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Considering going with backup only vs self powered

What are you growing down in Florida? Here in California, that load would cost you $1000/month from the grid

Its about 1/2 that in FL. when it is hot, it is hot. In Central FL we only get about 2-5 weeks of "cool" weather where the AC isn't coming on. and on some of those days it can be in the 40's at night and in the 80's during the day.
 
I went back and forth on three vs four powerwalls for a while, but after the tax incentives and Tesla’s discount for getting solar and powerwall at once it’s only $3400 to add the fourth. I’m actually even considering adding a fifth as they only want $1500 to go from four to five. I’d hate to go with less now and then have to pay considerably more to add another one or two later if I discover that I do want more power.

BrettS:
Sorry for being a bit off topic but I am also getting a 4 power wall system and I called Tesla to see how much adding a 5th power wall would be and they said it would be at least $6,500 more to add another power wall. Was that 1500 through them or am I missing something?
 

BrettS

Active Member
Mar 28, 2017
2,155
2,575
Orlando, FL
BrettS:
Sorry for being a bit off topic but I am also getting a 4 power wall system and I called Tesla to see how much adding a 5th power wall would be and they said it would be at least $6,500 more to add another power wall. Was that 1500 through them or am I missing something?

So I’ve got to admit that some of that price was just speculation on my part. I didn’t get an official quote from Tesla for the 5th powerwall, but what I was basing it on was the pricing details shown when you look at solar roofs at Tesla.com. By playing around with the home size and power bill amount you can get it to recommend different numbers of powerwalls and the following are the prices I was seeing for 3, 4, and 5 powerwalls:

3: $12950
8CA3BB13-68C6-4AAA-8378-4DBCDBA9EFC2.jpeg

4: $16354
ADF86CB4-FA59-4C18-BB69-70E462687E0E.jpeg

5: $17834

8728017C-FFD3-4DF0-BA68-C2C263038612.jpeg


Since the prices for 3 and 4 powerwalls did match what I was quoted from Tesla I speculated that adding a 5th would also be the same price. But I haven’t yet spoken to them about doing so since I’m still on the fence about it.
 
6500 is the retail price of the powerwalls, but obviously they are running a promotion of some sort...

i'm still waiting on what 6 & 8 powerwalls would be on a extra large system with 8 extra panels. 24 panels south exposure, and 24 on east/west exposure and i want to overdrive the east/west which is on 38 degree pitch. 8*315*cos(38)=1986 compared to 315*6=1890
 
I'n in Brett's situation, with PW's and 100% net-metering credit, in hurricane-prone south Louisiana. His initial post above asks all the right questions. I've alternated between backup-only and self-powered. One benefit of self-powered would be that "Storm Watch" will pump the batteries above 100% when it's activated, according to posts on this forum -- that will be a nice feature in an outage. I don't think Backup Only does that ... but it really should.

Apart from that, I wonder how much battery life you lose by having the batteries on Backup Only. If they're always at what Tesla calls 100% (which is something shy of true 100% capacity), it seems you'd lose some battery life. Lithium batteries are supposed to have longer lives if they spend more time in the 40-80% range, which suggests you'd want them to come down from the 100% high on a regular basis. In particular, lithium batteries aren't supposed to last as long when kept in a high state of charge combined with high temperatures, which would be the case for me and Brett for half the year, from about May until October.

So if Brett and I keep out batteries in Backup Only to avoid the lost 10% of running on battery power, are we shortening our batteries' lives in a way that will cost us more in the long run?
 
I just signed the documents for my solar/powerwall install yesterday, so this is probably a bit premature, but I’ve been spending some time looking at powerwall documentation and configuration options to try to see what makes sense for me. This wound up being pretty long, so I put a tl;dr at the bottom.

First, here is a bit of background -

-My average power usage over the last year ranged from about 70-75kWh in the winter to 105-110kWh in the summer. Over the entire year of 2019 I used just over 33,000 kWh. Much of the high power demand is due to car charging. I have a model S and I typically average 2000+ miles a month.

