Well you should only need the head to initially lift the water to the roof. Once you have flow established the height of the roof has no bearing as you effectively are only lifting to the surface of the pool. You create a siphon pulling down from the roof which cancels out the lift.

As for time of day pricing you will probably find that if you put a cover on the pool at night you will have no need for the solar panels in the peak summer months and will be able to run the pump at night as you have done in the past.