Powerwall Reserve - How Low Do You Go?

[jrweiss98020]

FWIW, the efficiency of charging the Powerwall and then using that power is around 90%. So if you want to maximize your sellback to the utility, keep the Powerwall in Backup mode. Do an occasional Reserve test and off-grid test if you want...

 

[jboy210]

Does anyone consider the effect on battery life of having a fully charged PW exposed to summer heat? My PW's are outside because I don't have a garage. I've read that it's not good to have a fully charged lithium battery exposed to heat, because it reduces battery life. I know "100%" on the app is not a true 100% thanks to battery management, but would it still be better to use the battery daily so it doesn't sit at 100# in the heat?

FWIW, I'm running 40% reserve right now, so it moves from 40% to nearly 100% most days. I was on backup-only during winter months. I'm still playing with it.

PWs has a radiator and coolant system to keep temps in line.

 

[KGM3]

I'm in PGE's EV-A rate plan which gives me TOU pricing of approx $.54 /KwH Peak ; $0.15 KwH Off-Peak and $0.30 Partial-Peak. I had always assumed that if I fed power back to the grid during those times, I'd be paid a similar (or slightly discounted) rate. However when I recently read this link on PGE's site, (see the "How much is surplus energy worth" section) I was disappointed to learn that we're only paid $0.03 per KwH for surplus energy returned back to PGE. Am I understanding this correctly? If so - there is a very small financial benefit to back-feeding surplus solar to PG&E, even during peak-hours. (although I make myself feel better by arguing that the environmental benefit is larger)

If I'm correct about the above financial aspects, I'm thinking it makes sense to set your reserve as low as possible so you lower your chances of ever using grid power (unless of course you have a high liklihood of power outages coming during wild-fire season). In keeping with this model, I've set my reserve to 10% so they don't completely discharge, mostly to maintain battery hygiene and extend their useful life.
Set to this low reserve, my batteries are only below 20% for a few minutes in the early morning after I've been running the Air Conditioning at night. If I got hit with a short term outage then - no big deal since the sun is about to rise an keep my refrigerator / freezer cold all day with solar power. By night time, I usually have 100% in my battery unless I ran the electric clothes dryer or the A/C all day - which I would not do if there were an outage, so I'm thinking I have a very small chance of not having any power at any point during the day.

My main goal has become to ensure I don't use grid - ever. I know my TOU plan makes things cheaper at night, but why even pay $.15/KwH for power that I can get for free from my PowerWalls? I'm confused by people who are setting their reserves to such high levels, unless they have very frequent outages in their areas, or unless they under-sized their systems and need to plan their discharges to avoid grid power during certain windows, while using grid power during other periods.

I'm open to other ideas if people disagree with this reasoning. Also - someone please tell me if I'm wrong about only getting paid $.03 per KwH of surplus energy supplied back to PG&E on the EV-A TOU plan with PG&E. Thanks

 

[Doom Man]

PWs has a radiator and coolant system to keep temps in line.

I know there's a fan that cools the PW's, and I'm not surprised that there's a radiator and coolant. But without a condenser of some sort, a radiator and coolant would only get the battery down to ambient temperature, right? So if it's 90 degrees outside, the battery will be at 90 degrees also. I've heard a slight fan noise from my PW's, but nothing like a refrigeration system.

 

[miimura]

I know there's a fan that cools the PW's, and I'm not surprised that there's a radiator and coolant. But without a condenser of some sort, a radiator and coolant would only get the battery down to ambient temperature, right? So if it's 90 degrees outside, the battery will be at 90 degrees also. I've heard a slight fan noise from my PW's, but nothing like a refrigeration system.

There's likely no refrigeration system in the Powerwalls. Tesla batteries are happy up to 50C. That's over 120F. I think that circulating coolant to a radiator rejecting heat to ambient is sufficient.

 

[jboy210]

There's likely no refrigeration system in the Powerwalls. Tesla batteries are happy up to 50C. That's over 120F. I think that circulating coolant to a radiator rejecting heat to ambient is sufficient.

I thought I read the same thing. And if I remember correctly the pre-conditioning in your car's systems when you enter a Superchargers as a destination will warm the battery to 40C or so for most effective recharging.

 

[Doom Man]

There's likely no refrigeration system in the Powerwalls. Tesla batteries are happy up to 50C. That's over 120F. I think that circulating coolant to a radiator rejecting heat to ambient is sufficient.

I didn't think so, but BrettS earlier in the thread mentioned a mini-compressor in there. I haven't heard anything like a compressor in mine this summer (south Louisiana - hot!), but maybe there could be a quieter thermoelectric cooling system?

 

[Zythryn]

In the summer, 25%. Although I should probably change that to 10%. I really don’t care if the power is out in the summer.
We do have storm-watch active so if a big storm is coming we will have maxed out PowerWalls in the event of a storm induced power outage.
In the winter, 100% as our heat is electric and we have no backup. Our 3 PowerWalls should keep us going for 6-10 days without sun. Any sun in that time will extend that.

 

[jboy210]

Anyone else's Powerwalls make a gurgling sound? I assume that is the coolant flowing to and from the radiator.

 

[BrettS]

I didn't think so, but BrettS earlier in the thread mentioned a mini-compressor in there. I haven't heard anything like a compressor in mine this summer (south Louisiana - hot!), but maybe there could be a quieter thermoelectric cooling system?

