In a detailed economic analysis, energy buy back rate and its dependence on time (if any), amount of backup, and the type of system (purely electrical or electrical and thermal or IRES) should be considered. However, a simple, but useful first estimate of generation cost can be obtained by summing the capital recovery cost and the O&M cost, both on a per kWh basis. In Fig. 9, the capital recovery cost in $/kWh is plotted against the total installed cost in $/peak watt for typical values of capacity factors and interest rates assuming an amortization period of 20 years. The associated equation is