-The system I’m putting in is Tesla’s XL size with 15.12kW of solar panels and 4 powerwalls. Tesla estimates that it will generate about 20,000 kWh annually, so it should meet about 2/3’s of my power demand. This system will pretty much fill my Southern and Eastern facing roof ares and nearly fill my Western facing areas, so even if I wanted to go bigger I really don’t have the roof space for it. Additionally, I don’t know that I will continue driving this much in the future, so if that happens I don’t want to wind up with a system that is way oversized for my needs.

-There is no time of use billing available in my area. My main goal for the powerwalls is to provide backup in the event of a power outage, and given that I’m in Florida we can have extended power outages after hurricanes. I lost power for more than three days after hurricane Matthew a number of years ago.

-In Florida net metering works on a 1:1 basis. For each kWh I export to the grid I can get full credit for a kWh I use later. Once every year they will true up and if I have excess credits they will pay me for those at their wholesale rates.

So with all that said, I’ve been looking at powerwall configuration options and at first blush it seems like backup only would be the right option for me, but the commentary here - Tesla Powerwall says:



I’ve seen similar sentiments in other people’s comments which is making me reconsider, but I still think backup only makes sense for me. Here are my thoughts -

-I have no TOU billing option, so I don’t need to worry about time shifting

-I get 1:1 credit for power sent back to the grid. If I generate more than I use then I would get back less in credit at the end of the year, but given that my solar system will only meet 2/3’s of my power demand I don’t think there is any chance of that happening.

-The powerwall efficiency is only 90%. I think this is the big one for me. Say I was in self powered mode and I used 40kWh from my powerwalls every night, then I would need to put 44.5kWh into them each day, losing 4.5kWh a day. On the other hand, I could put that 44.5kWh into the grid each day and get credit for the full 44.5kWh. 4.5kWh a day is 1642kWh a year. At $0.13c/kWh that’s over $200 a year.

-I feel like there may be some benefit in terms of battery longevity to not drain and recharge them daily. I believe the batteries will last longer without all the use.

-Backup only mode will allow the powerwalls to stay at a high state of charge, so they will be ready for an unexpected power failure. As I said above, that is really my primary goal.

I love the idea of being self powered (at least as much as possible), but I’m just not sure that it really makes sense in my situation. What do you guys think? Is there anyone else who just uses backup only mode?

tl;dr:
Given that I don’t have TOU billing, that my state allows 1:1 net metering, and that the powerwall efficiency is only 90% I think it makes more sense for me to use backup only mode, rather than self powered mode.
Superb post. I have the exact system size system, powerwalls and same power consumption as well as being in Florida. I have been having this debate regarding backup only Vs self powered. Are you still doing this? Do you discharge the powerwalls here and there?
 

BrettS

Active Member
Mar 28, 2017
2,155
2,575
Orlando, FL
Superb post. I have the exact system size system, powerwalls and same power consumption as well as being in Florida. I have been having this debate regarding backup only Vs self powered. Are you still doing this? Do you discharge the powerwalls here and there?

So after my system was installed I spent about 2.5 months running off grid before I finally got PTO, allowing the powerwalls to discharge at night and recharge during the day. Since I got PTO I’ve been keeping it in backup only mode. So far that has worked well for me and I don’t see any reason to change. I don’t have any plans to periodically discharge the powerwalls, but I suppose it wouldn’t hurt to do it once in a while.
 
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So after my system was installed I spent about 2.5 months running off grid before I finally got PTO, allowing the powerwalls to discharge at night and recharge during the day. Since I got PTO I’ve been keeping it in backup only mode. So far that has worked well for me and I don’t see any reason to change. I don’t have any plans to periodically discharge the powerwalls, but I suppose it wouldn’t hurt to do it once in a while.

I had my PTO 20 days ago and have been on self powered. I will probably switch to backup based on the information this thread provided since I am with FPL based on net metering 1:1
Sounds like powerwalls are money wasted for someone residing out of hurricane territory but the ability to operate solar in an outage is Very important plus you get the power in batteries as well.
I had a 25kwh generac propane generator that needed to be replaced (10k cost) and opted for this technology instead. I will still use my tanks for grilling.
BrettS would You still consider 10kwh more to cover the remaining 3rd of your power? Wife will be getting a model Y next year.
 
