Remember that the equation formula for this is y = ab^x, which a = initial value and b = growth/decay.
Since this is an increase, you will be subtracting 1 from the growth percentage, which is 1.15. The quantity becomes 0.15.
To convert that into a percentage, just move the decimal 2 places to the right, which will be 15.
In short, the rate of increase is 15%.