Solar Panels + Powerwall

[davidwpb]

I have learned so much in the past few hours reading through some recent posts. I appreciate all of you who provide insight and feedback. Even more so, I appreciate those that take the time to read through my crazy thoughts below and try to answer my questions.

I am hoping to get a 4 kW panel system and 2 or three Powerwalls from Tesla ASAP. Tesla did the site visit last week, but were unable to measure my roof so they are coming back next week to finish up prior to finalizing quote. I thought I had more control over how the system would work, but from reading posts, it seems there is a standard way for the system to be installed and run. The reason I am doing such a small solar job is that Tesla will not install on my barn and there is limited south facing roof surface area on my house, so I wanted to avoid paying to put solar on east and west facing roof that either get shade or require removal of 20 year old oaks. I figured I could have the system designed for additional solar (up to 8kW) to be installed later myself (maybe use the tax credit money to pay for the additional panels) with minimal permitting and design since it would just tie to existing system. The purpose of the Powerwalls is for back-up power for hurricanes. I want to keep the house running as close to normal as possible (no dryer or range needed but everything else to run) for comfort. I have a 200 amp main breaker on my main panel with all 40 circuits in use with 625 amps worth of breakers (150 of which were added after original house permit without permits) and two sub panels fed from the main panel located 20 plus feet from the main panel. I think I need 6-8kW solar system and 2-3 Powerwalls to meet my needs, but I worry Tesla will push me to too many (due to house electrical loads) or too few Powerwalls (due to solar system size they sell me). I typically use 50 to 80kW/day, but could get by with 30kw/day in backup power mode to keep quantity of Powerwalls low. Power here costs $0.11/kW up to 1,000 kW/mo and $0.13/kW over 1,000 kW/mo (same rate day and night). My daytime (sun hours) energy usage is as low as 1-3kW/hr. I'm concerned I can't upsize solar system too much or I wont be able to use it all (if no net metering) but if I go too small I cant generate enough energy to run my house when the power is out for days.

I have lots of questions:
1. If I am doing Powerwalls and live where net-metering is available, does Tesla do net metering or do they skip that and just deplete the power stored? Probably tied to the same question, is gateway in lieu of the net metering setup?
2. Does my solar system size of 6-8kW and Powerwall quantity of 2-3 make sense?
3. Will Tesla design for future solar expansion? Are there any considerations I should take into account, like inverter size?
4. Does Tesla provide separate inverter with the solar panels or use an inverter built into the Powerwall?
5. Does anyone have a Tesla created electrical plans for a similar setup they can share with me?
6. Does it seem I might need a bunch of extra panels due to my setup? From reading the posts about generation panels, it seems a reduction in main panel disconnect. I don't think I can reduce main breaker knowing we may want to add traditional load in the future (2nd electric car and pool equipment), but re-running wires seems like it could get time consuming and costly.
7. What happens if the load during outage exceeds the Powerwall max output, does it just shut-off power to everything?
8. Is there a conversion for kW used in one hour period to a max load (double?), my high usage rate per hour seems to be 6kW/hr.
9. Do I need to let Tesla know there are several circuits added that were not permitted or is that just handled on the electrical inspection? I can share the permitted plans from when the house was built.
10. How crazy am I for trying to customize my system to my situation? Should I just wait for Tesla to tell me what I should do?

 

[L-P-G]

1. Batteries fill first, then excess is sold back to the grid. the gateway is required for the PW to work.
2. What's you average daily generation (after what you consume is taken out)
3. Seems a bit small, go to pvwatts.com enter your address and see how much it'll produce in a day. Ideally you want to cover your usage plus the refill of the batteries in the daylight hours.
4. Separate. They AC couple the PW so you can produce when the grid is down by telling the GW to disconnect you from the grid
5. PM me
6. Why not hire another solar company that WILL do the install on the farm and tie it to that panel so you don't have to de-rate your main. Unless your farm is tied from your house meter
7. Yep. you have to cycle the on/off button to turn it back on
8. Can you word this differently? not really understanding your question
9. They handle it, during the survey they'll ask you to take pictures of your panels
10. You're not.