So after my system was installed I spent about 2.5 months running off grid before I finally got PTO, allowing the powerwalls to discharge at night and recharge during the day. Since I got PTO I’ve been keeping it in backup only mode. So far that has worked well for me and I don’t see any reason to change. I don’t have any plans to periodically discharge the powerwalls, but I suppose it wouldn’t hurt to do it once in a while.
That's the same thing I have done - multiple discharge cycles before PTO, then backup only since. I haven't seen any consensus that these batteries are significantly impacted staying near the nominally full charge state, so I only plan to discharge them as needed for backup power, or in the rare instances our utility runs what they call a Peak Savings Day where they pay you to use less energy for a few hours, compared to recent use. While they usually do one or two a summer of those, they haven't done any (at least so far) this year.
 
So after my system was installed I spent about 2.5 months running off grid before I finally got PTO, allowing the powerwalls to discharge at night and recharge during the day. Since I got PTO I’ve been keeping it in backup only mode. So far that has worked well for me and I don’t see any reason to change. I don’t have any plans to periodically discharge the powerwalls, but I suppose it wouldn’t hurt to do it once in a while.

what is PTO?
I've got a new 24 kW system with 3 PW's, so this thread is fantastic! Running backup mode only so far, but might try self-powered once in awhile to just test system. Performance of the PV has been about 20% better than projected, and overall we are delighted with our installation here in Indiana!
 
what is PTO?
I've got a new 24 kW system with 3 PW's, so this thread is fantastic! Running backup mode only so far, but might try self-powered once in awhile to just test system. Performance of the PV has been about 20% better than projected, and overall we are delighted with our installation here in Indiana!
Permission to Operate. Final OK from your utility to operate your system connected to the grid.
 
I operate in self-powered because I want to be off the grid. It reduces our carbon footprint but also frees us from the power company's whims. Maybe one day it will be common for people to have their homes completely off the grid.
That’s exactly thought I have had for a while (I’m sure it has been written about and I may have picked it up)
Imagine only businesses needing to be on grid ... the maint saved not having to maintain infrastructure of power lines to each individual home ... someday people will look back ... “ remember when each house had high power lines running to it ...crazy” :p
 

BrettS

Active Member
Mar 28, 2017
2,155
2,575
Orlando, FL
I operate in self-powered because I want to be off the grid. It reduces our carbon footprint but also frees us from the power company's whims. Maybe one day it will be common for people to have their homes completely off the grid.

The thing is there are a lot of advantages to being on grid. With solar and batteries you can easily (and reasonably inexpensively) get to the point where you are net neutral. However, if you are completely off grid and have a couple days in a row of rain and clouds or a lot of snow in northern climates then being connected to the grid means that you can still draw power from the grid on those few occasions where your solar and battery isn’t enough. If you were off the grid then either you would completely run out of energy or need to get a lot more battery storage which would add considerably to cost and only be useful like 5% of the time (or maybe even less than that)

Additionally your solar system and batteries would need to be sized such that you would be able to generate enough power during the lowest production month, which again would significantly impact the cost and likely lead to a lot of ‘wasted’ power during most of the year. If you sized your system such that it could support your house during the lowest production month, as you would need to do in order to go off grid, then during the rest of the year you’ll be producing more power than you need and during some months way more power than you need.

Maybe someday the cost of solar and battery storage will be low enough that getting the extra solar and storage you would need to stay off grid would be minimal, but certainly right now it’s going to be thousands or even tens of thousands of dollars extra.

In addition to the cost savings there is also reliability. As bad as the grid sometimes seems, I suspect that on average it is considerably more reliable than home solar equipment and batteries. And even when the grid does go down it is usually fixed within hours. With solar if you have an inverter go out it’s possible that you could lose all or much of your production for days or even weeks while you wait for a replacement. If you’re on grid your house can stay powered while you’re waiting for that replacement.

There are also environmental factors. If a significant number of houses did get enough solar power to stay off grid, then as I mentioned above they would be able to generate considerably more power than they were using during most of the year. If they aren’t on the grid then that power would go to waste. If they were connected to the grid then that power could be fed back into the grid to offset the grid’s dependence on fossil fuels.
 
