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Considering going with backup only vs self powered

BrettS

Active Member
Mar 28, 2017
2,155
2,575
Orlando, FL
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:

Backup-Only is, quite possibly, the least realistic option for daily use. This option charges Powerwall to 100% and only discharges in the event of an outage. Short of stormy weather or potential outages—negated by Storm Watch—I can’t see a use case where I’d personally prefer this option.

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.
 
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I'm in a similar situation - 22 kW system, five PW's and net metering. In the beginning, I used the PW's to power overnight, but eventually we would have cloudy/rainy weather and the batteries were not able to recharge completely, so I would end up with batteries hanging around 45% charge for a day or more. It would suck if we had a power outage in that situation, so now I allow the PW's to discharge to 70%, which is a more manageable number in the event of a power outage.
 

mspohr

Well-Known Member
Jul 27, 2014
12,081
16,156
California
I have a similar situation... net metering, no TOU so I opted for a backup only situation. I need it only for our power outages in California mountains. (However, Tesla was MIA for over a year so I gave up on them and installed an Outback Skybox which has more flexibility. It can use different kinds of batteries (I use AGM since I rarely cycle them and they are much cheaper), charge from the grid and has a generator input. Also has a solar PV inverter built in.
You might want to look at having fewer Powerwalls. Your 15 kW solar won't be able to fully recharge your four Powerwalls during an extended power outage. Also, in the rare emergency situation, do your really think you'll "need" to run everything at full blast.
 
So let me ask the basic question, do you really need backup power? Why spend the money on Powerwalls if there is no TOU cost benefit nor the interests to self consume stored power? At the very least, installing solar makes sense.

Also keep in mind that the batteries and gateway will consume energy while on standby and to maintain their temperature. I calculate my two Powerwalls (outdoors) consume about 1 KWh each per day. So roughly 700 kWh additional energy used.
 

BrettS

Active Member
Mar 28, 2017
2,155
2,575
Orlando, FL
You might want to look at having fewer Powerwalls. Your 15 kW solar won't be able to fully recharge your four Powerwalls during an extended power outage. Also, in the rare emergency situation, do your really think you'll "need" to run everything at full blast.

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.

I’ll definitely be able to do some load shedding in the event of an extended power failure, primarily by reducing the amount of car charging, but even if I can’t fully charge four (or even five) powerwalls during the day having the extra power should help me to be able to last through a longer extended failure.
 

BrettS

Active Member
Mar 28, 2017
2,155
2,575
Orlando, FL
So let me ask the basic question, do you really need backup power? Why spend the money on Powerwalls if there is no TOU cost benefit nor the interests to self consume stored power? At the very least, installing solar makes sense.

It’s a good question. I definitely do need backup power of some sort. Like I said above, I lost power for three days after hurricane Matthew 4 or 5 years ago and being in the heat and humidity of Florida with no air conditioner or hot water was pretty miserable. Additionally, I have a large saltwater aquarium and in an extended power failure I could potentially lose thousands of dollars worth of livestock. Right now I have a small portable generator to get by, but it requires me to manually pull it out of the garage and connect it and start it. Right now the aquarium is still at risk if the power were to go out while I was not at home to start the generator.

Obviously powerwalls aren’t the only solution here and I could get a standby generator for less than I’d pay for the powerwalls. Unfortunately I have no natural gas service at this house, so I would need to install a propane tank for it as well, which will add to the cost. But beside that, I really like the idea of not needing to burn fuel for my power and I’m willing to pay at least a little extra for that.
 

jackbowers

Jack Bowers
Aug 23, 2009
275
461
I use back-up mode in the winter (when there is not much difference between peak and off-peak rates and my main concern with a long outage is frozen pipes). In the summer I switch to self-powered because there's a lot of money to be saved by avoiding the peak periods, and enough sun every day that it doesn't matter what the state of charge is if an outage hits (I have seven Powerwalls backing up an entire panel). I'd agree for your situation backup-only makes the most sense. In effect you are getting a silent back-up generator that does not need any fossil fuel - ideal for protecting against hurricane outages.
 
It’s a good question. I definitely do need backup power of some sort. Like I said above, I lost power for three days after hurricane Matthew 4 or 5 years ago and being in the heat and humidity of Florida with no air conditioner or hot water was pretty miserable. Additionally, I have a large saltwater aquarium and in an extended power failure I could potentially lose thousands of dollars worth of livestock. Right now I have a small portable generator to get by, but it requires me to manually pull it out of the garage and connect it and start it. Right now the aquarium is still at risk if the power were to go out while I was not at home to start the generator.

Obviously powerwalls aren’t the only solution here and I could get a standby generator for less than I’d pay for the powerwalls. Unfortunately I have no natural gas service at this house, so I would need to install a propane tank for it as well, which will add to the cost. But beside that, I really like the idea of not needing to burn fuel for my power and I’m willing to pay at least a little extra for that.


Sorry. I missed reading that part of your long post. :oops:

For only $1,500 more adding the 5th Powerwall would be tempting. Much more expensive to add on to the system at a later date.

