It depends on the energy prices in any given year, but typically it's between three and four years, sometimes five years. With a geothermal system, for example, like the Verdant, the cost just for the system itself is a little higher. But usually, to keep the costs down for the occupants so they don't have to pay any additional costs, it could be around 10 to 15 years, but then after that your heat is essentially free. During that time you don't pay any more because it's paid from the energy savings.
On March 10th, 2009. See this statement in context.