The thing is there are a lot of advantages to being on grid. With solar and batteries you can easily (and reasonably inexpensively) get to the point where you are net neutral. However, if you are completely off grid and have a couple days in a row of rain and clouds or a lot of snow in northern climates then being connected to the grid means that you can still draw power from the grid on those few occasions where your solar and battery isn’t enough. If you were off the grid then either you would completely run out of energy or need to get a lot more battery storage which would add considerably to cost and only be useful like 5% of the time (or maybe even less than that)

Additionally your solar system and batteries would need to be sized such that you would be able to generate enough power during the lowest production month, which again would significantly impact the cost and likely lead to a lot of ‘wasted’ power during most of the year. If you sized your system such that it could support your house during the lowest production month, as you would need to do in order to go off grid, then during the rest of the year you’ll be producing more power than you need and during some months way more power than you need.

Maybe someday the cost of solar and battery storage will be low enough that getting the extra solar and storage you would need to stay off grid would be minimal, but certainly right now it’s going to be thousands or even tens of thousands of dollars extra.

In addition to the cost savings there is also reliability. As bad as the grid sometimes seems, I suspect that on average it is considerably more reliable than home solar equipment and batteries. And even when the grid does go down it is usually fixed within hours. With solar if you have an inverter go out it’s possible that you could lose all or much of your production for days or even weeks while you wait for a replacement. If you’re on grid your house can stay powered while you’re waiting for that replacement.

There are also environmental factors. If a significant number of houses did get enough solar power to stay off grid, then as I mentioned above they would be able to generate considerably more power than they were using during most of the year. If they aren’t on the grid then that power would go to waste. If they were connected to the grid then that power could be fed back into the grid to offset the grid’s dependence on fossil fuels.
I agree with all of this. I think as home generation and storage becomes more common, the way we think of the purpose of the grid (and how we pay for it) is going to need to change somewhat, but it will still likely be a critical part of our infrastructure for the foreseeable future, and since it is source-agnostic, it can be useful to transmit green power just as well. And a major challenge for solar does continue to be the seasonal factors, particularly in areas further from the equator where more energy for heating is required.

One interesting thing about backup-only mode is that all else equal, it is the most green option because sending excess solar to grid rather than the battery is less lossy given the 10% round-trip energy loss sending that power to the battery. Of course all else isn't necessarily equal, so if an area only operates certain polluting power plants during peak periods that don't overlap peak solar time, then using batteries to minimize grid demand during those times is likely the more green option. (And, if the area also has peak pricing, that lines up to being the most economical.)
 
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I just signed the documents for my solar/powerwall install yesterday, so this is probably a bit premature, but I’ve been spending some time looking at powerwall documentation and configuration options to try to see what makes sense for me. This wound up being pretty long, so I put a tl;dr at the bottom.

First, here is a bit of background -

-My average power usage over the last year ranged from about 70-75kWh in the winter to 105-110kWh in the summer. Over the entire year of 2019 I used just over 33,000 kWh. Much of the high power demand is due to car charging. I have a model S and I typically average 2000+ miles a month.

-The system I’m putting in is Tesla’s XL size with 15.12kW of solar panels and 4 powerwalls. Tesla estimates that it will generate about 20,000 kWh annually, so it should meet about 2/3’s of my power demand. This system will pretty much fill my Southern and Eastern facing roof ares and nearly fill my Western facing areas, so even if I wanted to go bigger I really don’t have the roof space for it. Additionally, I don’t know that I will continue driving this much in the future, so if that happens I don’t want to wind up with a system that is way oversized for my needs.

-There is no time of use billing available in my area. My main goal for the powerwalls is to provide backup in the event of a power outage, and given that I’m in Florida we can have extended power outages after hurricanes. I lost power for more than three days after hurricane Matthew a number of years ago.

-In Florida net metering works on a 1:1 basis. For each kWh I export to the grid I can get full credit for a kWh I use later. Once every year they will true up and if I have excess credits they will pay me for those at their wholesale rates.

So with all that said, I’ve been looking at powerwall configuration options and at first blush it seems like backup only would be the right option for me, but the commentary here - Tesla Powerwall says:



I’ve seen similar sentiments in other people’s comments which is making me reconsider, but I still think backup only makes sense for me. Here are my thoughts -

-I have no TOU billing option, so I don’t need to worry about time shifting

-I get 1:1 credit for power sent back to the grid. If I generate more than I use then I would get back less in credit at the end of the year, but given that my solar system will only meet 2/3’s of my power demand I don’t think there is any chance of that happening.

