Need help with energy caluculations

[retropaul]

I need the formula for the following problem, but I have no idea how to figure it out.

Part 1

A person pays an average monthly electricity bill of x. If energy prices increase each year at y percent, what will there total energy expense be for z years?

Easy, I know. But its been a long time since I've been in school and I need to know this for work.

Part 2.

The same person decides to invest in energy saving home improvements. The improvements lead to a percent savings each month in their energy bill x. If energy prices increase as described in Senario #1 at y percent each year. What would be their overall savings b in z years?

In short, I need the formula that shows how much they would save over time on their bills if they invested in energy efficiency, given rising energy prices. I have an energy calculator that does this for me, but I need the formula behind it.

Any help will be much appreciated.

 

[mmm4444bot]

Here's what I come up with.

Total energy expense for z years with y% increase:

f = 12 * x * (1 + y/100)^z

Total energy expense for z years with discount a% on previous:

g = 12 * x * (1 + [y - a]/100)

Total savings over z years:

b = f - g