As batteries do degrade, the 5th Powerwall can provide you a long term buffer and give you flexibility in the future. Electric rates and plans are always in flux. The cost of grid electricity goes up and your utility may switch to a TOU rate schedule in the future. In PG&E land (northern CA), most tied rate (E-1) users are transitioning to TOU rate schedules. Having Powerwalls is like insurance to mitigate the cost of this.
 
-The powerwall efficiency is only 90%
Exact same situation here in MA and I use backup only. My two PW need 600-700 watts a day to charge back to 99%. Another major benefit is ability to use PV during blackout. Without PW attached to PV, can't use PV if grid out. We have up to 5 day power loss after coastal storms here and sun is always out following day.
 

bob_p

Active Member
Apr 5, 2012
3,738
2,949
With 4 PowerWalls, initially set the system for about 50% reserve, and because we don't have net metering, we were throwing away electricity going back to the grid.

Have now adjusted the reserve to 20%, and on fully sunny days, we'll be able to go for days without any grid power with only a small amount of excess going back to the grid.

Of course, if there is an unanticipated outage, 20% won't run the house very long (though I can cut down power consumption by using apps to turn off HVAC and pool equipment).

Though if we anticipated a major storm/hurricane, we'd switch to backup mode or increase the reserve % to ensure we had more backup power.
 

BrettS

Active Member
Mar 28, 2017
2,155
2,575
Orlando, FL
With 4 PowerWalls, initially set the system for about 50% reserve, and because we don't have net metering, we were throwing away electricity going back to the grid.

Yeah, that’s the thing for me. Since I do have net metering I’ll be throwing away power if I use the powerwalls. I get 100% of the energy I send to the grid back, but I only get 90% of the energy I send to the powerwalls back, due to the powerwall (in)efficiency. 90% is pretty good, all things considered, but 100% is still better:)

Have now adjusted the reserve to 20%, and on fully sunny days, we'll be able to go for days without any grid power with only a small amount of excess going back to the grid.

Of course, if there is an unanticipated outage, 20% won't run the house very long (though I can cut down power consumption by using apps to turn off HVAC and pool equipment).

I mean if you’re able to run indefinitely with a 20% reserve and little to no power going back to the grid, then it doesn’t seem like a power failure is really going to affect you no matter where it catches you in the cycle, since you’re not using the grid for power anyway.

Of course a storm could bring clouds and rain for a while, which might affect your solar production, but that 20% reserve should still be able to get you though some cloudy weather.
 
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BrettS,

I am in a similar situation as you as far as usage. last year I was pulling on average 100KwH per day from the grid. since installing my Solar and 3 powerwalls I have pulled no more than 30 per day. last months bill was $17.00. I am running in self powered mode. on a sunny day which we have tons of in central FL, My 3 powerwalls are fully charged by 12PM, with the rest of the day going back to the grid. you will likely get better performance in sending to the grid than I get if you have panels facing west (all of mine are South and East).

As I am typing this we are having a TECO outage. My powerwalls were at 100% before the outage started at 1130AM this morning.
 
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jboy210

Well-Known Member
Supporting Member
Dec 2, 2016
7,406
5,029
Northern California
BrettS,

I am in a similar situation as you as far as usage. last year I was pulling on average 100KwH per day from the grid. since installing my Solar and 3 powerwalls I have pulled no more than 30 per day. last months bill was $17.00. I am running in self powered mode. on a sunny day which we have tons of in central FL, My 3 powerwalls are fully charged by 12PM, with the rest of the day going back to the grid. you will likely get better performance in sending to the grid than I get if you have panels facing west (all of mine are South and East).

As I am typing this we are having a TECO outage. My powerwalls were at 100% before the outage started at 1130AM this morning.

Sorry to hear about your outage,or maybe not since your PWs are doing their job! Anyway, I was curious is this outage due to weather or the power company issues.
 
Sorry to hear about your outage,or maybe not since your PWs are doing their job! Anyway, I was curious is this outage due to weather or the power company issues.
It was a power Company issue. power was out 3.5 hrs. But the Powerwalls maintained 100% and kept the house going. Wasn't sure this would happen, but looks like the solar kept the powerwalls charged.
 
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Gixx1300R

Active Member
Dec 30, 2017
1,026
1,654
orlando
Yeah, that’s the thing for me. Since I do have net metering I’ll be throwing away power if I use the powerwalls. I get 100% of the energy I send to the grid back, but I only get 90% of the energy I send to the powerwalls back, due to the powerwall (in)efficiency. 90% is pretty good, all things considered, but 100% is still better:)



I mean if you’re able to run indefinitely with a 20% reserve and little to no power going back to the grid, then it doesn’t seem like a power failure is really going to affect you no matter where it catches you in the cycle, since you’re not using the grid for power anyway.

Of course a storm could bring clouds and rain for a while, which might affect your solar production, but that 20% reserve should still be able to get you though some cloudy weather.
Do you have OUC?
 

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