Heaters

[ov10pat]

As heaters are the weakest link in our tanks, would it be a safer route to go and put the smallest heater that will keep the water up to the temp desired? Even if it stayed on all the time? I know that we could use controllers for extra insurance but that is at a higher cost. What I was thinking of doing was trying to see if a 150 W heater will heat my 90 gallon 11 degrees above room temp. My thinking is that if the heater fails it wouldnt be strong enough to heat it much past that.
Given that it takes a specific amount of energy(electricity) to heat a given volume of water to a given temperature, as long as the heater can supply that energy would it be a better(safer) alternative to buying a heater that is rated in the 3-5 watts per gallon rule. What about 1.7 watts per gallon? Has anyone had any experience with this? It seems to me that if we use a device that can cook our fish and plants that this device is maybe in excess of what is really needed. I know a larger heater can respond to temp fluctuation quicker. But if the room that the tank is in is always a certain temp where would the temp fluctuation come from? The lights?

Thanks,
Patrick