Payback on solar energy systems in Washington, DC and Maryland are already under 10 years for systems that are guaranteed to perform for 25+ years. In DC the payback can be as low as 5 years. The payback of any solar energy system depends on the local cost of electricity, the amount of sunlight that hits your roof, and how well solar is incentivized in your specific area.
By 2017, Clean Technica expects that both Maryland, DC, and VA will reach true grid parity. With the expiration of the 30% Federal tax credit at the end of 2015, the industry expects demand to surge over the next 16 months. See the article below for more information.