lighting question

[pandafishowner]

My guppy tank lightbulb blew out last night. I went to Wal-Mart and grabbed a set of 2 15w bulbs. They also had 25w bulbs and I wasn't sure what watt mine was so I had to check some of the tank they sell. Some tanks said they included 15w, some said they included 25w. Does the watt difference really matter? I went wth 15w because they're cheaper actually, but could I have gone with 25, or would that be too bright? My tank is about 5g's or so. Just has the 3 guppies and a snail, all happy now without dying and I've replaced the filter too