Have 48 panels, want to install (flat roof, SFH, SF CA)

[J.P.M.]

There are two heat pump issues that have been discussed. One is to replace the Water Heater and the other was a hydronic whole house heating using an additional larger HHPW that would only be used during winter..I agree before thinking about the heating issue more analysis would need to be done. The no brainer, at least from my perspective, would be to replace the water heater with a HPWH. I estimated that the OP will have significant excess production with a 11kW system even with some mostly foggy mornings. Using a conservative capacity factor an 11kW system might produce 10mWhrs of electricity or 900kWhrs per month. He mentioned that his electrcal usagecwas aboout 432 kWhrs per month.

The average 50 Gallon HPWH uses less than 100kWhrs per month based on EPA ratings. That would leave about 350 kWhrs excess generation even after installing a HPWH. The math gets even better with a TOU rate because the HPWH load can be shifted to a lower rate period increasing the financial payback.

Then do all that after getting some real usage numbers after doing all the more cost effective stuff of load reduction first that suits the situation and lifestyle choices.

HPWH may be more cost effective than some other methods, but I believe it's a disservice to this readership, who for the most part don't know a watt from a BTU, to suggest it is always or even sometimes the best alternative for residential heating water heating or HVAC applications.

As for EPA ratings, while better than peddlers promises, they are not much better if that's that only information used or relied on. A HPWH will draw as much power/month as it needs to meet the load.

As for estimates, PVWatts with input close to reality will be better than guesses for PV system output.

I'd respectfully suggest the OP get more information (self education0 about what PV capabilities might be for the application and the limitations as well as benefits vs. costs of HPWH before spending any money. We all have opinions. You like heat pumps. I'm less sold on the idea. People get screwed by not tempering and examining those and all other opinions with their own knowledge.

[Ampster]

Then do all that after getting some real usage numbers after doing all the more cost effective stuff of load reduction first that suits the situation and lifestyle choices.

Real usage numbers? The original poster said his usage was 432 kWhrs per month. I dont know how you can get any more real than that. I will always agree agree that conservation efforts should always be considered but long hot showers are a lifestyle issue that some prefer over cold showers.

HPWH may be more cost effective than some other methods, but I believe it's a disservice to this readership, who for the most part don't know a watt from a BTU, to suggest it is always or even sometimes the best alternative for residential heating water heating or HVAC applications.

Disservice to the readership? You may want to reread this thread. It was the original poster who said in his first post that he was considering changing out his gas water heater for electric. To be clear to this readership I was merely suggesting that he consider a heat pump water heater vs a resistive element water heater. It doesn't take a degree in engineering to understand that it is more efficient to move heat with a heat pump than create heat with a resistive element.

As for EPA ratings, while better than peddlers promises, they are not much better if that's that only information used or relied on. A HPWH will draw as much power/month as it needs to meet the load.

No doubt about that. How is that relevant to comparing a heat pump versus a resistive element? I agree with you that the EPA numbers need to be analyzed and that is why I did not use their estimates dollar savings figures. For example, the EPA assumes a cost per kWh of $0.12. The OP is in California which has an average cost of $0.20 so the comparative savings would be greater. Further analysis would only result in greater savings with more usage and less savings with less usage. Are you suggesting a Net Present Value analysis of a heat pump vs a resistive element water heater would provide a better answer?

We all have opinions. You like heat pumps. I'm less sold on the idea. People get screwed by not tempering and examining those and all other opinions with their own knowledge.

Yes the bottom line is that these are just opinions, nothing more. You have decades of thermodynamic engineering to bring to this forum. Context is important and the OP has a bunch of free solar panels and wants to utilize some of the excess production to heat domestic hot water. He will also be on a TOU rate plan which does enter into the equation. I own three HPWH and installed a fourth one in my sisters all electric home. I have been on various TOU rate plans for 9 years. I do have a strong opinion, based only on my experience, that a heat HPWH combined with solar and a TOU rate plan does offer many benefits. It all depends on where you are standing.

[chuckledog]

Thanks Ampster, J.P.M., Mike90250 and others for the helpful analyses and factors to consider.

We are still weeks away from making a decision on what size HPWH to buy, the decision that is now imminent is around the solar install. This is due to the need to re-roof: many roofers in our area are booked through the winter, so we need to pull the trigger on the roof pretty much this weekend.

