2xPowerwall whole house backup - HVAC sure start problem

[JSchnee21]

"3 tons of cooling = 36,000 BTU/hr.
10.5 kW electrical input to A/C unit ? You sure about that ?"

Yes, 36,000 BTU/Hr = 10.5kW (at least according to Google). But I realize his HVAC won't actually draw that much while running. My decrepit 4 Ton only draws 29 Amps @ 240V (last time I measured) or very roughly 5.5-6.0kW according to my Solar Edge consumption data while the HVAC is running.

But it's breakered at 50 or 60amps (I forget which).

Realistically, I would expect his 3 ton unit to draw ~20-25 amps continuously.

What is not clear to me, is what happens when the compressor tries to start while running on battery? Does it buzz and cause the lights to dim as one would expect if it was current starved? Is there a large voltage sag? Or does it not try at all -- suggesting the power monitoring circuity / control boards "don't like" the power quality coming from the inverters? E.g. bad ground, lack of neutral, issue with bonding, etc?

The controller for natural gas hot water heater is similarly "fussy."

[arf88]

25-30 continuous is about right I recall connecting an amp meter clamp on the common and that is what it was drawing. Tesla app reporting 5.2kW with compressor running.

In regards to what happens on battery power I believe every time the compressor tried to start it would shutdown the Tesla batteries for a millisecond bc the 2 different UPS alarms would both go off indicating momentary power loss. The microwave clock was on at all times. The house draw on battery was only 200watt could have probably got it down to under 100 watt but I doubt that would have made a difference. The one time the compressor did make any noise it appeared it was trying to spin up for half a second I heard it and the UPS sounded the alarm but at least once in that 10 min test window it tried to start but failed.

I'm wondering if it is worth it to try a sure start and make sure it is wired correctly as the Hard start kit made by Carrier didn't do it's job clearly and no specs what so ever are posted about it even from the mfg. At least with the surestart it gives you all the specs and says it reduces the LRA by 60%. Waiting to see what the solar company wants to do. They were supposed to contact Tesla to get some feedback.

[Mike90250]

What is the running and surge spec for 1 PW ?

[arf88]

Here is the specs sheet. Only 30a per PW2 is my u derstanding.

https://www.tesla.com/sites/default/files/pdfs/powerwall/Powerwall%25202_AC_Datasheet_en_northamerica.pdf

[Mike90250]

Here is the specs sheet. Only 30a per PW2 is my u derstanding.

https://www.tesla.com/sites/default/...rthamerica.pdf

I could not get link to work, but other web sites claim
Power: 5 kW continuous, 7 kW peak. So with 2, in prime condition, you only have 14kw to start a motor with. I think your sales droid just said anything to make his commission,

I have a 6kw inverter, so that I can reliably start a 1/2 hp motor for water pump. Seems like overkill, but isn't

[arf88]

Sorry about that I'm responding on the phone and the URL didn't translate.

I agree I think this is fairly new technology and they don't have nearly all the experience. Sadly I think this is partly Tesla's fault as well bc they used a Tesla calculator which included the AC is estimated consumption and they recommend 2 powerwall as well.

[arf88]

Here is the link

https://www.tesla.com/sites/default/files/pdfs/powerwall/Powerwall%202_AC_Datasheet_en_northamerica.pdf

[Mike90250]

What many installers fail to do, is size systems using Volt Amps (VA) and they simply use watts. When you get motors involved, that changes the game greatly, Power Factor comes into play and your inverters are not able to supply the out of phase power needed. Look at my chart and see the PF ratings for the different motors, and google how to calculate VA from power factor. Then THATS the number you use to find the stalled motor starting power. But sometimes it's easier to just use the $400 peak meter and record it, because you don't really know what your voltage sag is at starting time.

[arf88]

I think most solar companies look at Amps only. They see the AC is on a 50a breaker so they think 2pw with 60a max load should handle the load if other loads are minimal.

