Off-grid AC start test?

[ucmndd]

This might be a goofy noob question but a bit of searching hasn't yielded a satisfying response, so...

I have 2x Powerwall 2s and 7.5kw of solar in an "almost whole home" backup install. I have a 30 year old original to the home ancient AC unit that we're planning to replace this year but I'm not confident it will be done this summer.

I'd like to test things off-grid and see if the Powerwalls can actually start the AC unit or not, both mid-day when my solar is producing at the max and in the evening when there's no solar to speak of. I gather the PWs have overcurrent protection that will take them offline if they can't provide the juice but, like, how does it work? What do I have to do to get them back online if they trip? Do they just try to come back online after a period? Do they need a power cycle? Gateway reset? If documentation exists for this, I can't find it.

Thanks all.

 

[miimura]

Since it's only a test, you can just put yourself back on-grid and everything will go back to normal. However, if you just turn off the breakers to all large loads, the system should restart in a few seconds, even without the grid.

 

[Rocket_man]

If you remember providing the LRA number, or taking a picture of the AC nameplate, they would have looked at the current spike and decided it was okay with the two PW's or they needed to install a 'soft start' on the AC. I have two AC units and 2 PW's and I've never had a problem starting one or both on the PW's. I did not need soft starts. The new PW's can spike their output in the second or so that it takes to kick start your AC.

 

[ucmndd]

If you remember providing the LRA number, or taking a picture of the AC nameplate, they would have looked at the current spike and decided it was okay with the two PW's or they needed to install a 'soft start' on the AC. I have two AC units and 2 PW's and I've never had a problem starting one or both on the PW's. I did not need soft starts. The new PW's can spike their output in the second or so that it takes to kick start your AC.

Thanks. The AC nameplate is faded to the point of being completely unreadable/blank and I told the installers not to worry about it because I was going to replace it.

But it sounds like there’s little/no risk in giving it a go. Mostly a curiosity / worst case sort of thing for the next couple months… just want to know what to expect before there’s an actual outage.

 

[aesculus]

I gather the PWs have overcurrent protection that will take them offline if they can't provide the juice but, like, how does it work? What do I have to do to get them back online if they trip? Do they just try to come back online after a period? Do they need a power cycle? Gateway reset? If documentation exists for this, I can't find it.

When I had my PW-2s installed 18 months ago I too had ancient heat pumps that would not be able to start offline in most cases. Tesla knew this and there was no way to isolate them because they are on a sub off of my house panel which is a sub off of my main panel.

Tesla told me that the PWs would trip and try to restart every 15 minutes if I did not intervene to trip the breakers on the units. Maybe FW has changed the behavior but I have no way to tell.

Now I have replaced them with two variable speed units so I think I am in the clear, at least as far as the starting current. If they really got to it and turned on the heat strips I am sure I would be doomed, but that is another story.

 

[Fruitcake]

Rocket_man is on track - you need the LRA number for your compressor, otherwise you're flying blind.

If the nameplate is that faded, try to look up the LRA by make & model number of the compressor (if that information is known).

My guess - purely characterized by your description of "a 30 year old original to the home ancient AC" is that the LRA is sky-high and you will trip your Powerwalls.

I would turn those breakers off and live with the heat. Better to sweat a little bit until the night comes than have all of your groceries spoil in the refrigerator.

And BE SURE that your replacement compressor is "solar friendly" with a low LRA. Or get a soft-start capacitor installed with your replacement compressor.

 

[jboy210]

When I had my PW-2s installed 18 months ago I too had ancient heat pumps that would not be able to start offline in most cases. Tesla knew this and there was no way to isolate them because they are on a sub off of my house panel which is a sub off of my main panel.

Tesla told me that the PWs would trip and try to restart every 15 minutes if I did not intervene to trip the breakers on the units. Maybe FW has changed the behavior but I have no way to tell.

Now I have replaced them with two variable speed units so I think I am in the clear, at least as far as the starting current. If they really got to it and turned on the heat strips I am sure I would be doomed, but that is another story.

I thought modern heat pump system did not need heat strips since they could heat out of the air at low temps. Is this not true? Or are you someplace that gets colder.

 

[aesculus]

I thought modern heat pump system did not need heat strips since they could heat out of the air at low temps. Is this not true? Or are you someplace that gets colder.

They still have them when it gets real cold, when the unit has a huge recovery or when the unit needs to defrost. And they are huge energy hogs.

It may be that the new units can bring them on in stages or adjust the power going to them, but if it kicks on in my 4 ton unit, I think it would bury the PWs. Note there are ways to prevent this from happening when the outside temp is above a certain level but ...

Anyway all these use cases are so small I am not going to sweat it until it doesn't work.