Thanks (very much!) to the sage advice on this thread, we learned early that it's best to re-roof before installing the panels, primary reason being the chore of dismantling all the panels and racking to re-roof in say 3 years. However it's also pretty wise to put the stanchions / standoffs for the solar down during the re-roofing process. Just knowing that has likely saved us hundreds if not thousands of dollars. It has required a bit more up-front thought...

Here's a list of our questions as of two weeks ago, and the answers we've arrived at (I've been tracking these on a Google doc):

Q: How many stanchions / standoffs will be needed? A: Each row of portrait orientation panels will require two rails. Each rail requires one standoff at either end, plus one standoff every (this varies by rail type) 5 to 6 feet. So for our current design, about 70 standoffs.

Q: What panel tilt and orientation will be used? A: Contractors have quoted 0 degrees, 8 degrees, 15 degrees, and 30 degrees. All have quoted portrait orientation, matching our rafter orientation.

Q: Roofing material: is there a particular roofing material recommended? A: (not relevant to this forum)

Q: Will we REALLY get $0.0892 from our CCA for excess power generated? A: Yes, but their portion of the bill is less than half. TOU rates play a much bigger role. PG&E requires that we switch to a TOU rate once we start using net metering.

Q: How does TOU rate effect things like panel tilt? A: Optimizing panel tilt for TOU rates is somewhat in the weeds. South, southwest or west facing installations make the most sense, but since TOU rate schedules are prone to change, there's not too much to be gained by optimizing.

Q: San Francisco is foggy -- won't this kill power output? A: We will need to see. From posts on Nextdoor, this forum, and elsewhere, San Francisco is an excellent place to install solar panels, and the PVWatts estimates are not far from what people actually get.

Q: Our existing rafters are 2x6" dimensional from 1921, spaced 32" OC, running N-S, and spanning anywhere between 11' and 15'. Will this be sufficient to handle wind uplift for the panel tilt you are recommending? A: The answer has varied from each contractor. Some say "no problem", others advise caution. Being a responsible homeowner, I will add some reinforcement as soon as practical, especially on the 15' span. There may be a way to design 4x4 posts instead of standoffs at the time of re-roofing. We are proceeding with a sensible approach here.

Q: Should we just install half the panels? A: Putting up half the free panels will cost us around $3.25 per watt all told. Putting up the remaining 25 panels (we actually have 50, not 48, sorry for the error in the post title) will cost us around an additional $1.00 per watt, bringing the total to around $2.30 per watt. This has been a HUGE question for us since putting up only half the panels makes a LOT of sense. However, with the incentives drying up, and needing to do something with these panels, we've decided to go for it. The tipping point for us is around the water heater. There's a good argument that IF we are generating excess power, that we can use much of that excess power to heat our hot water and/or home, AND that doing so will likely happen at some sort of favorable rate. Plus, there's the global warming factor -- regardless of our own opinions on this, having more of a net zero home in our region will likely increase our home's resale value. So we're very likely going to go for it.

Q: Is it ok for the panels to be attached flat? A: Yes, this will decrease annual power generation by 14% relative to a 30 degree tilt, according to PVWatts. It will also result in a simpler system design with no shadowing to be concerned with, and an increased need to go up on the roof to wash debris off the panels especially around the edges. It will result in lower risk of wind damage in a hurricane. So yes, it's ok. Is it optimal? We'll let our contractor be the final judge of that.

Q: Should we buy a heat pump? A: Let's get this beast up on the roof first and see what it can do. Let's also see how the TOU and CCA rates actually shake out on our utility bills. Thanks again Ampster and J.P.M. for the valuable opinions here. There's a bunch more research to do in the area of radiant heating, hot water, plumbing, heat pumps, BTUs vs. watts, heat loss calculators, etc. That will take time to do a proper job. (Current bookmark for my research in this area: http://www.greenbuildingtalk.com/For...c/Default.aspx )

Here's our current proposed design:

Rooftop diagram 2.png
I've heard from contractors, PG&E reps and others that I've done my homework. That's in large part thanks to the help on this forum. Please let me know if there's other information / learnings I can share. Definitely still lots to learn.

