I see. I seem to be getting conflicting information from Tesla on this:
Powerwall Time-Based Control | Tesla Support says in balance mode (available with Solar only) that it does perform off peak charging. This seems to imply load shifting is possible. Maybe I am reading this wrong ?
Yet this page Combining Systems | Powerwall Support explicitly says you cannot charge from grid when using solar, supporting your assertion.
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
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.
3 years of bills, average is under $.20.