and left it as was I'd run out of power and that would be that.

In my case however, I can start the genset and "pretend" the grid is there so {sh,w}ould be able to kick-start the system such that the solar would kick-in and behave as described above. Ensuring I stop the genset and/or solar to avoid the issue described.

(I might get a chance to test this soon but Tesla should have already done this and published the results).

I did calculations of running costs of diesel and so-forth and to remain grid-connected is currently better value than trying to go off-grid (because I already have grid).

If I was installing a PW-2 in a grid-less site:

1. One solar array could be connected to the "grid" CT to provide a "grid" and an additional array could be connected as a "solar" input on the "Solar" CT. Having said that it would be necessary to have a "grid" solar array that does not care if it senses AC before enabling inverter output. -- How to ensure phase is in sync between these two; also back-feeding the "grid" {c,w}ould be an issue??

2. Perhaps if the PW-2 is set to "Backup-Only" mode and the "grid" CT is the only Solar array. This would use solar to charge the battery and the battery would provide power when the "grid" "fails". As long as you use less power than stored/available you'd be ok. The "problem" I see with this is the delay in switching as described previously - hence my option 1 above.

It would be nice if I could afford to buy some extra hardware to experiment with these configuration ideas because I'm very curious and I'm quite happy to embarrass myself contemplating these posts because I can't find answers elsewhere.

I do feel however that Mr Musk is on the right track and so far my system is behaving exactly as I believe it should. In the last week I (seem to) have been 97% self-powered (I see "glitches" in the mobile phone App) so I'd also speculate the glitches are part of the test/response of grid connectivity and so-forth and not necessarily reflective of the actual costs I'll see on the next bill [I wait with held breath ].


Cheers,

Harry E.
===

 

[abasile]

What I don't yet know but has been asked elsewhere in this Forum (with no answer yet) is what happens if the grid is down, the battery is flat and the sun comes up? Because the Solar system needs to detect the grid (or the PW-2 as a fake grid) in order to enable power output.

This should be easy for someone to test. Just turn off the main breaker and introduce a load that causes the Powerwall system to drain overnight. Then see what happens when the sun comes up. I promise to rate as "helpful" any post that describes the outcome of such an experiment!

 

[Parzival]

This should be easy for someone to test. Just turn off the main breaker and introduce a load that causes the Powerwall system to drain overnight. Then see what happens when the sun comes up. I promise to rate as "helpful" any post that describes the outcome of such an experiment!

I've been meaning to test it, but I've yet to exhaust my Powerwalls

 

[BobPaul]

@BobPaul, I would see this as a completely different issue. If you are referring to an off-grid system then your site is a stand-alone installation so you would not be using grid-connected inverters etc.
===

We have no idea how Tesla will set up the stand alone off-grid PW2. I imagined we would be able to use a normal grid tie inverter with no grid connected with the PW2 in the self consumption mode and it would behave like a normal PW2 that had a grid power failure.