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What is your reserve percentage set to and why?

What is your "Self-powered" reserve percentage set to?


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I'm on the PG&E E6 TOU rate. In the winter there is a partial-peak rate from 5-8 pm on weekdays. I set my reserve to 50% from 5pm to 6pm. All other times, I'm set to 100%. I also participate in OhmConnect's demand reduction program (only from 6pm to 5am, though), so if there's an OhmHour, I set my reserve to 50% for that hour. I'm not sure I'm actually breaking even on the self consumption because the difference between the off-peak and partial-peak rates is quite small (about 9%).

The 50% number isn't very significant - the Powerwalls probably won't ever go much below 85%.
 
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miimura

Well-Known Member
Aug 21, 2013
7,397
7,400
Los Altos, CA
When I want the battery to discharge I set it to 50% because I have two PowerWalls and I want to have one full PW worth of capacity in case of an outage. I have been setting it to 90% every night so that it doesn't discharge during my Off-Peak hours of 11pm-7am. I am on PG&E net metering with the EV rate schedule. My logic for this is that my solar generation has a nominal value equal to the Part-Peak rate of 19.8c/kWh (winter) because that is when most of it is generated. I want to use that energy to offset my Peak usage from 2pm-9pm that I would normally have to pay 32c/kWh. It doesn't make any sense for me to consume my solar energy during Off-Peak when I could just get it from the utility for 12.5c/kWh. Rates have even more spread during the Summer season when they are 45.4c / 25c / 12.2c/kWh. I am hoping that the upcoming Time-Based Control will do this for me automatically.
 
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A minimum of 50% in the winter when solar production is the lowest. Reserve is set higher since winter solar production will not fill up the battery in a day. The percentage is adjusted higher to maintain a relatively high state of battery readiness to account for the greater chance of an outage in the winter. Some TOU rate arbitrage is done, but within the confines of actual solar production and use.

For the summer, the reserve will be set to 30% when solar production is the highest. Reserve is set lower allowing more of the battery to be used for self consumption to mitigate for higher TOU rates. Solar production exceeds battery capacity. So I am more comfortable with a lower reserve. I consider 30% the bare minimum (20% for overnight use and 10% for a factor of safety (additional reserve)).
 
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Winter reserve level is currently set at 50% to allow enough reserve to make it through the night and into the morning at about 9 when I get meaningful solar generation (> 1kW). By this summer I plan on having a third PW2 courtesy of the referral program so I will likely set the reserve no higher than 25% since the added capacity and almost guaranteed sunshine for solar generation should cover any outages.
As the other members noted I will be counting on new TOU scheduling software to make all this much easier as it is all manual now by manipulating the reserve value to keep the battery at a preferred level.

I think it's worth noting that how much I hold over for period between 2300 and about 0900 gets modified depending on expected weather. If a very cloudy day is expected I often adjust the reserve to go fully on the grid at 2300 and re-adjust at 0700 to go back on battery.

Charging the Model S is currently scheduled on off-peak rate times (2300-0700) but I usually allow the car to discharge the battery the reserve value to assure that I meet the equivalent 52 full battery discharges per year required by the SGIP.

2X PW2 / 4.1 kW PV / MS 90D
 
It will vary depending on weather and time of day. If I need to use the aircon, then I'll set it to a percentage that I'll estimate will get me through the peak and shoulder periods and start using the grid when on off-peak rates. Then in the morning before peak starts again I'll lower the reserve to get me through peak and should rate times until solar kicks in and provides enough power to avoid pulling power from the grid.
 
I started this thread to get an idea of what people were setting their reserve percentage to and why. When my PWs were installed in December the reserve capacity was set to 20% but I had no knowledge basis or experience for setting it otherwise.

After playing around with various percentages, I discovered that setting it at 45% wasn't quite enough to get me through the night until the solar started kicking in the next morning. Now I have it set at 35% and unless it was a less than ideal solar day, I'm 100% self-powered. Currently, the batteries are getting charged to 100% around 1:30 PM, the excess is being exported to the grid, and once the sun goes down I'm pulling from the battery until solar kicks in again the next morning.

I'm grandfathered in on net metering 1.0 and elected to stay on SDG&E's original TOU schedule, so currently my semi-peak period runs from 6:00 AM to 6:00 PM and my peak period starting June 1 will still be 11:00 AM to 6:00 PM with semi-peak running from 6:00 AM to 11:00 AM and 6:00 PM to 10:00 PM. I'm curious how using A/C during the summer will affect this but Tesla should have TOU software by then.
 

WSE51

Member
Supporting Member
Aug 24, 2012
127
90
Park City, Utah
The ideal reserve percentage weighs taking risk of running out of power in a blackout vs the cost savings of using self-generated power, especially on TOU during peak cost period. Has anyone seen a formal method of making this calculation?

A lot of guessing is involved:

1) what % of daily use is air-conditioners, which my Powerwalls won't back up?

2) what % of my daily use is in the evening and night, when solar is no help?

3) seasonality of solar generation, stronger in northern locations

4) seasonality in risk of blackouts (summer higher due to strain on system, rainstorms higher due to falling branches on power lines)

5) portion of daily use that solar system will cover

6) amount of risk willing to tolerate

7) amount of cost savings from TOU credits.

If anyone wants to share their calculations, it could help us all.
 
70%'ish gives the best annual payback for me, although I'm testing 60%'ish to reduce NBCs.

I've modeled via SAM for my specific TOU times/prices and my average monthly annual usage-pattern, at half-hour intervals for real insolation levels measured in 2017 in my geographic area.

