Inrush - Current Limiter (NTC) design calculation

Hi All,

Input : 230Vrms
Power : 15W
Vsurge = 4K

I want to calculate the value of NTC resistance value which can meet above requirement.

1. How to calculate the NTC resistance value ?
2. What energy rating we need to meet this requirement ?
3. What energy rating we need to meet the surge requirement ? and how to calculate the energy rating for voltage surge per IEC61000
-4-5?
Thanks in Advance