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Back-up mode
- Thread starter Gunit1984
- Start date
If you have 1:1 net metering then backup mode probably makes the most financial sense for you. The powerwalls are rated for 90% round trip efficiency, so for every 10kW you put into the powerwalls you only get 9kW out. If you have 1:1 net metering then for every 10kW you put into the grid then you get the full 10kW back out.
Self power mode would definitely cause more cycling. In backup only mode the powerwall just sits there until you have an outage.
However I’m not sure that there’s really a definitive answer about what would be best for the batteries. Tesla warranties it for 10 years and an infinite number of cycles, so they don’t seem to be worried about cycles. There is also a school of thought that leaving the battery charged all the time without using it may also be bad for it. But again, doing that won’t affect the warranty either.
Honesty, Tesla put a lot of time and effort into logic that protects the batteries no matter how you use it, so I really wouldn’t let that make too much of a difference in how you decide to use your powerwalls.
However I’m not sure that there’s really a definitive answer about what would be best for the batteries. Tesla warranties it for 10 years and an infinite number of cycles, so they don’t seem to be worried about cycles. There is also a school of thought that leaving the battery charged all the time without using it may also be bad for it. But again, doing that won’t affect the warranty either.
Honesty, Tesla put a lot of time and effort into logic that protects the batteries no matter how you use it, so I really wouldn’t let that make too much of a difference in how you decide to use your powerwalls.
Just to clarify the Powerwall is warrantied for 70% capacity at 10 years for backup only with no other qualifications. For all other uses its 70% capacity at 10 years as long as there is less than 37.8 MWh of throughput.
https://www.tesla.com/sites/default/files/pdfs/powerwall/powerwall_2_ac_warranty_us_1-4.pdf
So @BrettS is correct about no cycle limits but there is a throughput limit on non backup uses.
Thanks for that clarification. I didn’t realize that there was a throughput limit. Although 37.8MWh is a *lot* of throughput for a single powerwall. That’s just over 10kWh per day, so basically it would be nearly completely draining and refilling the powerwall every single day for 10 years.
Honestly I hadn't done the math but you're totally right. You'd have to have a ton of solar and probably midday ToU (two discharges/day) to get even close to that.
Check out the (3) note on the warranty PDF:
3 Measured at the battery AC output.
You'd need every day to be like yesterday to get 10kWh AC output per Powerwall for 10 years straight to get to the warranty limit.
So, looking at my From Powerwall in the app for the month, it shows 317kWh. Divided by 26 days in the month so far, yields 12.19/day.
Yeah, I have that logged to a database. As of a few minutes ago the raw value was 918400. PWs installed ~3 months ago.
Napkin math on the top number would be 6.1kWh/Powerwall and napkin math on the bottom would be 5.1kWh/Powerwall.
You'll need to double your usage to hit the warranty limit
I havent read through to the end of the thread yet, so forgive me if someone said this already, but that throughput limit only applies if you are NOT charging your powerwall from solar, or at least thats how I read it. Its unlimited cycles when charged by solar.
the note from the link @gpez provided...
Key words are "For daily self consumption AND / OR (ii) use as backup. It seems pretty clear to me that as long as you are charging your powerwall from solar only (as "most" in the US would be) that its "unlimited cycles" so no need to break out the calculator on throughput if you are charging from solar, for warranty purposes anyway.
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You know I read that differently to start but after re-reading it with your notes I agree. Good catch.
NP! I havent put in a warranty claim so I dont know for 100% fact I am correct on this, but I have looked at this exact wording several times, having several different discussions with people both here on TMC and elsewhere about powerwalls and warranty.
Normally people start off with "Well, the powerwall warranty is only 10 years so....XXXXX" , so I have gone to that page you mentioned several times and looked closely at the wording.
Its also come up in the "but I wanna charge my powerwall from the grid when I want to!" discussions too, and I usually will mention the warranty on powerwalls NOT charged from solar with that cycle throughput.
I wonder though..... I am sure the powerwall is keeping track if this somewhere (those of you playing with the API could likely see if you see something), but /e musing.... does stormwatch charging go into unlimited cycle bucket or throughput bucket? Does any off grid charging throw you into the "cycle" bucket?
Common sense says "no" (stormwatch doesnt count) but businesses dont operate on common sense..... shrug.... lol
If you look at the footnote you highlighted it says “for daily self consumption and/or for use as backup power.” I suspect that stormwatch fits into there.
However, seeing that makes me wonder about these folks in the northeast who are getting paid to allow their powerwalls to feed into the grid to supplement the grid. That’s definitely not self consumption or backup power.
OP, on thread topic, the short answer is, we dont believe that 100% in the app is actually 100% of the battery, and we know that 0% in the app is not Zero percent of the battery. Self powered mode will cycle the battery. Backup only mode wont cycle it much.. likely a little but not much.
Saying "I have net metering" doesnt really say if you have full 1:1 net metering or not, but thats not really material to the "which is best for the battery" discussion. The "best" thing for these type batteries is to leave them at 50% usage and never use them, but thats not realistic.
It kind of depends on what your goal is. its rated at 70% unlimited cycles if charged by solar at the 10 year mark. less cycles might mean more life in it, but since 100% is not good for batteries either, if I was in your specific use case, with no real desire to run the battery down, I think I would still put it in self powered mode and set the reserve at like 80-90 percent.. and I would test it every so often by going off grid.
shrug.. I dont think there is a "right answer" on this.
Saying "I have net metering" doesnt really say if you have full 1:1 net metering or not, but thats not really material to the "which is best for the battery" discussion. The "best" thing for these type batteries is to leave them at 50% usage and never use them, but thats not realistic.
It kind of depends on what your goal is. its rated at 70% unlimited cycles if charged by solar at the 10 year mark. less cycles might mean more life in it, but since 100% is not good for batteries either, if I was in your specific use case, with no real desire to run the battery down, I think I would still put it in self powered mode and set the reserve at like 80-90 percent.. and I would test it every so often by going off grid.
shrug.. I dont think there is a "right answer" on this.
When comparing energy in vs out of PW...90% efficiency vs the grid at 100%, I'd factor in NBCs if you're trying to calculate ROIs.
