I have a steam heater of 357 kg/hr and i need to convert that into electrical heater so i need to know how much heater rating will be reqd.
If it is saturated steam, you have to know the temperature and or pressure out of the boiler. You also have to know the temperature on the return side. Then consult a steam table. You need to know the enthalpy difference between the two conditions, multiplied by the mass flow.
Let me say that this is enough steam that it should be evaluated professionally, by a heating engineer. (I'm not!!). A lot of heat is wasted in the steam distribution system, so installing the same electric capacity on a distributed basis may lead to excess capacity.
I will round your mass flow to 360 kg/h, which is 0.1 kg/s.
Heat transfer will be different in one-pipe and two-pipe systems, and depend on steam pressure or temperature. For a one pipe system, only the heat of vaporization is transferred, and pressure is generally only slightly above atmospheric. If we say 110 kPa (102.3 °C), the heat content would be 2250 kJ/kg. At 0.1 kg/s, this is
2250 kJ/kg x 0.1 kg/s = 225 kJ/s = 225 kW
(fuel usage would be somewhat higher, with heat going up the chimney)
In a two-pipe system with cooler condensate, this might be a little higher, around 245 kW being transferred.
These are crude estimates of the current situation and NOT an estimate of the electrical heating capacity required. Depending on details, which can only be evaluated onsite by a qualified engineer, I would expect a somewhat lower estimate of electric heat capacity required.