payback calculator for power wall???

[ABCDE]

hi guys,

watching a lot of videos I've come to realize it sounds like most would or should opt for just adding more solar then adding batteries.. which aren't cheap, install costs and such..

wondering if anyone knows a site or something for payback calculator to add all variables for payback period?
-electric costs
-selling electric back at
-normal battery usage levels daily
-price of battery and install costs
-efficiency loss ( 10% )

is there just a simple way of calculating this I'm assuming.. take costs , then take say 80% capacity daily ( I can run air con and charging car to easily drain battery every night ) then just take what power company charges and input that in to then find years till payback?? hahaha

 

[power.saver]

Perhaps you should review this thread: Solar financial payback

 

[jrweiss98020]

You will never get payback for batteries unless you have time-of-use electric rates, and you can buy low and sell high (or use your batteries during the high-cost periods).

The simplest calculation is: How much will you save on your monthly bills? Divide that into the net cost of the system. Result is the number of months for payback. To estimate the first part, total up last year's electric bill and divide by 12. Find the minimum monthly charge from your electric company. The difference is your max monthly savings.

If you are also switching to an EV, you might also add in the savings in gas costs. A reasonable estimate may be about 10¢/mi for gas ($3/gal, 30 mpg) vs 2.5¢/mi for electricity (4mi/KWh, 10¢/KWh). Use your best estimates for your local costs and yearly mileage.

Yes, you can get VERY involved with return on investments, loan rates, etc. But then you would also have to KNOW what you are going to do with the money you save, or the result means nothing...

 

[wjgjr]

If you are able to get your historical data by hour from your utility (ours provides this in a nice CSV/Excel-ready format,) you could relatively easily estimate how much money you could theoretically save by using powerwalls to shift that usage to off-peak times. Of course, you would have to take into account things like weather that might mean your powerwall doesn't recharge fully every off-peak cycle and variability in your daily usage. For example, if you average 10kWh daily during peak hours but sometimes use 20kWh on hot days due to A/C, one powerwall (13.5 kWh max storage) would be drained, so you could not shift the entire load those days (without getting a 2nd powerwall, which obviously adds to the cost side of the calculation.)

 

[ABCDE]

thanks guy, I felt like just adding it into total costs, but people seemed to think you cant calculate it that way... not sure why or who's right or wrong, but I'm just seeing that its simple to just take total system costs, and as you said divide by max bill savings and figure out years for payback..

I guess I just feel my calculation is off when everyone says a power wall would have a crazy time for payoff as I have a system at $40,000 with power wall, and shows me in my math ( could totally be wrong ) that payoff is in 6.5 years.. maybe my math is wrong, maybe because our electric is so high... I don't know..

my electric for year is $4,000. yes Hawaii.. so monthly max save is $308. so payback on system after incentives ( $25,900 ) is 84 months or 7 years.. that's sounds fine for me with a battery and our panels are charged really high at a rate of about $5 a watt being in Hawaii...

 

[wjgjr]

I feel like there may be two different calculations being asked about - solar payback and powerwall payback - or maybe I misunderstand what you are meaning by max bill savings. Although installing both can bring added benefits, they really should have their payback calculated separately. For solar, quick payback is definitely possible, and relatively easy to calculate - and I think most installers provide an estimate for payback.

For powerwall, you need to determine the benefits it brings over solar alone. And, generally, that is by load shifting - moving your relative grid usage from peak to off-peak times with utilities that have TOU pricing and sufficiently advantageous net metering rules. There are certainly other benefits - we included powerwalls largely to provide uninterrupted power during outages, even though in our case these are unpredictable enough that it would be difficult to assign a specific value to them.

I'm not familiar with the net metering rules for Hawaii, but if there is a minimum monthly charge regardless of your generation, it is possible powerwalls will bring no added monetary benefit, depending on the size of your PV system.

 

[SoundDaTrumpet]

Mahalo @ABCDE ,

This is what I came up with for you:
1. You live in Hawaii. Assumption Oahu.
2. You are on some TOU or EV schedule. Assumption TOU or EV. Makes no difference anyway.
3. Rate arbitrage for solar from peak to partial-peak is only $0.03/kWh. Assumption based on residential TOU & EV, and that you plan to "Cost Savings" mode for Powerwall.
4. The peak rate only applies Monday thru Friday. Hence, no rate arbitrate on weekends.
5. Add ~85% efficiency.

I have exactly 1 year of PW data: (weak assumption that your usage is the same as mine in California). No EV charging captured in data. My annual usage is 7000 kWh.
1. Total PW discharge (only for peak shaving): 2500 kWh.
2. 2500 kWh * $0.03/kWh = $75 per year.

That's your answer. (skipped efficiency, but you get the idea.)

I hope this helps.

 

[miimura]

In Hawaii, solar & Powerwall payback is complicated if you have a non-export interconnect agreement. In this situation, there is no credit for surplus solar because you're not allowed to export at all. The solar inverters have to be configured to curtail the solar so that it never produces more than you're using at that moment. This greatly improves the payback of Powerwalls because you can time shift your generation to capture all your solar.