It's an interesting idea, but your calculations are not quite correct. A 200w heater is not powerful enough.
To raise 1kg (1 litre) of water 1 deg C requires 4186 joules of energy.
therefore, to raise 300 litres 1 deg c requires 4186 x 300 = 1,255,800 joules
to raise 300 litres by 15 deg c would require 18,837,000 joules
Say 19 megajoules for ease. You need to deliver that amount of energy in about 6 hours.
Since 1watt = 1 joule per second:
6 hrs = 21,600 seconds
so you need a device able to deliver 879 watts of power for 6 hours.
So for just 300 litres, you'd need AT LEAST a 900w heater, not a 200w one.
Anyonw know how much it costs to run a 900W heater for 6 hours?