 

[miimura]

1. Net metering is dependent on your utility and their metering hardware and policies. This has nothing to do with Tesla's hardware or any other company's solar hardware.
2. 6-8kW sounds like a minimum recommended size for your stated requirements. 10kW would probably be better if you had roof area to install it.
3. No, I don't think anybody will design for future expansion. You should do it right the first time, especially since the 30% tax credit is still in effect and will decrease starting Jan 1.
4. Solar and Powerwall inverters are separate and are AC coupled through a breaker panel.
5. I don't.
6. Since you have loads that were added after the fact, without permits, you may need significant panel upgrades to bring things up to code and add the solar and powerwalls.
7. Yes. Overload causes shutdown. I have no first hand knowledge about restart procedure.
8. kW is power, kWh is energy. You max load would be expressed in power like 15kW or current like 62.5A. If you interval meter reads 10kWh for a specific one hour interval, the average load is 10kW. However, the peak load could be much higher.
9. They will just look at what is there and figure out how to permit everything together.
10. You should always get a system that meets your needs if you can afford it. However, many people don't fully understand and communicate their needs.

 

[dotbombjoe]

It doesn't seem like a 4kW system is going to produce your low-end target of 30kWh/day. I'm in SoCal and my 6.175kW system doesn't produce 30kWh/day all year round...we've had some bad weather this week for example, and I've only exceeded 30kWh on 3 days...2 were less than half that...

Agree with previous poster re: checking out pvoutput.org and putting in some numbers or even just looking up some neighbors solar systems to get a very good sense of what you can expect.

What time of year do you expect outages to be most common? How long do they typically last? Do you know if you often have cloudy weather leading up to or after the storm?

I just don't know if an undersized solar system feeding batteries is your best bet. There was a recent discussion about this in another thread: PSA: Tesla solar panels $1.70-$1.99/watt

In short, I suspect you'd save a lot of up-front money and have a more reliable system if you went with a whole home, natural gas generator instead. It would likely be more maintenance and louder, but I think it may meet your need better unless you go with the other recommendations above re: putting a lot more solar up. Given your electricity costs though, it seems like a lot of $ and a long pay-back time.

Don't get me wrong...I love solar+batteries, they are just hard to justify unless your utility rates are exorbitant like they are out here in CA.

 

[davidwpb]

1. Batteries fill first, then excess is sold back to the grid. the gateway is required for the PW to work.
2. What's you average daily generation (after what you consume is taken out)
3. Seems a bit small, go to pvwatts.com enter your address and see how much it'll produce in a day. Ideally you want to cover your usage plus the refill of the batteries in the daylight hours.
4. Separate. They AC couple the PW so you can produce when the grid is down by telling the GW to disconnect you from the grid
5. PM me
6. Why not hire another solar company that WILL do the install on the farm and tie it to that panel so you don't have to de-rate your main. Unless your farm is tied from your house meter
7. Yep. you have to cycle the on/off button to turn it back on
8. Can you word this differently? not really understanding your question
9. They handle it, during the survey they'll ask you to take pictures of your panels
10. You're not.

2. 17 kW/day average per the tesla website, PVwatts and other solar installers indicate 20kW/day for 4kW system
3. confused. why do i need to produce more than I use in a day?
6. I'm open to that, its a metal structure with a corrugated metal roof. I know from experience asking for quotes that solar installers shy away from anything that isnt a typical wood truss residential roof. I guess I need to keep asking. Its all on the same meter though,
8. I know my hourly kW usage from the power company website. I was asking if there is a rule of thumb for determining the max kW at any given time withing the hour. 2x, 3x?

 

[L-P-G]

2. 17 kW/day average per the tesla website, PVwatts and other solar installers indicate 20kW/day for 4kW system
3. confused. why do i need to produce more than I use in a day?
6. I'm open to that, its a metal structure with a corrugated metal roof. I know from experience asking for quotes that solar installers shy away from anything that isnt a typical wood truss residential roof. I guess I need to keep asking. Its all on the same meter though,
8. I know my hourly kW usage from the power company website. I was asking if there is a rule of thumb for determining the max kW at any given time withing the hour. 2x, 3x?

2. Add up 12 months of power bills, then divide by 12. This will give you your average monthly power usage (summer vs winter has different values so in the winter when your AC runs less it'll offset the over use for the summer). This will give you the kwh you need to produce per month in order to fully eliminate your power bill. If you produce less, you'll have to pay the power company.

3. You ideally want to produce enough power with your solar to account for your usage plus filling the PWs (unless the PWs have enough capacity to cover all your night time use, at which point solar will mainly be used to fill your PWs but the end will be the same result). If you don't then you'll have to use the grid for your loads and your PWs won't fully charge for that day (PW can only charge from solar, they won't charge from the grid).

