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2xPowerwall whole house backup - HVAC sure start problem
- Thread starter af88
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So is there a resource, once you determine your A/C unit, which technology works with it? With just the number of posts on this thread its clear the issue is not some sort of a secret. Its also clear that one can find installers or customer service people who do not know, but who does know?
Nothing? Hmm I’m going to have to wait until the CT comes out to refute that on levels from energy to spaceX to multiple gigas.
They made a push to take the market and then when the losses are too high, the parent company will figure it’s way through it, as all fortune 500companies do.
lol, that will just about solve any issue at a cost $$$
You need to educate yourself first and foremost. You know what you have in your home, do your own due diligence. Sadly the best way is to build your own system and depend less on sales people and incompetence at these companies.
1. Is your compressor single or variable stage/2-stage compressor
2. Measure the Inrush LRA current with a reliable RMS Fluke or similar meter, don't waste your time with Max, you need a meter to measure inrush. If you have a scope even better. Do the same exact thing with a soft-start in place. Use the delta and find out the real % efficiency vs the max advertised.
3. Assume you have no PV (night time use or dawn) so leave the PV out of the equation when determining the total home load and number of PW needed.
4. Measure total load at MSP at dawn and daytime with PV off, load your home with worse case Realistic scenario for load measurement and that should give you an idea of load without AC, than factor in your inrush AC current you measured earlier and determine how many PW ~28.5A you need to support your system.
I can guarantee you no installer or Tesla will do this or has done this b/c they use the MFG spec and take a best guess just on specs.
Which I think is mainly a matter of having enough people. Back in the day their customer service was great. Then they ramped up production but didn't ramp up the service department to match.
Next week I'm having my A/C serviced. This house is seven years old, I bought it last year, and I doubt the unit has ever been serviced. When the service tech is here I'll ask about a soft-start adapter. I figure if it can be done reliably, the A/C people should know about it. I'll also ask what is the start-up amperage. I don't expect that they'll have good news about soft-start, but maybe I'll find out how many Powerwalls it would take to start it without grid power. I probably wouldn't add another PW, because lack of A/C here is not a life-or-death issue, and power failures are not all that common. But it would be interesting to know.
The sure way is at minimum follow the steps I provided above. You need facts and real world numbers. Most HVAC companies do not use soft start devices bc there is no need to use them in a home. Battery backup is generally considered new, the dozen experienced HVAC techs had never installed a sure start bc PW isn't common to them.
In regards to customer service competency is more important than quantity. If you get 10 people who don't know anything it is no better than having 100 inexperienced people who answer the phone quick to only elevate your request to someone who does know or put you on hold to transfer you anyway. Hire competent knowledgeable support and I'll wait to speak to them rather than get bounced around. L1 might as well be sales people with minimal training. Heck even Tesla L2 is as bad.
In regards to customer service competency is more important than quantity. If you get 10 people who don't know anything it is no better than having 100 inexperienced people who answer the phone quick to only elevate your request to someone who does know or put you on hold to transfer you anyway. Hire competent knowledgeable support and I'll wait to speak to them rather than get bounced around. L1 might as well be sales people with minimal training. Heck even Tesla L2 is as bad.
Education is definitely required if designing your own system. Its a good idea regardless with a large purchase regardless.
Some choose to pay someone to go the extra mile and look at these design questions. Low cost comes at a price. Doing your own system design and measurements is one option.
Your idea to measure at dawn and dusk is the hard way to do it. All smart meters report the actual draw every 15 minutes. All you need to do is buy the interval data set, and dump it into a spreadsheet. From there you can pretty well see what your house pulls as a max. Also likely you will see the deltas from when the AC turns on during the mid day and afternoon.
Good luck with the free Powerwall. Regardless, I hope you get to a place of satisfaction.
It also appears, from some very inexact research, that the number of residential (and probably commercial for that matter) installations dwarfs the number of homes with battery back up and solar.
Unlike the auto division, Tesla energy does not make every component of the system, and of course they actually acquired a solar company. I need to educate myself on this issue and af88s outline looks great, but I would say that its not surprising in any field where the the particular problem is limited to some small fraction (it looks like whole home battery + solar was less than 1% of all solar installations) of the area.
With the recent price drop, its only now that enough people are considering solar systems to run an entire house that this issue has a chance of making it to the level of common knowledge of the industry.
Indeed, research is required. Especially since once a system is installed whether or not the A/C is connected is not something to easily turn on or off.
This is the second such issue I have found, the first being the fact that if you don't have "enough" powerwalls, the whole system can shut off in a grid outage.
Unlike the auto division, Tesla energy does not make every component of the system, and of course they actually acquired a solar company. I need to educate myself on this issue and af88s outline looks great, but I would say that its not surprising in any field where the the particular problem is limited to some small fraction (it looks like whole home battery + solar was less than 1% of all solar installations) of the area.
With the recent price drop, its only now that enough people are considering solar systems to run an entire house that this issue has a chance of making it to the level of common knowledge of the industry.
Indeed, research is required. Especially since once a system is installed whether or not the A/C is connected is not something to easily turn on or off.
This is the second such issue I have found, the first being the fact that if you don't have "enough" powerwalls, the whole system can shut off in a grid outage.
I'm not sure of the granularity of those reports but that is certainly another option. I would caution though the inrush will always be higher than max so a sure way to know is to actually flip the breaker at the MSP and compare it to the max. This obviously depends on those appliances etc you would normally use if they power on immediately or not. But to be on the safe side you want to measure both. At the end of the day the only thing that matters initially is to ensure the Inrush is <= PW(s) current.
