Another thing to consider is
Another thing to consider is improved battery technology and falling prices. I'm sure many Leaf owners would swap out their batteries much sooner than 2020 if a new battery in the same space could provide more range for a reasonable price. Let's say in five years the power density is increased by 30% and the cost is down to $125 per kWhr. A 38 kWhr pack would travel 125 miles instead of the by then degraded 85 miles and cost about $5000. The residual value might be $90 per kWhr for home use, or about $2000, so the upgrade would have a net cost of $3000. If in 5 years solar is down to $1 watt installed, then 6 KW of panels should be enough for most people, so going off grid allowing for $2000 worth of used car battery would cost about $8000. 1200 W ave at 8 cent per kWhr is 10 cents per hour, 2.40 per day, $75 per month, $900 per year, $8100 per 9 years. Since such a system would last at least twice as long, maybe three times as long, so the cost of power would be about 4 cents per kWhr, allowing for occasional upgrading of the battery and panel capacities.