I just got 2 Powerwall 3s installed. The app recommends 20% Backup Reserve. What % are you using?
2? I have 2 PW2's as well as 5.6kW nominal solar since 2017 (some stats at
Solar, Battery, Home and Grid PowerFlow until they disabled LAN access), and for me, that's a woefully undersized system (i.e., it does not cover 100% of my usage on a cold dark winter day for no electric cars, much less adding EVs to the equation).
I like 50% or above in summer and 80% to 90% in winter. According to studies, under 50% and over 80% seem to "damage" the longevity of the battery, so I avoid going below 50% or above 80%, but quite frankly, 80% seems like leaving too much battery capacity on the table so to speak in winter.
That way when the power goes out, I have at least half of my battery capacity available for actual backup use. I live in Theftofornia, so most of our electric utility money has been stolen for illegal aliens (through rate discount programs), churches (that organize the illegal aliens coming here to get more contributions), regulators that pocket huge taxes, corrupt utility executives, and corrupt officials, rather than maintaining or undergrounding our power lines. As a result, the same system and same lines which used to be nearly perfect when I was a kid running for decades without even a single glitch now glitches or goes down several times per month due to lack of maintenance. All the tree cutting has seemed to increase the number of times the electricity goes out, not decrease it. Our undergrounding tax funds were never used to underground our lines, but they raise our rates to chop down our trees and leave the ugly falling apart lines in the sky ruining our view.
All that to say, our electric utility is not what it was. Your experience where you are could be way different than ours, as a result. Soooo, I'd rather keep my battery above 50% all the time in summer and above 80% to 90% in winter, so that when the utility power goes out, I still have my home electricity up and running. I also have solar. Unfortunately at the time of purchase, I was "frugal" and selected undersized solar and undersized PowerWall sizing, getting half a summer daily amount of use without EVs (less in winter or with EVs). With conversion of several appliances and cars to electric, that means it's fast becoming quarter sized of what our usual use is, if not a lot less. I should have gotten the entire roof covered with solar, and I'm still trying to get permission to put a checkerboard of solar on the rest of the quarter acre we have here to pull in even more. Rather than two PW2's, we should have gotten six, or at least four.
All this to say, I can never hope to cost shift our usage with the undersized solar and undersized batteries we have. The most I can hope for is using it as backup during the many power outages. Letting solar charge a small portion of the capacity up in the day and then letting it drain a small portion of the capacity in the evening seems like good use in summer, but in winter, I raise the minimum backup a lot more, and there's very little load shifting going on, nothing worth making any money off of, which is a fool's errand with the way the rates are set up anyway.
Also, you need to factor in other concerns:
- Does your area experience power outages ever? If never, then you have less need of backup.
- Did you size your battery and solar at 20% or 200% of your usual darkest coldest winter use? If so, then this should be factored in that calculation. For any particular season, whatever % more you have excess of energy from your solar and your battery for the entire 24 hour period is something you can lower the amount of backup reserve, and still be able to keep your energy on during a power outage, if you get enough power outages to let this be a concern.
- How important is continuous electricity to you? How important is electricity to you? If you run always-on computer servers or have a tenant that lives on a breathing air machine that has no battery backup system or requires an elevator to get to and from necessary parts of the house, then never letting power go out is primary goal number one, and you should increase your battery backup reserve % quite generously. If you are willing to put skiing clothes on, turn the heaters off, stop washing clothes, leave the cars unplugged, and turn off every other high user of electricity (pools? etc.) every time the power goes out and turn your usage down to 5% of your usual, then perhaps having a less generous backup % is acceptable to you. If you like lighting candles and have good fire insurance and lots of firemen nearby that actually like to put out fires without letting your house burn down that aren't busy during any power outage (which would be a wrong assumption in most areas) and you don't own any valuables, then perhaps you can turn backup battery % all the way down to 10% and not worry about that feature, and play with fire to light your way (or go to bed early and twist and turn in bed). But if you notice your battery staying at 10% too often since you don't get much solar input, you should try to keep your average battery charge around 50% throughout the day, so I'd really recommend bumping it up 5% more every day until the average is around 50%, just for battery longevity. The various usage preferences in the settings and weather will wildly change this behavior, so I have learned to continuously monitor this and adjust it whenever conditions or settings change.