I started looking at a system just to charge the Leaf, but then added 2 Volts.
I ended up with a 6kW system for just under $10K as a prepaid 20 year lease. The economics of a smaller or even a larger system didn't make sense, we had to find the sweet spot with regard to cost/benefit.
Since November here in Plano it has averaged just over 24kWh/day with about 2.5kWh going back into the grid for a net for 21.5kWh.
I don't know if the prepaid lease deals are still there, but it was very easy (they worry about all the rebate/subsidy paperwork, etc.). I know SolarCity said they weren't able to do those in Texas at the time, but were doing them elsewhere in the country. Ours was coordinated by a local installer but NRG wrote the lease.
My electric rates get real low in the winter (4.2c/kWh anyone?) so the ROI is kind of tough but I basically prepaid 20 years worth of electricity at less than 7c/kWh so it has the potential to pay for itself.
White 2014 BMW i3 - Aerovironment EVSE
White Diamond Tricoat [2012 Volt - Blink EVSE
Black 2014 Volt - Blink EVSE
6kW Solar array