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Just trying to figure out the economics, thanks in advance
- Thread starter Southpasfan
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pdx_m3s
Active Member
Even with big TOU price variance, this is very hard to believe. A break-even ROI would mean that, over the life of the system, on a discounted cash basis, the up-front cost of the Power Walls offsets future TOU charges (not the total charge, just the TOU premium).
Even on an undiscounted cash flow basis, I don't see how a ~$15k investment (rough guess) can save you over $15k in TOU premiums.
I didn't say the TOU differentials would would bring the net cost to $0 during the product life. I said it helps the ROI.
My personal ROI calculation deducts a huge SGIP rebate, Federal ITC, pre-budgeted nat-gas generator applied to PW cost, and finally TOU arbitrage. After all that, my net cost will actually go to zero in about 7 years, assuming utility rates follow current patterns. PG&E did recently screw me on my solar ROI, but the TOU arbitrage is still roughly the same on an annual basis.
pdx_m3s
Active Member
Interesting. I haven't seen your math, so I'm not going to say you're wrong. But your scenario aside, I do think it's difficult to get the ROI to pencil-out in most net-metering scenarios.
There seems to be different types of "net metering". I'm on net metering with off peak, partial peak and peak rates. If you can shift your peak rate with PWs, then there is a benefit. With PWs, I'm able to use the PWs during Peak and send solar to grid at the peak rate.
What type of net metering does not do that?
What type of net metering does not do that?
pdx_m3s
Active Member
Again, you have to actually do the math to see if the up-front cost of PWs actually more than offsets future TOU premiums to determine if PWs are worth it from a cash-flow/ROI standpoint. Tesla will not do this math for you. They just want to sell you a PW. And the math isn't simple.
That is my general understanding of how true net metering works.
Where we are, it is unfortunate that they don't have TOU pricing for regular customers and the pilot program was closed before I could join. The pilot had about a $0.30/kWh difference between peak and off peak. Combined with the various discounts (Tesla credit, federal credit, state credit) this would have generated a positive return in under 10 years on the two PWs we got. We still wanted them for other uses, but it is definitely possible to generate a decent return, depending on the structure of the TOU pricing.
My understanding is the precise answer depends on your utility and its exact rules when you over-produce. Some will pay you the full amount while others may pay only a lower/wholesale amount or nothing at all.
But, assuming you have full net metering, even if you are over 100%, you can still make money by shifting from peak to off-peak. The basic strategy is to re-fill your PWs from solar during off-peak time (and, depending on your usage, this may mean pulling power from the grid to power your home) and then drain the PWs to power your house during peak hours (with solar being sent either to the grid or to supplement home power, if needed.)
I guess the thing to remember with full net metering is 0 is just a number - if you can go negative on usage, the utility pays you and if you can shift that negative usage more towards peak periods, the utility pays you more for that power. So, just as you save more money by getting closer to 0 usage, you make more by going more negative.
This doesn't address the ROI question, which depends on things like the difference between peak and off-peak pricing, as well as how much power you can generate and store during off-peak times, but, in principle, the ROI calculation should not be limited by the production number, depending on the utility's rules.
In parts of Florida served by Duke, net metering is allowed on the tiered rate plan so customer-owned generation is credited based on kWh rather than dollars as production * rate-when-produced. We pay a slightly higher rate from 1,000 kWh to infinity, 7.856 energy charge + 4.067 fuel charge cents over 1,000 vs. 6.167+3.067 for the first 1,000.
Duke has ToU, it's 19.045 peak, 1.058 off-peak cents, plus fuel charge. That would be a case for PWs, but when a watt = a watt regardless of when it is absorbed by the grid, Duke is the battery.
Duke has ToU, it's 19.045 peak, 1.058 off-peak cents, plus fuel charge. That would be a case for PWs, but when a watt = a watt regardless of when it is absorbed by the grid, Duke is the battery.
I don’t believe that TOU is an option for Florida Duke customers now. At least it’s not for me here in Volusia county. I believe they did offer it at one time and if you were on it you could stay on it, but I don’t believe that anyone can switch to a TOU plan on Duke in Florida right now.
It is hard to do much analysis without understanding the rate plans available to you. I can almost guarantee that the power walls will destroy the ROI proposition unless SGIP returns in a meaningful way. Totally agree with earlier posts that you should really ROI the solar and power walls separately.
It is critical to understand when your energy consumption is to really understand potential ROI. If you have EVs, you are scheduling their charging overnight during super-off peak hours. There won't be any value from the power walls in that equation and possibly limited value even with solar in that regard. SDGE has a rate plan for EV owners at about $0.09/kWh...that is a small gap between what solar will provide so that would dramatically push out your ROI. But I have no idea what rate plans are like in LA.
