Short Answer:
Things are working out as calculated, for our 4.1kW PV and 4x Powerwall system. Some comparisons:
July 2020 Bill: $169.53; 2,883kWhr July 2019 Bill: $411.70; 3,410kWhr
Winter Bills (Dec-April) were around $45-70, where as previous years those months were 100-160. Our peak summer bills were close to $500 in one or two months last year. This year I'm expecting no bills over $190.
I'm calculating my break even point to be 4.6 years from when we installed our PW & PV system and it became truly operational last November, on a system that cost us $43k.
That's the short answer, but it not very informative in my opinion, but for more info on how I'm calculating our particular break even point and how I use the system, read on.
TLDR Answer:
A few factors changed significantly when considering our 2019/2020 July bill comparisons, we are in a much heavier usage scenario at our house as compared to last year, so if we did not have solar now, for this years July bill, a "no-solar" version of our bill would have been much closer to $500 or higher.
- This year, my daughter and her spouse are living with us while they work on their masters degrees (and save for a house), we now have 4 people living/working 24/7 at hour house in 2020 since early march, COVID driving us all to be working at home full time, where as in 2020 it was only my wife and I and we worked outside of the home at least half the time during the weekdays, and usually we ate out together in the evenings somewhere around town with each other and/or friends, depending on where we were working, often not returning until after peak, so we'd have the AC slightly higher until then through much of the peak periods.
- Also, for the last 3 weeks of our July bill we were driving full time on only electric since we added the Model Y to the garage (we love it!!), in addition to our existing Chevy Volt. In 2019, we only had the Volt, and it usually only got a single 12kWhr charge per day at most.
Even with heavier usage this year, we're on track to shave about $2000 off our annual $3000 SRP bill, even with much higher home living/usage and car charging. I'm estimating the SRP billing savings are conservatively more like $2500+ without solar if we added our extra usage this year to last years bill.
And that's before I account for the $3000+/year in truck gas ($65-75/week) that I don't buy anymore now that we've added the model Y and charge it at the extremely low SRP off peak rates. It's looking like the Model Y will only cost about $350 in electricity each year to run. So yeah, the Model Y is adding about $20/month to the electric bill I didn't have last year, but it's saving around $300+/month in gas we're not buying each month. We were going to be buying a new SUV for my wife no matter what, and were looking at slightly larger mid-sized SUVs in about the same cost range as the model Y, so that's all a wash as far as I'm concerned. The Model y is a good size for us.
Typical Solar/PV Usage Weekday Scenario:
July 7th was a fairly typical peak summer day in terms of usage, with mostly clear weather, and you can see (attached photos) we only used the PW during the SRP Peak period which has stiff "demand" penalties. We've been traveling for the last week, so those are not very typical days, so I had to go back to the 7th. So the PWs allow me to easily zero out any Peak usage while still keeping my 3700sqr ft home reasonably cool, I don't like being hot so we don't suffer in a warm house. At most we've had little 0.1 kWhrs "Peak" period blips for what ever reason, not sure what the PWs are doing when that occurs.
I only schedule the use of the PWs during peak periods, otherwise the SRP solar off peak rate is so low it's not worth the wear & tear on the PWs to cover off-peak home usage. In addition with the electric cars starting charging after Peak period, and AC's turning down to the "supercool" levels that I like in the evening, it means we're using up to 20kWhrs non stop for 1-2 hours immediately after the peak period, so at most we'd get an hour or less out of the PWs if I used them during off-peak. I'd rather save that 20-25k of reserve kWhrs for emergencies and/or to cover peak periods safely during a series of cloudy days.
Most weekends I turn the PWS off after our Friday night peak period at about 40% SOC, and turn them back on Sunday night or Monday morning - that results in 104 days each year that I'm not cycling the PWs at all (52*2). That should result in significant PW life extension, and value/investment extension. So on weekends all solar goes to power the house, or goes back to the grid for a 1:1 offpeak credit. This also avoids the expected round trip losses to/from the PW on the weekends. If we're traveling and I can't physically turn them off on the weekend, I just set the PWs to balanced mode on the weekends with a target reserve of 45%, and the PW will generally in the summer never peak much higher than 55%, and end the day at my set level of 45% reserve as it powers the house first, and then sends any extra to the PW through out the day, only drawing down to 45%. Again, this minimize depth of charge cycles and cycling losses, while keeping the lithium batteries in their optimal cycle depth range for capacity loss purposes.