[Ampster]

Q: How many stanchions / standoffs will be needed? A: Each row of portrait orientation panels will require two rails. Each rail requires one standoff at either end, plus one standoff every (this varies by rail type) 5 to 6 feet. So for our current design, about 70 standoffs.[

Q: What panel tilt and orientation will be used? A: Contractors have quoted 0 degrees, 8 degrees, 15 degrees, and 30 degrees. All have quoted portrait orientation, matching our rafter orientation.

Q: Roofing material: is there a particular roofing material recommended? A: (not relevant to this forum)

Q: Will we REALLY get $0.0892 from our CCA for excess power generated? A: Yes, but their portion of the bill is less than half. TOU rates play a much bigger role. PG&E requires that we switch to a TOU rate once we start using net metering.

Q: How does TOU rate effect things like panel tilt? A: Optimizing panel tilt for TOU rates is somewhat in the weeds. South, southwest or west facing installations make the most sense, but since TOU rate schedules are prone to change, there's not too much to be gained by optimizing.

Q: San Francisco is foggy -- won't this kill power output? A: We will need to see. From posts on Nextdoor, this forum, and elsewhere, San Francisco is an excellent place to install solar panels, and the PVWatts estimates are not far from what people actually get.

Q: Our existing rafters are 2x6" dimensional from 1921, spaced 32" OC, running N-S, and spanning anywhere between 11' and 15'. Will this be sufficient to handle wind uplift for the panel tilt you are recommending? A: The answer has varied from each contractor. Some say "no problem", others advise caution. Being a responsible homeowner, I will add some reinforcement as soon as practical, especially on the 15' span. There may be a way to design 4x4 posts instead of standoffs at the time of re-roofing. We are proceeding with a sensible approach here.

Q: Should we just install half the panels? A: Putting up half the free panels will cost us around $3.25 per watt all told. Putting up the remaining 25 panels (we actually have 50, not 48, sorry for the error in the post title) will cost us around an additional $1.00 per watt, bringing the total to around $2.30 per watt. This has been a HUGE question for us since putting up only half the panels makes a LOT of sense. However, with the incentives drying up, and needing to do something with these panels, we've decided to go for it. The tipping point for us is around the water heater. There's a good argument that IF we are generating excess power, that we can use much of that excess power to heat our hot water and/or home, AND that doing so will likely happen at some sort of favorable rate. Plus, there's the global warming factor -- regardless of our own opinions on this, having more of a net zero home in our region will likely increase our home's resale value. So we're very likely going to go for it.

Q: Is it ok for the panels to be attached flat? A: Yes, this will decrease annual power generation by 14% relative to a 30 degree tilt, according to PVWatts. It will also result in a simpler system design with no shadowing to be concerned with, and an increased need to go up on the roof to wash debris off the panels especially around the edges. It will result in lower risk of wind damage in a hurricane. So yes, it's ok. Is it optimal? We'll let our contractor be the final judge of that.

Q: Should we buy a heat pump? A: Let's get this beast up on the roof first and see what it can do. Let's also s inee how the TOU and CCA rates actually shake out on our utility bills. Thanks again Ampster and J.P.M. for the valuable opinions here. There's a bunch more research to do in the area of radiant heating, hot water, plumbing, heat pumps, BTUs vs. watts, heat loss calculators, etc. That will take time to do a proper job. (Current bookmark for my research in this area: http://www.greenbuildingtalk.com/For...c/Default.aspx )

.

I assume the top of the diagram is North but if not please annotate.I do understand the rafters run NS but they are not noted in your drawing but I assume the are the dark lines running from top to bottom. The rail direction may be a function of the panel layout and and tilt. Engineering may need to be resolved before any quantity bill of materials for rails and stanchionns can be developed.

I will try to answer your questions in the priority becauseI see the most important thing is locking in an installation date for the roof. .I am a big Fan of TPO membrane roof. You can generally get as much as a 25 year life and it has the benefit of not weighing as much which goes to the next issue which is the engineering. Depending on the strength of the roof you may need to tear off the old roofing material if the existing roof construction will not support adding more weight.
Your building authority is going to want to see stamped engineering drawings. I am not an engineer but am somewhat familiar with lumber span tables and 2x6's on 32" centers spanning 11 to 15 feet looks like it might be a little bouncy. It may depend on the sheeting and if any benefit from the sheeting and rafters workiing together as a diaphram. I don't know if an engineer would give 100 year full size 2x6's any benefit since they are probably free of knots and much tighter grain than todays skimpy 2x6s. If there is a layer of plywood that would contribute to the strength of the roof and its ability to support the weight of the panels and the roofing. The other calculation will be the uplift loads and that will be a function of the tilt of the panels and it will drive what size rail to use and how closely spaced the stanchions need to be. I unfortunately have not answered most of your questions because they are engineering issues that will drive the design. The simplest installation from a structural standpoint is horizontal panels. You will have to find out if the 14% efficiency gain is worth the cost of any structural improvements if any that may need to be made.

