Online Calculator / Summary for Joomla

I need a calculator for an electrical business using a Joomla! website. The calculator will need to show the cost of a business's lights (they enter information about the amount of lights, type of lights) which will tell them how much it roughly uses and costs. The calculator will show a comparison between how much it will cost with a new energy efficient program being installed. It will show the savings/difference, and how quick their investment will be paid off. It need to sit inside a Joomla! website. It has to work using a mathematical equation and variable number (can be changed, if say cost of electricity goes up). It must be simple, effective and visually engaging, as well as informative to the prospective buyer.