if you get a 11 inch immersion its usually around 3kw.
i have a 400 litre tank and i put it on a timer for approximately 4 hours,4.00am till 8.00 am and this will get the water hot.
You may get a few cold spots in the water this i think is down to the baffle in the tank,it stops the heat transferring thru the whole tank one way of getting around this is to buy a small agitator pump from a pet shop its like a small pump,actually i think its a small propeller and it will move the water around the tank,they are only 12 volt you put it into the top of the tank,if you dont get cold spots then dont bother.
i have used mine twice in the last week or two and it does make a lot of difference