8. No, for this you'd need a whole house energy monitor to find out what is using that hourly power. I could charge my car for 30 min and that would be 15kW or I could use the water heater and the dryer for an hour and also add up to 15kW.

 

[NuShrike]

2. 17 kW/day average per the tesla website, PVwatts and other solar installers indicate 20kW/day for 4kW system
3. confused. why do i need to produce more than I use in a day?
6. I'm open to that, its a metal structure with a corrugated metal roof. I know from experience asking for quotes that solar installers shy away from anything that isnt a typical wood truss residential roof. I guess I need to keep asking. Its all on the same meter though,
8. I know my hourly kW usage from the power company website. I was asking if there is a rule of thumb for determining the max kW at any given time withing the hour. 2x, 3x?

Your numbers is the maximum under ideal conditions.

My experience is it's more accurate to multiply by 1/3 to 1/2 for average conditions. That's 5.6 kWh under adverse conditions in a day. If you can live with that, then everything after that is icing.

 

[SoundDaTrumpet]

Welcome to the balancing act. It is good you are assessing needs vs. wants. Establish what you want, and Tesla will design you a system based upon what you want. With quite a bit of patience, you can remove customer requirements with cost as a factor.

3. Will Tesla design for future solar expansion? Are there any considerations I should take into account, like inverter size? Generally no. The design heavily based on photographic evidence. Put the loads in first, then contact Tesla. I would even hint to drop in unconnected circuit breakers of future loads. (yeah this is a hack)

5. Does anyone have a Tesla created electrical plans for a similar setup they can share with me? Generally no, there is a confidentiality statement printed on the drawings. I suggest sketching something out. I do not advocate copy and pasting designs to this forum.

6. Does it seem I might need a bunch of extra panels due to my setup? From reading the posts about generation panels, it seems a reduction in main panel disconnect? The reduction in main disconnect is ONLY ONE way to do it, albeit, the easiest way. Adding subpanels with sum of breakers less than the bus bar is one approach, and Tesla said they do this when the main disconnect reduction limits the design. The almost never talked about one, is a main lug only (MLO) meter panel with a brand that has huge 200A branch breakers. (I am on a MLO meter with 125A max size breakers for PW system, and did zero panel service upgrades to get here). The aforementioned service panel idea definitely is NOT current practice, but sure avoids a service upgrade to 320/400A. If you have aerial/overhead service, this is cheap towards 320/400A service. Underground (fully explored in my case) was money pit, so I bailed out. Even the 400A panel discussion splits ways with dual 200 buses (which isn't much better for back-feed, but good for unbacked up loads) vs. a single 400A bus.

9. Do I need to let Tesla know there are several circuits added that were not permitted or is that just handled on the electrical inspection? Go to the city and pull those permits yourself. Doubt Tesla will permit existing not permitted loads. Tesla didn't even want to plug in a Ethernet cable into the existing inverter. Tesla will relocate existing loads... they don't check unless it's obvious like a 12ga wire connected to a 50A breaker.

10. How crazy am I for trying to customize my system to my situation? Should I just wait for Tesla to tell me what I should do?
In my case they spit out a boiler plate design when some engineer hit a brick wall. It took months to remedy until I was given the Design Hotline number and spoke to highly knowledgeable engineers who were one office area away from the designers.

 

[davidwpb]

Thanks everyone. Not sure I understand all the responses yet, but I'm trying to get there. PV Watts indicated I would get 12,000 kW per year with south facing panels and 10,000 kW per year with east or west facing panels. I think it might be best to install the right size system now as some have suggested. I think I found a way to make it almost optimal and a back-up plan if that doesn't work. Below are some pics of my roof and trees if anyone wants to give feedback on my solar panel design. I'll probably hold off on posting again until i get my design and have my call with the Tesla rep late next week.
Option 1 - Let Tesla install (assuming they agree with my layout) 8kW system on south, east and west facing roofs as high as possible to avoid shade from oak trees in SE and SW corners of house. This gives a 15% output reduction vs all south facing.
Option 2 - Hold off until I can build a covered patio with south facing roof and install all 8kW south facing. This may delay solar install by 1 year or longer. Also, my wife hate the roof design since the house and patio are sloping toward eachother.

Please note the trees south of barn have been removed (original solar panel preferred location). Any tips on trimming trees for solar, maybe just trim the top of the tree?