[Ampster]

It is the startup loads that are the issue which many don't consider because the grid is more forgiving.

[arf88]

Agree, already proved if compressor is running and it switched to battery backup it still ran. It's just starting that is an issue

[J.P.M.]

"3 tons of cooling = 36,000 BTU/hr.
10.5 kW electrical input to A/C unit ? You sure about that ?"

Yes, 36,000 BTU/Hr = 10.5kW (at least according to Google). But I realize his HVAC won't actually draw that much while running. My decrepit 4 Ton only draws 29 Amps @ 240V (last time I measured) or very roughly 5.5-6.0kW according to my Solar Edge consumption data while the HVAC is running.

But it's breakered at 50 or 60amps (I forget which).

Realistically, I would expect his 3 ton unit to draw ~20-25 amps continuously.

What is not clear to me, is what happens when the compressor tries to start while running on battery? Does it buzz and cause the lights to dim as one would expect if it was current starved? Is there a large voltage sag? Or does it not try at all -- suggesting the power monitoring circuity / control boards "don't like" the power quality coming from the inverters? E.g. bad ground, lack of neutral, issue with bonding, etc?

The controller for natural gas hot water heater is similarly "fussy."

3 tons of cooling does indeed = 36,000 BTU/hr. Google is right in that respect.

And 10.55 kW of power expended continuously for 1 hour is the work equivalent of 36,000 BTU of heat.

So what ? Where does the C.O.P. of the refrigeration equipment or the P.F. of the motors or any startup transients come in ?

I appreciate this is about startup loads and what those loads can do to batteries, etc., but as you stated it, I'm pretty sure most any modern 3 ton A/C won't draw anywhere near a 10.55 kW when running under steady state. I also wouldn't use that 10.55 kW to estimate startup power or current draws.

I think you may have forgot to divide by the C.O.P. and also the P.F. of any motors.

[pete4]

3 tons of cooling does indeed = 36,000 BTU/hr. Google is right in that respect.

And 10.55 kW of power expended continuously for 1 hour is the work equivalent of 36,000 BTU of heat.

So what ? Where does the C.O.P. of the refrigeration equipment or the P.F. of the motors or any startup transients come in ?

I appreciate this is about startup loads and what those loads can do to batteries, etc., but as you stated it, I'm pretty sure most any modern 3 ton A/C won't draw anywhere near a 10.55 kW when running under steady state. I also wouldn't use that 10.55 kW to estimate startup power or current draws.

I think you may have forgot to divide by the C.O.P. and also the P.F. of any motors.

Exactly, AC doesn't create heat/cold, like for example electric heater would, where 36000BTU = about 10.55 kW, AC transfers heat, totally different process. My 3.5 ton AC uses about 4kW, OP is good for steady state, it's the startup surge, that overloads the system and it would be helpful if OP could somehow measure the current surge, to see how much it is. Maybe AC is drawing more current than it should and since grid is much more forgiving, it doesn't affect it.

[J.P.M.]

Exactly, AC doesn't create heat/cold, like for example electric heater would, where 36000BTU = about 10.55 kW, AC transfers heat, totally different process. My 3.5 ton AC uses about 4kW, OP is good for steady state, it's the startup surge, that overloads the system and it would be helpful if OP could somehow measure the current surge, to see how much it is. Maybe AC is drawing more current than it should and since grid is much more forgiving, it doesn't affect it.

Yea, - the heat just gets moved around.

Sometimes I've had some luck over the years explaining residential A/C and heat pump operation and the electrical energy requirements to power them by first differentiating the electrical power used to produce (work) energy that is then expended and used to power the equipment that produces the cooling/heating effect as separate from the transport of (thermal) energy that is the commodity moved out of (when cooling) or into (when heating) a dwelling.

As a loose analogy, kind of like the fuel needed and burned by the ICE (or a truck powered by electricity if/when that evolution comes along) that delivers the commodity (fuel) to the gas station.