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The Solar/Powerwall Checklist

Southpasfan

Member
Jun 2, 2019
461
587
Pasadena
We will see how long this gets. :) From these boards and my own experience through somewhere in permitting on Los Angeles:

1. What is your usage? Its easy enough to get a year's worth of bills together if you have them. You will need to convert dollars into Kilowatt hours (Kwh) used. A couple of things right off the bat. First, there is every possibility that you will need to know not only your total Kwh used, but when (as in time of day) you use. This impacts cost generally (see Net Metering, below) and Powerwall use specifically. Second, don't forget to increase estimated usage for an EV. Let's say your house uses 35kwh in a day. The Tesla Model 3 battery is 75 Kwh. Often, even if you already have an EV incentives have masked how many extra Kwh you use.

2. Do you want to cover all of that usage? Note that photovoltaic (PV) systems are rated in Kwh, but that number has to be converted. For example, my neighbor's "7.2 Kwh" system is producing an average of 40 kwh per day on sunny days. Also, the "7.2" Kwh is aspirational - a system virtually never reaches its maximum rating in any given hour. Both Tesla and a site named PVWatts will estimate yearly usage, however .......

3. What can your roof hold? Everyone's roof is different, and two identical systems will produce significantly different results in different locations and different orientations. I have a relatively flat roof, but you can see other layouts on these boards where the system is divided between several sections of roof, facing different directions. Any annoying factor at this point is some utilities actually limit the size of a system they will approve.

4. Do you want battery back up? If you are in a fire or storm zone, Powerwalls can run your house in an outage. If you are in a place where you normally don't lose power much, Powerwalls are more like a combination of insurance policy/green upgrade. Because powerwalls can charge when the sun is out and your usage is low, and then be discharged at night, they allow for some savings because many utilities charge a premium per Kwh in the evening, so Powerwalls can save money but typically not as much as they cost.

5. If you want battery back up, learn about sizing the powerwalls to the overall system. You want about one Powerwall per every 5kh of expected solar production, because that is what Powerwalls can accept. Allowing for the fact that there is loss. In my case I expect three powerwalls (15kwh max) to be well matched to a Tesla 16.32Kwh system.

6. So, far, so reasonably complicated. Next, it starts to get a bit ugly. If you want Powerwalls, how much of your house to you want them to back up? This requires acquiring a rudimentary knowledge of electronics. One super basic way of starting this is to realize each powerwall can put out about 30 amps. Amps are useful because many of us understand our breaker switches. However, it is also immediately apparent that 30 amps is nowhere near enough to run an entire house with any significant electrical appliance. The accepted rule is that three or more Powerwalls truly can power an entire house. Two powerwalls could be close. The reason this is critical is that there is significant electrical engineering involved in separating the "backed up loads" from the "non-backed up loads" in a partial home back up.

7. Next, there is no way of avoiding the math. Figure out the cost of your bill, then applicable rebates, including ones for powerwalls. Its pretty likely you will be saving money regardless, but because you can pay with cash or finance, there is some relatively complex math to do to understand what you are buying.

8. By this point you would have a good idea of what you are actually going to buy for the money. Comparing Tesla to other companies for equivalent systems is next up. Tesla compares well on price, but many say that Tesla is poor on customer service.

9. Get ready for the process. Because solar and battery systems require permits from a utility, and approval of plans, they take time. During that time, if you go with Tesla (or even if you don't) there is a lot of waiting around. Plus, you need to decide where to locate stuff (panels, conduit, powerwalls, extra breakers, etc). Location by itself is a significant design decision. Tesla generates a layout of the panels very quickly. Final approval of plans takes longer.
 
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gpez

Member
Apr 25, 2019
724
592
USA
This is a good start!

There's a lot to get in to regarding the different electricity pricing structures. My local utility is quite simple: $x per kwh up to 600 per month then $y per kwh after (where $y/kwh is marginally higher than $x/kwh). For me that makes the ROI change at that break point. Beyond that there is potentially ToU shifting, maximum daily use charges, and of course net metering (which you say is below but didn't see any more discussion). The Powerwall can help with those as it's not just a backup solution.
 

jrweiss98020

Tessa's Tesla
Jan 9, 2020
468
335
Edmonds, WA
6. can be a lot simpler if you just back up the entire house, then manually shut down anything you don't need when there is an outage. maybe there will be plenty of sun, and you want to keep your A/C and pool running; maybe it will be dark & rainy, and you need to keep the refrigerator & freezer going as long as possible...
 