An Example Normal Daily Usage Scenario: On a day that hit 106 degrees, during peak (see attached photos) we drained 19kWhr during our SRP 2-8pm utility peak period and used NO grid power, mostly just cooling the house to keep it reasonably comfortable, 2 AC units, 3 zones in the house. Without AC we would have only used about 4-5kWhrs during that peak period as I turn just about everything else off via home automation (Hubitat and Smartthings) during peak periods. As you can see we used 19.4 from the PW and were able to fill it with 20.8kWr before the peak period, at which time the solar started powering the house, and only adding a trickle to the batteries after 2pm as the production starts dropping pretty fast down for us after about 3-4pm. Total solar production was 26kW, so about 6kWhr went directly to powering the house during the day after the PWs hit my target SOC, and during the peak period.
Again, this is for 4 adults living/working 24/7, in a 3700 square foot house. Also, I have a 3rd AC unit on my garage, that cools about 1000 square feet of garage, but mostly I don't cool that during the peak periods - so all that power usage for the shop is just using off peak power 3-4 days a week including mostly weekends.
PV Production can go as high as 28kW on rare and unusually cool summer days, but with our heat over 109ish most days now, it's usually around 25-26 of production at most as the heat noticeably reduces production.
Cloudy Days and PW sizing:
We did have a cloudy day last week, and production dropped to ~15kWhrs for the day, and on the very rare dark day I've seen production get as low as 8-10kWhrs, but that's happened once since November. It is exactly because of these cloudy days, and because of monsoon season here in Phx that I wanted to insure I had 4PWs to handle our summer AC peak needs across multiple cloudy days, and have just enough Solar PV production to fill them enough to handle my SRP peak periods with reasonable AC usage during those peaks. As eliminating peak usage is were 90% of my savings and return on investment come from. So right now most days I have the PW settings set in such a way that by the time my peak period hits at 2pm the PW is hitting around 80-85% SOC at most, at which point I have it configured (via @Darwins smarthings app) to start powering the house but continue sending any left over PV to the PW, in this scenario often the system will often add another 5% to the PWs from the remaining PV after powering the house until PV ramps down for the day. When the forecast shows cloudy days coming, I reconfigure the system to charge up to 95+% leading up to the cloud days to maximize buffer for upcoming cloudy days.
Normally, on clear days the PWs are draining down to around 50-60% from my target peak SOC of 80-90% each day during my peak week day periods. However, if we have (for example) two or three cloudy days in a row, I might only get back to 80% SOC the first cloudy day, and it will drain down to ~50% SOC by end of that first cloud day peak period, and on the 2nd cloud day I might only get back to a high 70% SOC and by the end of the 2nd cloudy peak day I might be at ~40% drain-down, and with 3 cloudy days I'd probably drain down ton 30%, and so on.
Ideally, in normal operation I like the idea of having 30-40+% capacity remaining (so about 20kWrs total in the 4xPWs) for emergency backup, or to handle say 3 or 4 cloud days in a row during the week. So far the system is behaving essentially as I had modeled, actually slightly better, but I'm program/project manager in my day job, so I usually estimate and model pessimistically. I thought once temps regularly hit around 110 each day, I'd be falling behind by 5-10% each day and would have to catch up on the weekends, but it turns out if weather is clear, we're usually restoring what we use each day to the PWs with a little left over to power the house directly from the PVs before diving into fully powering the house on only the PWs during the peak period.
Could I have done this with 3 PWs & thoughts on PW Cycle depth?
Yes, I think I could have! But....
I probably could have accomplished all this with only 3 PWs, and seen essentially the same savings, but with just a bit more risk having less capacity "margin". Also, I could make all this easier by allowing the system to charge to 100% everyday and not need custom automation do keep the PWs in my prefered 30-75% SOC range most days. However, I've worked with DIY lithium batteries extensively (mostly 18650s and larger format flat package batteries) over the years for my own hobby/projects, RC cards and planes, started to build an electric car, etc - and I know that keeping the the usage of these batteries in the 30-75% SOC window can potentially double the life of the batteries based on my actual experience. Given my experience, I had a hard time not spec'ing my system to allow me to do this during normal usage with the PWs. The side benefit being also having some significant back-up capacity on all but the most rare weeks where we have lots of cloudiness.