 

[L-P-G]

Trim the trees. If you go that high you violate IFC and won't pass code. You have to have setbacks on the roof for firefighter access. Also after this year federal tax break goes down from 30% to 22%

 

[Dan123]

Thanks everyone. Not sure I understand all the responses yet, but I'm trying to get there. PV Watts indicated I would get 12,000 kW per year with south facing panels and 10,000 kW per year with east or west facing panels. I think it might be best to install the right size system now as some have suggested. I think I found a way to make it almost optimal and a back-up plan if that doesn't work. Below are some pics of my roof and trees if anyone wants to give feedback on my solar panel design. I'll probably hold off on posting again until i get my design and have my call with the Tesla rep late next week.
Option 1 - Let Tesla install (assuming they agree with my layout) 8kW system on south, east and west facing roofs as high as possible to avoid shade from oak trees in SE and SW corners of house. This gives a 15% output reduction vs all south facing.
Option 2 - Hold off until I can build a covered patio with south facing roof and install all 8kW south facing. This may delay solar install by 1 year or longer. Also, my wife hate the roof design since the house and patio are sloping toward eachother.

Please note the trees south of barn have been removed (original solar panel preferred location). Any tips on trimming trees for solar, maybe just trim the top of the tree?
View attachment 411544 View attachment 411548 View attachment 411552

It doesn't seem that you have too much shade. I would just install as much as you can on the south side, and then the rest on east/west sides.

 

[SoundDaTrumpet]

I would pitch it to Tesla at this point. They may need to actually use a tape measure. They will know Florida's current roof setback rules. If the rules are like California, one requirement is the driveway/street side needs at least one passageway to the top ridge. This will cut into the south array. This is okay as rules have relaxed in the past year or two in California and are more practical to allow the West and East to be covered up completely. I did find at one point the official guidance on this on-line.

Don't be surprised if Tesla puts panels on the north as you have more space and live in a more southern latitude.

 

[Dan123]

I would pitch it to Tesla at this point. They may need to actually use a tape measure. They will know Florida's current roof setback rules. If the rules are like California, one requirement is the driveway/street side needs at least one passageway to the top ridge. This will cut into the south array. This is okay as rules have relaxed in the past year or two in California and are more practical to allow the West and East to be covered up completely. I did find at one point the official guidance on this on-line.

Don't be surprised if Tesla puts panels on the north as you have more space and live in a more southern latitude.

I don't think Florida has any setback rules. Anything goes here.

 

[L-P-G]

I don't think Florida has any setback rules. Anything goes here.

Maybe in Miami, but in CFL we definitely have to follow IFC. Best bet is to call your county building office ask them.

 

[davidwpb]

Trim the trees. If you go that high you violate IFC and won't pass code. You have to have setbacks on the roof for firefighter access. Also after this year federal tax break goes down from 30% to 22%

• 2016 – 2019: The tax credit remains at 30 percent of the cost of the system.
• 2020: Owners of new residential and commercial solar can deduct 26 percent of the cost of the system from their taxes.
• 2021: Owners of new residential and commercial solar can deduct 22 percent of the cost of the system from their taxes.
• 2022 onwards: Owners of new commercial solar energy systems can deduct 10 percent of the cost of the system from their taxes. There is no federal credit for residential solar energy systems.

 

[CSFTN]

1. Batteries fill first, then excess is sold back to the grid. the gateway is required for the PW to work.
2. What's you average daily generation (after what you consume is taken out)
3. Seems a bit small, go to pvwatts.com enter your address and see how much it'll produce in a day. Ideally you want to cover your usage plus the refill of the batteries in the daylight hours.
4. Separate. They AC couple the PW so you can produce when the grid is down by telling the GW to disconnect you from the grid
5. PM me
6. Why not hire another solar company that WILL do the install on the farm and tie it to that panel so you don't have to de-rate your main. Unless your farm is tied from your house meter
7. Yep. you have to cycle the on/off button to turn it back on
8. Can you word this differently? not really understanding your question
9. They handle it, during the survey they'll ask you to take pictures of your panels
10. You're not.

pvwatts.com brings you to a virus-laden site. "Click here to install updated version of Flash" Did you mean PVWatts Calculator ?

 

[davidwpb]

pvwatts.com brings you to a virus-laden site. "Click here to install updated version of Flash" Did you mean PVWatts Calculator ?

I went here, but thanks for the clarification. PVWatts Calculator