How much wattage for heater?

I have a tak of dimensions 1.5 feet length. 1 feet breath and 1.5 feet depth it holds 18 to 20 liters water. I don't have time to switch the heater on and off regularly so please suggest wattage according_to that

I am having a new tank of 16.83 US gallons of dimenssions as follows
leangth = 18 inches
breath = 12 inches
height = 18 inches
I am going to aquascape the tank and want to know the wattage rating of the heate runit to be purchased as I won't be having much spare time due to my college and tutions to switch the filter on and off regularly and no else one at my house is interested in fish keeping so I have to do everything myself so suggest a wattage which will not harm my aquascape as well as fishes in the tank even if it is kept on for the entire day.
thankyou

 

for an 17 gal you could likely get away with as low as a 50 watt, but I'd shoot for a decent 100 watt with a proper thermostat built in. You don't want to overheat or underheat the tank. That being said what is the general ambient temp of the room the tank is in? If you need to heat the tank more you will need more wattage. If the ambient temp is around...say 70...you won't need to heat the tank as much.

 

for an 17 gal you could likely get away with as low as a 50 watt, but I'd shoot for a decent 100 watt with a proper thermostat built in. You don't want to overheat or underheat the tank. That being said what is the general ambient temp of the room the tank is in? If you need to heat the tank more you will need more wattage. If the ambient temp is around...say 70...you won't need to heat the tank as much.

the room temperature is 32°C

 

you don't switch the heater off and on. The thermostat does that all by itself. Anyway with a room temp of 32c you don't need a heater. You need a chiller!

 

32C is about 90F. So it is just a 'tad' too hot. You need to cool the tank, not heat it. Where are you located?

 

the room temperature is 32°C

You mean 22 C?