|Photo from abb.com, original from Edward Csanyi of Electrical Engineering Portal|
Solar energy must interface with existing power systems. Alternating current is used for transmission because there are fewer losses because it is very easy to transform between different voltages. Heat is dissipated when large currents heat wires, increasing their resistance. With an AC signal, currents can be kept low by transmitting high-voltage signals, decreasing losses.
Solar cells do not produce a consistent amount of energy throughout the day. Additionally, they produce little energy at night when one often needs electricity. Consequently, the energy from the solar cell must be either stored in a battery or fed back into the grid. It is more common to feed the energy back into the grid as an AC signal due to existing payment schemes and challenges/costs and safety risks associated with storing large amounts of energy. Since these solar cells are connected to the larger power grid, the DC signal that the solar cells produce must be converted to an AC signal.
Furthermore, since many household appliances are designed to operate with an AC signal, it would not be feasible to completely remove a house from the grid and operate the building on DC energy. Already, since computers run on DC signals, you need an AC to DC converter. If one switched to all DC circuits, one would need DC to AC converters. Since most of what we have now either relies on an AC signal or has an incorporated AC to DC converter, it would be expensive to switch to DC electronics.