@JayClark, how much electricity do you use during the SRP 2-8pm peak period? I’m also planning to try to mitigate peak demand via Powerwall here in AZ but figure it should be possible for me to get through peak with 8 KW and 2 PW on ~3200 square feet, even on most cloudy days. For example on Tuesday (mostly cloudy) my house used 23.4kWh during peak, so two full PW with 27kWh could have carried the peak.
The system
@Darwin uses generated 7.5kWh before 2 pm that day, 11.2 during peak and 18.8 total, so an 8 kW system could do maybe 11kWh before 2 pm, 16 during peak, and 27 for the day. It would be close, but I think 8kW and 2 PW might sustain repeated days like Tuesday indefinitely when it comes to peak mitigation if the PW software did the right thing.
Obviously your situation is different which is why I’d like to compare your peak demand. When it comes time for selling the house, I also figure solar could boost perceived home value more than batteries, but who knows?
Seems like you should be fine. I think I lean towards "overkill" usually, so that's just me. And the battery is where I did the overkill since that seems to be the harder thing for me to add later, but also avoiding peak charges, and prioritizing toward erring toward extra battery capacity are my top priorities. I feel like I have more control over battery capacity and doling out the usage, vs the control (or lack there of) that I have over solar production based on weather and such. So I'd rather build the buffer into the battery end of things and use production averaging (including weekend production) to size the solar. For instance, jumping to 6k system but with only two Powerwalls actually had a negative impact in terms of payback for me based on my usage patterns as compared to doubling the battery size from 25kWhrs to 50kWhrs, and sticking with a 4k pv system, given the my particular house and power usage needs. Having said that, I will add panels eventually regardless, just for my own peace of mind. Sorry for the preamble, so to your actual question...
So, when using pre-cooling during the worst of summer, I still end up using about 14-18kWhrs during the 2-8 peak on these 105+ degree days, but with the 54kWhrs of battery I'll have more than enough to cover peak, and peak is primarily what I want to cover 100% no matter what. Our off-peak is so cheap it makes no sense to use up battery cycles to cover off-peak usage. If after the battery is full I'm fine with extra solar going back to grid but charging battery for use only during Peak coverage will always be the priority.
When we're hosting/entertaining during peak summer days (usually Fridays),
I find we're closer to 20-22k (depending on how hot it is outside) of peak usage no matter how careful I try to be - the house simply has to be cooler with lots of people, and cooking going on, usually all the lights are on, a couple fountains are running, etc. Most of this type of stuff is only off peak usage otherwise. I have a "Peak" mode, and a "Party" modes configured on my home automation system. Peak mode aggressively turns all high watt items off, well really almost everything off, even if somebody turns them on during peak, my system turns them right back off. If my wife and I are both out of our home office, which happens once or twice a week during the day, then the house temp is allowed to rise, and
on those days my 2-8pm peak usage is only about 10kWhrs, but that's usually only once or twice a week, and on those days we end up eating out together since we don't want to go home until we can cool the place down! Ha!, and we eat out a lot anyway since we both work and all the kids are out of the house.
In the winter, during the 5-9am and 9-5pm peak times (am/pm), we use 7-10kWhrs at most (usually between 400-900 watts per hour) because I don't have to run the AC at all with my house during those hours. Although if my wife turns on her hair dryer or straighter in the morning I might have 30 minutes that average 1500-2000 watts in there somewhere during the morning hours.
If anybody is curious about pre-cooling in the desert, I've been doing some dry runs and measuring my usage under different scenarios, because I can't have the house warm at all during summer since I work at home as does my wife, and we have clients at the house at times, so I end up pre-cooling around from 5 or 6am until 8am (hotter forecast days I start it at 5am). The house cools down faster when it's coolest outside. then I pre-cool again from 12:30-2pm (SRPs peak starts at 2pm). With the pre-cooling, when peak hours hit, because of the battery I'll be able to run the AC in the house for 30-40 minutes every hour (when it's 108-115) outside and we've found the house stays reasonably cool until 8pm, maybe gets up to 77-78 degrees by 8pm because 30-40 minutes of AC running each hour isn't quite enough to keep up so the inside temperature creeps up on average from starting at or near 70 degrees at 2pm up to about 78 worst case usually by 8pm. I know this sounds crazy, but I pre-cool to 69-70 degrees in the early am for 2-3 hours - cools the house slab and granite countertops tops very nicely, and quickly, during the cool morning hours and it seems to help the house feel cooler the rest of the day. When the temp outside during early hours is around 70-80degress outside vs 110-115 at peak afternoon, the air-condition works quickly when it's cool outside, and not so well once it gets above 108-110. I don't pre-cool again until the next days early hours.
Most importantly, with 54kwhr battery I should be able to cover "entertaining days", and/or make it through a couple cloudy days, or unexpected high peak usage days (usually Fridays when we have family and friends over as often as not). It will requiring charging all weekend probably to get back to 100% with only a 4k pv system, and during the worst of summer that may not be quite possible to get back all the way to 100 over the weekend - we'll see (if so I'll add more panels). If it seems like after Monday or Tuesday we're trending high on our battery usage during peak hours, then maybe I should be able to do minor rationing of power on for a day or so if needed and cut AC back to 25-30 minutes each hour vs closer to 45 minutes for a day or so - which again, I can do with a toggle on the home automation system. I'm not doing the demand averaging plan because the off-peak hourly usage charges are higher (almost what the non-solar ez3 plan rates are), so I calculated the best value for my $s to be insuring I use 0 watts during peak hours Monday-Friday no matter what. One bad day blows the bill for the whole month with SRPs aggressive demand plan, but the payoff is huge if I can zero the demand hours out, so I'm going a little overboard on the batteries for that reason. Also since for 6 months of the year I'll hardly exercise the battery I figure it will last a longer than for folks that fully cycle their battery(s) every day based on my experience with electric cars so far. I should only be fully fully cycling the battery the equivalent of 2-3 times per week, and only for about 12-14 weeks out of the year - plus have significant power outage backup when needed most of the time - but which is rarely needed around here to be perfectly honest. In 3 years of living here the powers maybe been out 2 times, an only once more than a few hours, the other time it was minutes.
Last bit of info and "theory" about the way I spec'd the system is in regard to
lithium battery health. It's well established that topping off lithium batteries has implications in terms of degrading battery life expectancy more quickly (though still great compared to other chemistries). Fully cycling the battery (from empty to full) also has battery health implications. It's also well established that running the battery to the low end of it's capacity also has life expectancy implications for lithium batteries, but maybe not as much so as always topping them off. Final factor for me, is lithium batteries can be safely charged faster at the lower ends of their capacity with less impact to the health of the battery, therefore can better make use of the incoming solar charge when less than 65-70% full. For all these reasons, I've heavily tilted the design of the system at my house so that with each passing day over the course of a single week the battery is slightly "less full" than the day before so that I'm at most only topping the system off once a week on the weekend. Also, on any given day I should only normally (on average) be cycling about 20-25 or less of the battery capacity, with every day that cycle being shifted ever more toward the middle to lower end of it's capacity range (on average) where the battery would prefer to be for best wear and tear. Take that with a grain of salt but there is fairly extensive research to back these principles up, and this is one investment worth maximizing.
Installation day for me is Sept 11th!
