70 miles x 0.4 kw = 28kw x $0.0522 per kilowatt hour = $1.40 per night to charge
$1.40 per night X 5 nights (typical work week) = $7.00 a week.
$7.00 week X 52 weeks = $364 a year / 12 months = $30.33 a month
Compared to:
70 miles a day / 20MPG (average of both trucks) = 3.5 gallons X $4.00 a gallon = $14.00 a day x 5 days = $70 week
$70 week x weeks = 3,640 a year / 12 months = $303.33 a month
$270 a month savings divided into $10,000 (est build cost) = Pays for itself in 37 months.
The next year will be gravy and the following year batteries will need replacement at a cost of $1000. 4 months later I am back in black.
The big bonus here which you cannot put a price on, I am not using gasoline anymore.
Source of my kwh rates:
http://www.fpl.com/rates/pdf/electric_tariff_section8.pdf