-The powerwall efficiency is only 90%. I think this is the big one for me. Say I was in self powered mode and I used 40kWh from my powerwalls every night, then I would need to put 44.5kWh into them each day, losing 4.5kWh a day. On the other hand, I could put that 44.5kWh into the grid each day and get credit for the full 44.5kWh. 4.5kWh a day is 1642kWh a year. At $0.13c/kWh that’s over $200 a year.

-I feel like there may be some benefit in terms of battery longevity to not drain and recharge them daily. I believe the batteries will last longer without all the use.

-Backup only mode will allow the powerwalls to stay at a high state of charge, so they will be ready for an unexpected power failure. As I said above, that is really my primary goal.

I love the idea of being self powered (at least as much as possible), but I’m just not sure that it really makes sense in my situation. What do you guys think? Is there anyone else who just uses backup only mode?

tl;dr:
Given that I don’t have TOU billing, that my state allows 1:1 net metering, and that the powerwall efficiency is only 90% I think it makes more sense for me to use backup only mode, rather than self powered mode.
I have 4 PowerWalls and a 16.32kW array. In CT, we also have net-metering so I was thinking along similar lines that it might be better to go Backup-only instead of Self-powered, but there's one little gotcha. We have been generating far more energy right now than we are consuming, so when my bill came in today and there was a $13 charge on it, I was surprised and called Eversource, our utility, to clarify why, They explained to me that they charge a Distribution Service Fee for kilowatts they send me when my solar system doesn't meet my energy needs (at night and during bad weather) even though I sold them more energy this month than I purchased. When I sell them energy, I don't get a credit for this service fee, even if I my "sales" at the end of the month are higher than my "purchases." So, while the energy is 1:1 here in CT, the service fee is not. You may need to add that into the equation (i.e. round-trip battery loss vs. Distribution Service Fee). Based on my calculations, the distribution service fee they are charging me is approximately .03 a kWh. So, to really understand the pros/cons, I would need to quantify the difference between wear and tear on the PowerWalls in Self-powered vs. Backup-only mode, round-trip battery loss, and my utility buying energy back at wholesale at the fiscal year-end to really figure out what option is best. Currently it looks like we might generate more energy than we will consume on an annual basis. I will "geek-out" and create a calculation for this but I fear, in my father's words as a Disney Engineer, that, "I am placing a gnat on an aircraft carrier and trying to measure the change in waterline."
 

h2ofun

Active Member
Aug 11, 2020
4,128
1,072
auburn, ca
I have 4 PowerWalls and a 16.32kW array. In CT, we also have net-metering so I was thinking along similar lines that it might be better to go Backup-only instead of Self-powered, but there's one little gotcha. We have been generating far more energy right now than we are consuming, so when my bill came in today and there was a $13 charge on it, I was surprised and called Eversource, our utility, to clarify why, They explained to me that they charge a Distribution Service Fee for kilowatts they send me when my solar system doesn't meet my energy needs (at night and during bad weather) even though I sold them more energy this month than I purchased. When I sell them energy, I don't get a credit for this service fee, even if I my "sales" at the end of the month are higher than my "purchases." So, while the energy is 1:1 here in CT, the service fee is not. You may need to add that into the equation (i.e. round-trip battery loss vs. Distribution Service Fee). Based on my calculations, the distribution service fee they are charging me is approximately .03 a kWh. So, to really understand the pros/cons, I would need to quantify the difference between wear and tear on the PowerWalls in Self-powered vs. Backup-only mode, round-trip battery loss, and my utility buying energy back at wholesale at the fiscal year-end to really figure out what option is best. Currently it looks like we might generate more energy than we will consume on an annual basis. I will "geek-out" and create a calculation for this but I fear, in my father's words as a Disney Engineer, that, "I am placing a gnat on an aircraft carrier and trying to measure the change in waterline."
IMO, not worth it. I produce more than I use. I have fees to use grid. I just leave my 5 PW's in balanced. With no heat or AC on now, they fully charge by noon. Then solar is running house and rest to grid. At 3, house goes 100% to batteries. I am off grid at least 95% of the time now. Fun to watch
 

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