Calculate the real rate of return
The rate of return expresses on a percentage basis how much an investment’s value has changed compared to its original cost. The higher the ROR, the better the investment. The ROR can be expressed in annualized form to make it easier to compare different investments on an equal basis. The better calculator for this problem is the internal-rate-of-return calculator. The IRR calculator calculates an annualized rate-of-return when there’s a cash flow involved – in your case, that would be the savings per year. But just a note, calculating the ROI might be more complex than just looking at the savings.