ROI and profitability or feasability tool for a non financial decision maker

Help - I would like an excel type spreadsheet to allow me to input a number of variables that affect the decison to install a solar power generation system. Once the basic data is input I would like to see a summary that could also be used to present the feasibility of the project.

Variable are Capital, Loan facility or ratio, interest rate and amortisation period. There will be things such as RPI and an electricity price index which affect the long term cost benefit. The presentation is for up to 25 years forecast.

May be easy for a finance wizard but I am a sales person that would like to show customers how viable or not a Solar power system may be.