Julie is considering installing solar photovoltaic panels on the roof of her house. Her monthly electricity bills currently average $80. The cost of installing a photovoltaic system is $17,500; however, she expects a 35% reduction in this cost due to tax credits and local rebates. Assuming all of her electrical needs are met by the new system and neglecting possible revenue when the system puts electricity back into the grid, what is the approximate payback period for the photovoltaic system