Try this math:
I get 15-17 miles per charge on my 330e and ~37 mpg driving without charging. If you choose the right SCE plan, you can charge between 10pm-8am for 13c per kWh in winter and 12 per in summer. Your ~15 miles will cost about $1.10 then. Without charging ever, using a gallon of gas you’ll go about 37 miles at a cost of $3.75 each gallon (at Costco in SoCal)
37/15x$1.10=$2.71 in electric cost for those 37 miles. If you save 3.75-2.71= ~$1 per 37 miles by charging at home and drive 10000 miles a year that’s 10000/37x$1=$270/year.
If you’re only getting 30 mpg on gas alone, figure $2.20 to charge what will cost $3.75 in gas to go 30 miles, then your annual cost savings (charging with the cheap after-hours plan) is about $517/yr