You have the right approach, educate yourself and rely less on others to get it right. There is no substitute for facts at hand.
Good luck in your venture.
Certainly the inrush current is higher than the operating. We typically trust the LRA of the manufacturer spec, and don't make any assumptions about the Sure start reduction, or even the compressor compatibility. We also do not install the sure starts ourselves, for the reasons pointed out in this thread. If 29A x #of powerwalls is not larger than the LRA of the compressor we advise it wont start that AC, but it may with the addition of a Sure Start if its within 20-30%.
We have an HVAC shop come out and install any sure starts. We do not promise that any particular load will start after being bitten by a home elevator throwing a whole 2 Powerwall system into fits. We ended up fully refunding that customer, and removing the system. 4 months later they signed up again with 4 Powerwalls which does start the elevator. We applied for SGIP resiliency program funds, so most of it was totally paid for under medical exemption.
There is too much variability in a customers load profile over the year. The customers typically not understand these nuances either. Also equipment will draw more with age and temperature, so that's another factor out of our control.
It sounds like you are trying to have it both ways. You are making the assumption within 20-30% with a soft start device it "may" work, that is a fallacy for the reasons described. One, you don't even know if the compressor is compatible with a sure start, even if it falls within 20-30%. Second, you are assuming the sure start can cover the gap without knowing exact compressor efficiency or how well the sure start will optimize LRA. LRA also goes up with aged compressors, you are not testing any of this and relying on specs. With 28.5A x PW(s) just enough to cover the compressor LRA spec leaves no room for regular home load so I assume you are not covering just the LRA and have some idea of inrush/max regular non AC load is.
Details and numbers matter, relying on specs alone is how installers get in trouble. Onsite testing would mitigate all problems and avoid future headaches.
You certainly did the right thing by refunding the customer. I commend you for doing that. This could have all been avoided however. Sure start is a simple device to install, if your techs are competent and can read schematics it could save you some money in the long run but it certainly absolves you of liability if they make a bonehead mistake and fry something by using an HVAC company.
Your job is to gather the details for the current conditions with max realistic loads at the given time as I mentioned prior. This covers aging equipment and load variability.
Simply put onsite testing could have avoided the situation you had with the customer and their home elevator. I'm willing to bet the cost to completely remove the system was far more expensive than to do some preliminary onsite testing before you undersized them with 2 PW and finally ended up with 4 PW.
I agree most customers have no clear understanding of any of this, it is your job as an installer to have a candid conversation with them and provide an honest assessment of how many PW they need for worse and and best case scenarios. A general blanket statement most make only to realize they have an angry customer on their hand could easily be avoided. Just my .02 but I"m not in the solar business just an honest engineer.
Well, I have to say, I am no electrician, but: (1) the LRA on my Rheem 5 ton is 134. (2) It looks to me like three Powerwalls is, as someone said, 30 amps each, (3) 3 x 30 is 90, which is less than 134, but (4) the Tesla website clearly says, without beating around the bush or hedging at all, that you look at the type of compressor, it gives two pictures, and then it specifically states how many powerwalls are needed to start each type.
I better hope I have a "scroll" compressor, because if I have a "reciprocating" compressor I need 5 powerwalls.
I better hope I have a "scroll" compressor, because if I have a "reciprocating" compressor I need 5 powerwalls.
Not only am I not an electrician, but I am a lawyer, and the Tesla spec sheet must have been looked at by in-house counsel, and the representations on a less than 139 LRA scroll compressor don't leave much wiggle room. It says 2 powerwalls cover it. Unlike, of course, the reciprocating compressor, which appears to require 5 powerwalls for the same LRA number.
They could have easily said, "this is what we think will work, test it first" - that's only 11 words. They could also have said, "don't rely on how the compressor looks, etc...."
I fully believe its an issue, but it does appear to be Tesla's problem. Of course, getting a company to fix a high four figure problem is in itself a problem. Kind of like two problems, if you will.
Counsel, could not agree with you more
You are armed with this information now and probably know more than the average sales person or possibly even the PV architect. I would get everything they promise in your contract documented so there is no ambiguity of expectation, in case it ends up in court it's a simple contracts case. Also make sure your final payment is predicated on final testing not just install. Test everything first after install before making final payment. If they don't fix the issue like Vines company above than you have everything you need in your favor. Hopefully they correctly size the system based on your expectations from the start and don't under size and over promise using just "specs" without coming onsite to do some real world testing.
Good luck
You are armed with this information now and probably know more than the average sales person or possibly even the PV architect. I would get everything they promise in your contract documented so there is no ambiguity of expectation, in case it ends up in court it's a simple contracts case. Also make sure your final payment is predicated on final testing not just install. Test everything first after install before making final payment. If they don't fix the issue like Vines company above than you have everything you need in your favor. Hopefully they correctly size the system based on your expectations from the start and don't under size and over promise using just "specs" without coming onsite to do some real world testing.
Good luck
For what it’s worth, my installers seemed to take this issue seriously. I have four powerwalls, so it’s perhaps less of an issue, but I happened to be outside while they were discussing my heat pump compressor. They looked at the LRA number on the label and they even connected an ammeter to the compressor and watched as it cycled on, then based on those numbers and the fact I was getting four powerwalls they determined that I would not need a soft start unit. Additionally, when they had completed the install they verified that the compressor was able to successfully start while the house was running on the powerwalls.
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