9

1 if you use a bucket as tub

5000w runs an electric water heater, 1300w per S9 so on paper it'd take three 👀

Watts are watts. Take the wattage of your existing hot tub heater divide it by the wattage of the S9 (1323). If I had to guess, I would say 7.

If you put a S9 straight in your tub, you might get fried instantly...

just be careful