The "important solar calculation" below has been turned into a handy online calculator. You can access it here.
Once you have had your site survey, you will receive a quotation for the installation and an estimated annual generation figure. The brochures for your panels will include figures for degradation in performance due to ageing. With these figures, you can determine the system output over its lifetime; and thence the equivalent cost per kWh treating the purchase of the generating system as a single bulk purchase of electricity.