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BrettS

Active Member
Mar 28, 2017
2,129
2,540
Orlando, FL
6. can be a lot simpler if you just back up the entire house, then manually shut down anything you don't need when there is an outage. maybe there will be plenty of sun, and you want to keep your A/C and pool running; maybe it will be dark & rainy, and you need to keep the refrigerator & freezer going as long as possible...

That’s true, but there can also be issues with this. If you are only getting a small number of powerwalls then you need to make sure that everything in your house can be supported by that small number of powerwalls. If your whole house would seriously overload the powerwalls, or you have appliances with very large power demands then you might not be able to do a whole house backup even if you plan to manually load shed.

You also need to consider the fact that manual load shedding is just that... manual. What happens if the power outage starts when you aren’t at home and you can’t turn off the pool pump and the A/C? Your powerwalls could have lost a lot of power or even be dead by the time you get back home to turn things off. Or if the power outage happens at 3AM are you going to want to wake up (or will you even wake up?) to turn off some appliances to conserve power?

It certainly can be done within reason, but there are definitely can be some advantages to just going with a partial home backup model, particularly with a smaller number of powerwalls.
 

gpez

Member
Apr 25, 2019
724
592
USA
What happens if the power outage starts when you aren’t at home and you can’t turn off the pool pump and the A/C? Your powerwalls could have lost a lot of power or even be dead by the time you get back home to turn things off. Or if the power outage happens at 3AM are you going to want to wake up (or will you even wake up?) to turn off some appliances to conserve power?

Home automation to the rescue :)
 
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DoyleS

Member
Jul 10, 2020
65
18
El Dorado Hills, Ca
Great post. I am in the waiting around period. I already have solar but 5 weeks ago I submitted the order for 2 Powerwalls. My screen says I have completed the home tssessment and it is checked. Next step is
Confirm Order: Coming Soon
We're preparing your payment details
And that is a black dot with no check mark.
I sent Tesla a request for update 3 days ago but have yet to hear back. So clearly in the waiting around period.
 

Southpasfan

Member
Jun 2, 2019
461
587
Pasadena
Each of the points in the initial post has at least one thread on the boards here. As for point 9, I would say that the specific thing to be ready for is that Tesla's "progress page" is nowhere near detailed enough. As far as I can tell, Point 9 is really more like this:

A. Give Tesla $100, and pick a system size. You upload a bunch of photos, but the main issue I believe (at this stage) is that Tesla looks at a satellite photo of the house and the photos of the main panel. You also send a utility bill, but that is probably most needed for utilities who limit the size of systems. It would obviously be needed if the customer was way off on what they thought they needed.
B. Tesla fairly quickly (in a day) sends a design back. Its a design only of the layout of the panels.
C. You can comment on this. Frankly, one can comment all along. In my case the layout was incorrect due to Tesla thinking that there was a vent pipe which was no longer there.
D. Tesla sends someone out to look over the house, including getting up on the roof. I would note that at this point physical problems with the actual roof begin to emerge (as in "your roof is too old - surprise, you need to replace it" or "sorry, those appear to be clay tiles.")
E. Based on that initial inspection, Tesla contacts the utility. You will probably not "see" this happen (it will be an email just to the untility).
F. In my case, the utility came out for a quick look. The main reason was to verify where the new meter would go. In my case, the guy, who is assigned to the project, did not seem to care about the panels, the conduits, or even where the powerwalls were going to go. This, for me, was about two weeks in.
G. A week after that, I could see that the guy from LADWP set up an official "project" Tesla had me sign a form interconnection agreement.
H. I believe Telsa submitted some actual "plans" at this point. I am not sure of that. So at the moment, this weeks follow up is to figure out who is waiting on whom. As a editorial note, this is a fair amount of work for $100.
I. There will be a permit issued.
J. In my case, there will be an extra charge to upgrade the main panel.
K. There will be an install date set.
L. The day of the install will be one more chance for something in the design to go wrong. Look up threads here for examples, such as changes to location of panels, or a decision on location of powerwalls.
M. You don't get to turn the system on immediately (you can run off grid tests. BrettS did so for weeks. :). Eventually you get permission to operate from the utility.
N. A month or so later you get an electric bill to see how your estimates of metering worked out.
 
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DoyleS

Member
Jul 10, 2020
65
18
El Dorado Hills, Ca
From my call today it looks like they are running about 6 weeks for the designer phase. So I am probably out another 2 weeks. Sounds like the fire and hurricane power outages spiked their orders. At least now I have the name of my dedicated point contact at Tesla who hopefully will update me and I can call direct. Was surprised this morning as I got through to someone with no wait time.
 

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