If it were my house and I found out that I had to tear off the old roof and I planned to live there more than 20 years I would bite the bullet and spend the money on making sure my roof could support what ever loads I would foreseeably want to put up there. I would add insulation and move vents and other protrusions so that I could maximize the coverage of the roof bv solar panels at optimum tilt. Also I would reserve an area for future mechanical equipment. If I found I was space constrained versus my expected loads I would sell or get rid of my free panels and get some higher density ones that would generate more Watts per square foot. The price of $3.25 per Watt for installing your panels isn't that far from the cost of an installation of entirely new panels that would also give you the 30% tax credit. New panels are below $1.00 a Watt.

I think there is still some confusion about how the CCA works. I will try to answer that in another post. That answer may only serve to inform the decision about how many kWs of panels to install. The engineering issue is the critical issue that will dictate most of your choices.

[chuckledog]

Confirmed, the top of the diagram is north and the rafters run N-S. Structurally the roof has been deemed fine to add the dead load of the new panels and racking (only 3 lbs/sq ft, less than the tar and gravel roof we will be tearing off). The wind uplift (live load) is the concern and we will have to deal with this regardless. We are working with contractors who have experience installing on very similar roofs (all these houses were built in 1915-1925 when San Francisco development rapidly pushed west, in the wake of the 1906 quake and fires).

Point taken about moving vents. One GC we talked to was saying the vents might pop up on the roof anywhere, however after reading up on venting it appears possible to tie into existing vents, meaning we don't have to leave giant gaps between panels "just in case".

What future mechanical equipment would you envision?

Definitely still confused about the CCA billing, are there any posts that might explain that further?

[Ampster]

Confirmed, the top of the diagram is north and the rafters run N-S. Structurally the roof has been deemed fine to add the dead load of the new panels and racking (only 3 lbs/sq ft, less than the tar and gravel roof we will be tearing off). The wind uplift (live load) is the concern and we will have to deal with this regardless. We are working with contractors who have experience installing on very similar roofs (all these houses were built in 1915-1925 when San Francisco development rapidly pushed west, in the wake of the 1906 quake and fires).

Great news. If you are doing a full tearoff that will save some weight. Maybe a previous owner put plywood downthat gave additional strength to the old diagonal 1x6 sheetng. One of the flat roof I mentioned earlier put a layer of high density foam down under the roofing material which will give you some benefit in terms of heat retention..

Point taken about moving vents. One GC we talked to was saying the vents might pop up on the roof anywhere, however after reading up on venting it appears possible to tie into existing vents, meaning we don't have to leave giant gaps between panels "just in case".

Speaking of vents, if the vent in the lower middle of the diagram is for the gas water heater it looks like you could put 5 or 6 panels in that space if you went with a HPWH. Clearly I will use any excuse to pitch HPWH because my experience has been great. I have had less experience with hydronic systems but in both cases they were gas fired. There are extremely efficient gas fired modulating boilers that accommodate zoning which allows you to heat just the space you are occupying. If I were to build a house today where I live in Sonoma I would go with electric heat pump split units like bcroe describes. You live in a city that is air conditioned by nature but I am north in Sonoma and need a little A/C from time to time. I have forced air and I did consider retrofitting it for a heat pump unit and abandoning the gas part of the FAU but it wasn't worth it at the time. Long term it may be a project, especially if my A/C compressor goes out.

What future mechanical equipment would you envision?

I was thinking of the compressor part of a split unit.

