Good news,good news.
We have the portable solar chargers available in 12,16 and 21Watt versions now.
These are for parked cars.
So may you have a car you don't drive so often you might want to consider to have one of these, unless you want to call for a jump-start and reduce the life-time of your battery significantly.
Not to mention fleets like rental companies, dealerships...etc.
More about garage equipment and solar chargers,click here