Are you in California? Who is your power company? Aren't you on Time of Use Billing?
TIME OF USE BILLING IS THE PROBLEM !
The cost per kw is highest exactly the hottest moment of the day when your AC is running full blast... In july/august we had 100 to 110 degrees F for 2 months straight... so that dings enormously into the powerbill.
Here in the central valley you blast through the baseline allowance in the first week alone.
Tier 1 allowance is 20.9 kWh per day. @ 0.24373 per kilowatt. . That gives you a baseline if 152 $ per month. anything over that is billed at 0.30672 per kilowatt. In September that cost me another 188 $ ... for a total of 340$
In July my 'overage' was 1070 kWh ... cost : 328 $ My total consumption was 1721 kWh ... bill : 490$. In august i got dinged another 482$ ...
They ran an analysis and the two tier system was the cheapest ... the others would have come out higher.
Note that during this time i have not once charged my cars at home. I went to the superchargers cause they are free.
My array can theoretically produce 17Kw.
Taking into account a severe efficiency drop and reducing that to 10kW production and on top limiting that to 8 hours a day (even though in peak summer there is more than 12 hours of sunlight) . that is 80kWh / day
80 kWh x 30 days = 2400 kWH. More than what i use ! That extra 600 kWh is ending up in the storage.
The key in my system is the battery storage. In a traditional solar setup the array produces only during daylight. What you do not 'consume' is pure waste. So you still need the grid during the night. so if you base your efficiency on that it goes to snot !
That is why i opted for 4 powerwalls. What i do not 'consume' during solar production time is not lost : it ends up in storage. During the time my array is idle i run off the 'tanks'. The cars will be used as storage as well. Anything the array produces in excess of what i need goes into storage.
The biggest efficiency loss with solar is the nighttime... that's where the storage kicks in and washes that away.
On a winter day (shortest daylight , low sun ) the array should still be able to cover my consumption (my winterconsumption should be 60% of summer) and fill the storage array to 75%.. That is plenty to make it through the night and put plenty of mileage on the car every day.
If i do need powerdraw from the grid : fine. the net metering will reclaim that in summer.