Definitely still confused about the CCA billing, are there any posts that might explain that further?

i would start with becoming familiar with the TOU schedules and figure out which TOU rate schedules might be best for you. For simplicity just focus on those the TOU periods of Peak and Off Peak.
The fundamental concept of working the TOU rates is to sell high and buy low. I try to move as much consumption to the Off Peak rates so that the majority of my generation gets credited at Peak rates. The difference can be as much as three times. For example I get credited in the summer at $0.45 per kwh and charge my car and heat my water at $0.15 per kWh. As far as the future is concerned there has been a shift in the Peak Period. For example the Peak period has shifted as late as 3 to 9PM whereas previously it started at 11AM. The result is that less of the Peak Period is available during times when solar is producing. The mornings are becoming Off Peak so that generation is being credited at lower rates. I am grandfathered for another year but what that means is there is a benefit to building a larger system as a hedge against this erosion of the effective rates during optimum solar production times.

SF Clean Power is just starting so I don't know the details of their true up payout. When I previously lived in Southern California, I was involved in the startup of the Los Angeles County CCA and there are clear benefits to being in a CCA. The benefits are not significant enough to drive any planning decisions. I may have mentioned that Sonoma Clean Power subsidized the cost of a charging station and had reviously provided rebates for the purchase of a Nissan Leaf and another car that I can't remember. ( I already owned a Tesla so I didnt take advantage of the car rebates.)

Sounds like you are off to a good start.

[chuckledog]

I'm working on a spreadsheet that takes as inputs:
- PVWatts Hourly output (watts generated, and ambient outdoor temperature)
- PG&E's current TOU rates (EV-B and E-TOU-A, hourly including peak, off-peak and shoulder)
- a heat pump's energy consumption and BTU output (varies by ambient temperature)
- a home's heating BTU needs

and outputs:
- Total kW consumed annually
- Total annual $ bill

It's functional, but likely wrong in some calculations.

if anyone is interested to help me develop this spreadsheet, please PM me (are PM's possible on this forum)?

[mutantblack]

There's no PM on this forum.

[Ampster]

if anyone is interested to help me develop this spreadsheet, please PM me (are PM's possible on this forum)?

There is a guy on the Tesla Motors Club forum that has done that. His handle is Miimura and he often posts on the Tesla Energy subforum..He lives on the Penisula and is in one of the CCAs on the Peninsula as well. He is quite knowledgeable about PG&E rates and his CCA.
EDIT
Here is a link to a discussion of rate plans but it give some insight into other rates.

https://teslamotorsclub.com/tmc/threads/pg-e-rate-schedules-home-charging-ev2-a-goes-live-vs-others.158153/page-2#post-3824463

I will keep looking for the post where Miimura posts his spreadsheet.

[Mike90250]

Links, relative to helping someone who asked , are fine.

[chuckledog]

Thanks Ampster. Super helpful, lots of good info to parse there about PG&E TOU rates + CCAs.

[Ampster]

You are welcome. That site is useful for California issues because there are a lot of California users there. I have learned a lot about overall issues that affect California public policy by reading that forum. My comments about overpaneling are a result of insights gained there into the future of energy pricing in California.

[chuckledog]

So -- we pulled the trigger and decided to go with a one-stop-shop company that will do both the re-roof and the solar (with our panels). The people there have been good to work with, and they've come up with what looks to me like a great plan for layout.

They just sent over plans and I've posted them here in case someone might want to take a look and comment. We're still several weeks out from the install.

My main comment on these plans was to ask that they put the standoffs under the new roof, not over it, they were willing (I still need to ask them exactly how they plan to do the flashing, I'm not familiar with Q base mounts). I really like the layout they chose, it's much more functional than the ones I'd come up with, routing around our various roof obstructions and providing clear safe access for equipment maintenance (as well as firefighters).

[Ampster]

It looks like a good plan. How long is the roof guaranteed for? Did you get a service panel upgrade or can your existing service panel handle that large inverter?
I did notice a reference fo an Envoy monitoring system which is an Enphase product but the Inverter is Solaredge which also has a monitoring system built into the inverter.

[chuckledog]

Thanks for taking a look! Yes we're getting a service panel upgrade. We'll go from 100A up to 200A service (with a 225A bus for solar).

I'm pretty sure the Envoy is a copy-paste issue, I'll have to ask them how the SolarEdge monitoring will be hooked up (from what I gather, Ethernet is the way to go if possible).

The roof labor is guaranteed for 10 years, as is the solar labor and the inverter/optimizers.