I just had my 10-year old State 50 gallon, 40K BTU water heater repaired this morning. There was water on the floor in front of it and I thought the worst based on its age. It turned out to be a coroded Pressure Relief Valve.
Anyway, my 2 story + basement home has 2 full baths, 1 half bath, + a full bath in the basement (stall shower hardly used, toilet and sink used constantly as I work from home with my office in the basement as well). In the Master Bath (2 floor above basement) there is a large tub that we've never really used because the hot water would run out (pumped from the basement located water heater) before it got filled up. I have 2 kids: 1 in 8th, 1 in 10th plus my wife and I. The reason I mention that is that starting next year, for 2 years, the kids will both be showering at the same time in the morning as they'll both be in high school). Probably 15-30 minutes after they finish showering either my wife or I will shower, one after the other, and I will be shaving.
At times in the past the hot water has not lasted long enough. Seeing as my water heater will probably have to be replaced within the next couple of years ("normal" failure, I suppose) do you feel that I should replace it with the same 50 Gallon/40K BTU or go larger? The plumber who was here feels that it is the right size for my home and that a larger i.e. 75 Gallon tank would just waste energy. If my present tank fails under my service contract they will replace it but I have no control over brand, efficiency, etc.