What is the lowest latitude for which if the ideal solar energy received each day for a year is graphed, the graph would contain one peak?
Any area south of the northern polar circle will have one peak per day,and the same goes for the southern polar circle any area north of this will have one peak per day,the polar circles is at 66,33 north/south.

The areas whitin the polar circles will have only one peak per year but it will last for half a year.

A solar panel will only produce significant power for a maximum 12 hrs even in the area of the midnight sun,This is due to the very low angle of the sun during the "night",or at the poles it will produce energy for 24 hrs but due to the suns angle a solar panel will produce a lower level of power than if the solar panel was at the equator.

Solar panels will be most effective if placed close to equator and less effective as you move away from equator,The suns rays have to penetrate less atmosphere at equator and a lot more closer to the poles.