yes but i don't know for sure its 2.5KW, its an assumption
Use a better assumption and assume it has to use less current then the fuse in your fuse box. I'm not sure of the standards in Isreal, but that we be a 15A fuse in the states. If you can look in your fuse box, find the fuse for the circuit your water heater is on, I would imagine it is on a single circuit, and get a relay rated for that current. If it tried to draw more current the fuse would go first.