Also, strangely, the money I'd have saved on only buying 3 PWs but the same amount of solar would have reduced my 4.6 year pay-back period by only about 10-12 months.... but I'd be working the 3 PWs much more extensively across their full SOC/capacity range, I'd have little or no "left-over" capacity for emergency back-up on a daily basis, and based on my experience I'd be reducing the life of the PWs on what is a relatively large investment. For less than a year of additional payback period I can potentially can add up to 5 years, maybe more, to their 10 warrantied life.
There are no guarantees, but I've seen the difference between batteries worked hard daily, and those that are managed more carefully, and it's an overwhelming difference.
Will I get more PV capacity (beyond our current 4.1kW system):
Maybe. I do financial analysis almost every day as part of my job calculating ROI on development investment opportunities, and then working with program teams to execute on those plans. And I can't help doing the same in my own life. So if I take my investment of $43k for the 4.1k PV system, and 4xPWs, break even is in about 5 years if I were to sell my house - since the system adds around 50% the cost of the system to the value of the home, up to about $20k. It seems home buyers won't calculate much more value add than about $20k even for very expensive systems (wife is in a Realtor). (Per year savings $2000bill savings+$3000gas savings+20k added home value). We all know the value, but there are limits. Just like somebody's 100k pool might be worth it to them, but to most buyers they'll only add 20-30k at most for the value of the pool, no matter how much it cost originally. So, my ROI focus is on when do I effectively get my money back, break even, if for any reason I need to sell this house that I otherwise plan to retire in. The answer, based on our actual observed savings since last October is 4.6 years or less from when I installed the system.
Beyond the base system we already installed, right now the better payoff for us is to send every extra dollar toward paying off our house mortgage, before spending another 12k adding solar. So I'm going to wait a couple years, and see how much progress we make on paying down our mortgage, and maybe by then solar prices will have come down more too, and we'll look into adding more solar because I still think it's the right thing to do, and the idea of getting closer to zero annual electric bill is attractive to me. But for now, with off peak Solar plan rates around 4cents/kWhrs the payoff for that additional solar becomes a much longer time frame. I think that adding 8k of solar costs about $12k (after tax incentives), so it would take about 18 years to break even on since I'd only be offsetting my offpeak usage (at under 4cents per kWhr) as my on-peak usage is already at zero, and it really won't effectively add much more value to the house than what the base system already adds (just purely from a real estate resale point of view).
I would love to add the PV, just because I do think it's the "right" and responsible thing to do for my defendants and those that come after us, but I'm also near retirement and need to be practical, and focus on eliminating all my monthly costs & bills by the time I turn in my retirement resignation some 5-10 years from now. Our initial solar was definitely part of reducing those costs, but the payback was more immediate, and ongoing, since I didn't finance or lease the system.
How Would I configure Additional Solar? (if I ever added it)
One last thought, I think if I added more solar, I wouldn't want/need any of that solar going to the PWs to charge them any faster. They are already operating in the ideal range covering all of our Peak usage needs with some buffer at all times already. So if I added Solar, I'd want it to only go toward powering the house first, and any additional be sent to the grid for a 1:1 credit offset for my off-peak power usage since I'm already more than covering 100% of my peak coverage for the entire year.
I'm not sure if I added 4 or 8kWs of PV, if Tesla and SRP would allow the additional 8kW of PV (over and above my existing 4.1k) to only go to the house or as feedback to SRP for credit. I kind of think the way they have it setup with the meters, I'd be required to configure it to go the PW first, and I don't want or need the PWs charging at a faster/higher rate - it's optimal for longevity and wear & tear to charge them only at the minimal rate needed to achieve capacity for covering 100% of our peak needs, which is already occurring with our 4.1kW PV system.
But I vaguely recall reading somewhere Tesla can set that max charge rate, so if they can do that, maybe the point it mute.