The reason I said that was because one of my powerwalls was making kind of a strange noise on startup. It was a rattling sound that sounded almost, but not quite like a fan blade hitting something as the fan ran at a slow speed. The installer (who seemed reasonably knowledgeable) was a bit puzzled by the noise and said that he couldn’t tell if the noise was being caused by something hitting the fan blade or a problem with the compressor.

I actually just spent some time googling and I couldn’t find anything that definitively said that the powerwalls do or do not have a compressor. Tesla tends not to publish that sort of detail.

It is certainly possible that my installer from Tesla Energy was mistaken, but he was definitely under the impression that there is a compressor in there.

 

[garrett5688]

Highly doubt there’s a refrigeration cycle. Most likely it’s just a pump and radiator to cool to ambient. Electronics operate happily at much higher temps than we do.

edit: oops. Exactly what @miimura said above

 

[aesculus]

I'm open to other ideas if people disagree with this reasoning. Also - someone please tell me if I'm wrong about only getting paid $.03 per KwH of surplus energy supplied back to PG&E on the EV-A TOU plan with PG&E. Thanks

If you send back more than you receive then yes, you only get $.03 kWh. But you will get full credit (nearly so) if you offset energy you consume. And it can get even trickier when you shift power between rate periods.

Bottom line for me is to shift as much load as I can into off peak, over produce during peak and minimize partial peak. I still have about a $1500 bill at true up (I have massive heat pumps and live in a hot area that needs AC too), plus 10k of car miles.

 

[180r]

I'm in PGE's EV-A rate plan which gives me TOU pricing of approx $.54 /KwH Peak ; $0.15 KwH Off-Peak and $0.30 Partial-Peak. I had always assumed that if I fed power back to the grid during those times, I'd be paid a similar (or slightly discounted) rate. However when I recently read this link on PGE's site, (see the "How much is surplus energy worth" section) I was disappointed to learn that we're only paid $0.03 per KwH for surplus energy returned back to PGE. Am I understanding this correctly?

(SNIP)

I don’t think you understand this right. You are mixing up Net Metering (NEM) credits and Net Surplus Compensation payments.

You are getting your $.54/$.30/$.15 in NEM credits for your future usage until your true up where your NEM credits are either spent or you are out of credits and pay the true up. This is net metering and has nothing to do with the $.03 you refer to. This is great because the potential to use more power than the PV produces and still pay basically nothing for the excess power usage. But these are credits. Unused credits at true up go up in smoke, no check in the mail.

What those $.03 per kWh refer to is if you have created a net surplus of energy during the year (that is the same year as your NEM year). This has nothing to do with NEM. Basically what PGE says is that you are a net supplier of surplus energy in a year as a whole they will pay you the wholesale rate for that energy which is around a few cents per kWh. That’s what they pay the power plants on average or according to some formula. For the few with this surplus, likely a tiny check in the mail *if* the solar customer requests a check.

I really don’t think many residential systems are designed with the net surplus compensation in mind. It’s NEM that matters.

TL;DR You’re good. You’re getting the credits you are hoping for.

 

[GaitherBill]

Currently have the reserve set to 65%

Seems like a good balance between sending power back to the grid in the morning versus using that solar to recharge the Powerwalls.

 

[RKCRLR]

I haven't figured out a good way to estimate this yet. I just got my Powerwalls up and running but my rate plan hasn't been switched to EV2A yet and I don't yet have PTO.
The way I look at it (for TOU customers) is your objective is to have an energy bill for $120 at the end of the year that is offset by the $120 you paid in minimum distribution charges while putting the fewest cycles possible on the Powerwalls. The fewer cycles on your Powerwalls the longer they will last and extra cycles aren't worth the $0.03/kWh you may be paid for surplus energy production.
Do I have this right?
Can you use excess generation to offset your minimum distribution charges at a rate greater than the $0.03/kWh for surplus generation?

 

[getakey]

you cannot offset minimum generation charges other than $.03/kWh which is not worth the cost of excess Solar capacity

 

[RKCRLR]

I guess the challenge is to come up with a reserve level (or an automated way of changing the level) that gets my energy charges to $120 at the end of the year.
Anyone know how to do that?

 

[getakey]

I haven't thought this through in detail so I may get corrected.
My solar was sized to be ~NEM zero for the annual true-up. My actual true-up was $50 and I slightly under-produced on a net kWh basis. This does not include the minimum distribution charges which are paid monthly. If you install PWs, you are not changing your solar production. In my case, I installed PWs recently and the PWs will change my cost basis, but not production. So I expect my next true-up to be less than $50 if not negative (which I won't get paid). However, I would expect that I still under produce on a net kWh basis

 

[RKCRLR]

I haven't thought this through in detail so I may get corrected.
My solar was sized to be ~NEM zero for the annual true-up. My actual true-up was $50 and I slightly under-produced on a net kWh basis. This does not include the minimum distribution charges which are paid monthly. If you install PWs, you are not changing your solar production. In my case, I installed PWs recently and the PWs will change my cost basis, but not production. So I expect my next true-up to be less than $50 if not negative (which I won't get paid). However, I would expect that I still under produce on a net kWh basis

Close, but my understanding is there is about a 10% energy loss when the Powerwalls are charged and discharged. That may also need to be taken into account.

 

[getakey]

in that case, my net cost will go down and my net production will go down