24 hours a day, obviously it's not working 24 hours a day, in the summer 2-4 hours is normal, 50 Watts should be sufficient for a 14 Gallon and yes a few Watt's her nor there really doesn't matter.
yeah...go for the 50w to be on the safe side. i always work in litres...sorry. i looked up a convertion calculator and it came out at 40something litres so will need to be a 50w. think they mostly come in 25/50/100/200w anyway so go for a 50w.