What would you use the hydrogen plant for? For the same amount of solar energy, a Tesla is going about 3x further than a Mirai, and a heat pump would provide much more heat your house.
Open fuel cells storage can be much cheaper than battery based storage. You can store energy for a long period, paying by the maximum power, not storage capacity.
That said, molten salt cells look way more promising than hydrogen.
Mirai is one of the first production HFC vehicles, well below what's feasible. Total solar energy is not really a constraint, refueling time and amount of stored energy are.
Total solar energy is a constraint, as someone has to pay for the cells and to have space to set them up. If you want to power your own car, don't you think it makes a difference whether you need 1x or 3x the area covered with solar cells?
Open fuel cells storage can be much cheaper than battery based storage. You can store energy for a long period, paying by the maximum power, not storage capacity.
That said, molten salt cells look way more promising than hydrogen.