-
Want to remove ads? Register an account and login to see fewer ads, and become a Supporting Member to remove almost all ads.
To Powerwall or Not to Powerwall?
- Thread starter Fragger
- Start date
The balanced mode states that "Powerwall will charge from excess solar during off-peak and shoulder (if required)." This is consistent with what @getakey stated that you "cannot charge the PW at night during low rate. It can only charge from Solar. You can use PW to power house during Peak from charging during morning from solar."
The issue is that the peak/shoulder time can run into or past sunset, meaning there is no evening off-peak time for PW charging. So, you may find that your PWs are partly (or mostly, depending on the system and your settings) drained from the end of peak until the morning, when your PWs can charge during the off-peak period from sunrise until you hit peak or shoulder.
In your specific case with 4-9 peak, that seems a likely scenario. Every morning, your PWs will charge so that by 4:00 they are full. Then, from 4:00 to 9:00 they will discharge during your peak. They will remain at that lower level until sunrise the next day, when they can begin re-charging from solar. What Tesla claims they do is figure out how much you use during peak and how much you will generate in order to optimize PW use, consistent with your selection of balanced or cost savings.
Here is the equation.
1. Figure out how much per kwh you are paying. That's not quite as simple as it sounds, since there are so many different ways utilities charge. In my case its $.25 avg.
2. Next up, figure out what the size of system you are considering will produce per year. This is not easy either, since everyone lives in different places and has different roofs, but PVwatts estimates are good. In my case the XL system I have on order will do 25k kwh per year.
3. You have to assume something as to time period, I have assumed a 20 year window. Over that period of time the system will produce 500,000 kwh. If I were to pay for those kwh, at 25 cents per kwh, it will cost at least $125,000 over the 20 years.
4. If a person had 1 to 1 net metering (I don't) as the cost of solar is well under, WELL under $125k, simply click "order."
5. In my case, solar and powerwalls are BOTH under $125K. However, this is where you might pause. The Powerwalls cost extra. Its still cheaper than buying the power, but Powerwalls do enable you do use all of the power your panels generate and are a hedge against Time of Use Billing, where the utility buys from you at a low rate and sells back to you at night at a higher rate. If the utility does that, the reality is you would not (without powerwalls) get the full benefit of the Kwh your system produces.
6. Finally, you have to put a value on actual independence from the utility, and actual back up, like our posters from Florida.
1. Figure out how much per kwh you are paying. That's not quite as simple as it sounds, since there are so many different ways utilities charge. In my case its $.25 avg.
2. Next up, figure out what the size of system you are considering will produce per year. This is not easy either, since everyone lives in different places and has different roofs, but PVwatts estimates are good. In my case the XL system I have on order will do 25k kwh per year.
3. You have to assume something as to time period, I have assumed a 20 year window. Over that period of time the system will produce 500,000 kwh. If I were to pay for those kwh, at 25 cents per kwh, it will cost at least $125,000 over the 20 years.
4. If a person had 1 to 1 net metering (I don't) as the cost of solar is well under, WELL under $125k, simply click "order."
5. In my case, solar and powerwalls are BOTH under $125K. However, this is where you might pause. The Powerwalls cost extra. Its still cheaper than buying the power, but Powerwalls do enable you do use all of the power your panels generate and are a hedge against Time of Use Billing, where the utility buys from you at a low rate and sells back to you at night at a higher rate. If the utility does that, the reality is you would not (without powerwalls) get the full benefit of the Kwh your system produces.
6. Finally, you have to put a value on actual independence from the utility, and actual back up, like our posters from Florida.
If the Powerwalls are connected as a backup source without Solar, then you can charge from the grid. However to tax credit or SGIP would apply.
One minor change. You can add the 26% (2020), or 22% (2021), tax credit into the mix and go even higher on price and still come out ahead.
In my case, 16.32 solar plus 3 powerwalls including finance charges over twenty years is .20 per kwh.
Solar alone would have been roughly half, or .10 per kwh (the total amount of power is the same whether you get powerwalls or not).
That's why the first step is figuring usage and the utilities rates. Someone who only needs a smaller system has a lower cost. Then the math also depends on estimated production.
That's the real kicker. Two 16.32 systems might have vastly different production results for different locations and different roofs. Yet, the system cost is the same.
I've done a reverse type of calc. I've taken my Investment on Day 0 as a negative number. Then took my annual savings based on real production numbers. Over 20 years, I have an 18% rate of return and thats with no risk compared to other investments. Plus, if rates go up, my return will be higher
