This is the most frequently asked question by any customer considering any renewable technology. What they want to know is how long will it take to repay their capital investment based on the energy savings that a system will offer them. Inevitably this is not a simple question with a straightforward answer.
Talking to most customers, they seem reasonably satisfied if they can achieve a payback period in less than 10 years. The simplest way to calulate payback (ignoring the effects of interest rates) you will need to understand how much energy the system you are installing will save the customer in unburned fuel and a prediction of future energy price inflation. Of course we can't know exactly what energy is going to cost in the future, but a reasonable forecasting method would allow for a spread of constant annual increases at various percentages from 5%-30%.
Of course the simplest way to enhance the payback period is to be able to offer systems at a competitive and affordable price to everyone.