I'm guessing you have summer/winter rates and that during winter rates cost doesn't vary much between TOU. Summer rates likely have a much larger gap, so again, in terms of power wall ROI, that's really the only important thing to look at...summer peak usage.
I'd expect AC is your only significant draw during peak summer hours. Find out how much you used over the past few years if you can. Figure out the $ difference between peak and off-peak for that amount of power and you have a decent baseline of what the power walls may recover for you each year. Unless you have terrible plans, it won't be anywhere close to worthwhile. Again, if SGIP is around and you can get the power walls largely subsidized, it becomes a no-brainer.
Also, if you have frequent power outages or are particularly sensitive to them, power walls are freakin' awesome. Four power walls is a massive setup...I can't imagine that making any financial sense at all.
It is critical to understand when your energy consumption is to really understand potential ROI. If you have EVs, you are scheduling their charging overnight during super-off peak hours. There won't be any value from the power walls in that equation and possibly limited value even with solar in that regard. SDGE has a rate plan for EV owners at about $0.09/kWh...that is a small gap between what solar will provide so that would dramatically push out your ROI. But I have no idea what rate plans are like in LA.
I'm guessing you have summer/winter rates and that during winter rates cost doesn't vary much between TOU. Summer rates likely have a much larger gap, so again, in terms of power wall ROI, that's really the only important thing to look at...summer peak usage.
I'd expect AC is your only significant draw during peak summer hours. Find out how much you used over the past few years if you can. Figure out the $ difference between peak and off-peak for that amount of power and you have a decent baseline of what the power walls may recover for you each year. Unless you have terrible plans, it won't be anywhere close to worthwhile. Again, if SGIP is around and you can get the power walls largely subsidized, it becomes a no-brainer.
Also, if you have frequent power outages or are particularly sensitive to them, power walls are freakin' awesome. Four power walls is a massive setup...I can't imagine that making any financial sense at all.
For us, its not going to be four powerwalls now that I have done some research. That number is only for someone who is in a situation where going off grid for days is a regular occurance. In Los Angeles, outages have been measured in hours, not days. If we end up going with two Powerwalls its really a luxury. I don't think there is any other way to realistically look at it. Everyone's time is worth something, the amount of energy arbitrage one would need to do to work the difference in a TOU plan does not seem worth it either.
This article suggests that those of us who live in Tier 2 & 3 fire risk areas can potentially get the full cost of the system reimbursed (up to 14k)?
The diff between winter ($100/month) to peak summer is drastic for me ($500/month). However, I still have a hard time wrapping my head around how this works. Let me explain what I *think* is happening and maybe you can tell me what I'm missing:
* solar generates electricity shoves it into SCE's grid. It doesnt matter what time this electricity is generated, it generates "credits" per kW
* your house continually uses electricity straight from the grid, despite generating electricity from solar
* even though you generate electricity credits, these credits aren't 1-to-1? Meaning a credit of kW generated doesn't equal to 1kW used during peak summer months?
* by pulling from batteries during those peak TOU hours, you can get the trading rate of kW credits to real consumed kW closer to 1-to-1?
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its not based on kWh, its based on cost
easier to think about it with real numbers
my partial peak rate is $.27/kWh; peak is $.53/kWh (2pm to 9pm)
With solar only, I generate more than I consume from sunrise to 2pm and send bac grid at .27/kWh. From 2pm to sunset, I do generate more than I consume and get credit at .53/kWh, but I'm using my solar to power the house so its not the full solar going back to the grid. After sunset to 9pm I consume from grid at .53/kWh
Now if I add PW, I charge PW at the partial peak rate instead of sending to grid until 2pm. (I'm usually fully charged by noon). At 2pm, I start consuming from PW and I'm sending 100% of solar to grid at .53/kWh. I can do this all the way to 9pm unless it gets hotter than 95 degrees. In this case, I make it until about 7:30pm. So I have exchanged PW charged at .27/kWh for solar credit at .53/kWh
easier to think about it with real numbers
my partial peak rate is $.27/kWh; peak is $.53/kWh (2pm to 9pm)
With solar only, I generate more than I consume from sunrise to 2pm and send bac grid at .27/kWh. From 2pm to sunset, I do generate more than I consume and get credit at .53/kWh, but I'm using my solar to power the house so its not the full solar going back to the grid. After sunset to 9pm I consume from grid at .53/kWh
Now if I add PW, I charge PW at the partial peak rate instead of sending to grid until 2pm. (I'm usually fully charged by noon). At 2pm, I start consuming from PW and I'm sending 100% of solar to grid at .53/kWh. I can do this all the way to 9pm unless it gets hotter than 95 degrees. In this case, I make it until about 7:30pm. So I have exchanged PW charged at .27/kWh for solar credit at .53/kWh
pdx_m3s
Active Member
It depends on utility. Here in Portland it is net-metered based on kWh. TOU is an opt-in.