Basically, it's nice to use the PW to cover most of your usage by draining it as much as possible, but recharging the PW is also marginal cost in lost solar FiT.

SAM can give me an annual summary for varying this minimum SOC, and then I target whether I want:
* lower electricity bill with the system
* highest net present value, performance ratio, energy yield, capacity factor, or annual energy
* lowest simple payback period
* lowest levelized COE
* or a preferred combination of such
 
I'm using 0% for mine also (and just regular Self-Powered mode). On a typical day it'll discharge to ~45-50% from sundown to sunup, but if I do dishes/laundry or run a portable AC it'll get lower. But I figure why ever draw from the grid unless I have to, and so far the only time I've had to was when I had the reserve set to 20% and the AC pulled it down to there by early morning. I caught it within <1kWh from the grid and immediately changed the reserve to 0% and kicked the Gateway by logging-in to it, so it wasn't a big hit at least.

I also disabled Storm Watch, again to prevent any grid charging to avoid NBCs, etc. My PW is typically fully recharged by 10am-noon, so there's a very limited outage window where I'd be below 20-50% anyway. But I realize my setup is out of the ordinary since my production is high vs. my (current) consumption. Since ideally the PW would never be fully-drained, seems like the Advanced modes are also useless to me, they would only increase the chance that the house would sometimes draw from the grid when it should never need to.
 

daniel

Well-Known Member
May 7, 2009
5,456
4,977
Kihei, HI
I figure that an outage would be an unpleasant nuisance, but not a disaster. So I'd rather avoid using grid power even if it means that during the very rare outage I could be without power until sun-up. The big power demand here is A/C, and if I need A/C there'll be sun. Back in North Dakota, it was critical to have heat. The worst that could happen here is that I get so hot (if I didn't have A/C) that I'd have to walk the 4 minutes to the beach and cool off in the ocean.
 
I figure that an outage would be an unpleasant nuisance, but not a disaster. So I'd rather avoid using grid power even if it means that during the very rare outage I could be without power until sun-up. The big power demand here is A/C, and if I need A/C there'll be sun. Back in North Dakota, it was critical to have heat. The worst that could happen here is that I get so hot (if I didn't have A/C) that I'd have to walk the 4 minutes to the beach and cool off in the ocean.

I set mine to 0. A power outage is not the point of a powerwall for me. It’s a nice side benefit. The only time I would be without power is early in the morning. The rest of the time if you average it’s at 40%. Enough to keep the power on for a few hours with no sunshine.
 

jjrandorin

Moderator, Model 3, Tesla Energy Forums
Moderator
Nov 28, 2018
16,650
21,919
Riverside Co. CA
Since we are now replying to a resurrected thread (lol).

I use self powered and move the percentage quite often depending on what I expect my daily usage to be. If I can help it, I will never again be without power in this home, so I set the reserve at a percentage that ensures that when I wake up in the morning, there is still power in the powerwall when the sun comes up. Right now I have it set to 20-25% depending on how the solar does that day.

I am trying to both maximize my time off grid, while still ensuring that I have power, so I micro manage it almost daily. I enjoy looking at it daily so have not looked into automating anything. My powerwall gateway is hard wired with ethernet to my network, and when I change the percentage, it takes effect within 10 seconds, usually (actually usually almost instantly, I can watch flow change directions if I change modes etc).
 
I'm at 40%. May set it higher over winter. Right now, it's a tricky balance between the ~10% round-trip loss to the battery vs ??% loss on net metering. Although we get 100% credit on the energy service and transmission charges from the utility, we only get 25% of the distribution charge, which I think results in >10% loss overall making batteries more efficient.
 
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I set mine to 0. A power outage is not the point of a powerwall for me. It’s a nice side benefit. The only time I would be without power is early in the morning. The rest of the time if you average it’s at 40%. Enough to keep the power on for a few hours with no sunshine.
Previous thread/post somewhere said/proved that if a PW drops below 10% charge, it will auto-shutdown at some point near 0%. Then, it will NOT restart/power back up without manual intervention upon any outage nor restart when grid/solar-power resumes.

Manual intervention involves opening up the Gateway, and jumping some undocumented contacts.
 

bob_p

Active Member
Apr 5, 2012
3,738
2,949
The Tesla app hides 5% of the charge - 0% reading in the app is actually 5% of charge in the PowerWalls. The TEG's web interface shows the actual charge level - 5% in the TEG is 5% actual charge.

Since the Power Reserve % is set in the app, that means you can't set it below 5% of the actual charge level, evidently providing the reserve power needed to keep the PowerWalls and TEG operating.

We've settled on 30% as our Power Reserve level, providing us enough stored energy to operate our house for several hours during an unplanned power outage (when we may not be at home, and the pool pumps/air conditioners could be running).

We have a Free Nights electric plan - providing free power between 9PM to 9AM, so our goal is to use the solar/PowerWalls to minimize grid power usage during the day (when we're charged about 2X the fixed price rate for electricity). During the summer with high temperatures outside, we don't generate enough solar power to fully power the house between 9AM to 9PM, so we end up at 30% of charge for overnight power outages. Now that it's cooler out, we are fully charging the PowerWalls during the day, and have 60-80% of charge at 9PM.

Under the Advanced Time-Based Control settings, the TEG will try to guess how much solar charging will happen the next day and will provide power from the PowerWalls over night to bring that charge level down before the sun starts recharging the PowerWalls. Because our electricity is free overnight, I wish there was a setting that would turn this feature off - because we'd prefer to store as much excess energy as possible in our PowerWalls, to provide power during very cloudy days, when there isn't much solar energy generated.
 
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