I think you need a 75 gallon model or, if your water supply isn't too hard, a tankless heater. With a 2.5 gpm shower head mixed at 2/3rds hot and 1/3 cold, a 50 gallon tank gives you 30 minutes of showering. At that point, you have 50 gallons of (at best) lukewarm water in the tank. You'd probably need to wait 30 minutes from that point until the water in the tank is again hot. If two are showering at once and no other hot water is being used, it'll start getting cool after about 10 minutes. If you're using a lot of hot water, there isn't going to be that much difference in operating expenses between the two sizes, you're going to be burning BTUs to heat water either way. You can easily cover the cost difference with the savings you'd have by replacing your old washer with a front loader.

if a shower is taken for 30 minutes.thay would be the 75 gallons of water.. sounds right but its not..the heater will actually give you only about 50-60 gallons of not water because the tank is filling with cold as you use the hot it is also mixing so..if you take a shower for 30 minutes you will deplete the heater, then it needs to reheat the water. but the next person jumps in the shower. he will get cooler hot water and less of it..it all boils down to the length of showers and how lon inbetween them. when your house was built a misunderstand of a water heater system happenrd instead of a huge heater seperat heaters for different baths would have been the way to go this is also the reason of your high gas bill you have a hugh vessal of water, the gas has got to keep that vessal at the temp you want it..every time you use hot water the heater has got to come on and heat up the water to thee desired temp. a larger vessal will cost more than a smaller vessal to keep warm.. my opinon..water heater to big...need to split the system upinto 2 systems,and..with the price of fuel.the way it is now. i suggest to switch to 2 50 gallon electric heaters AND the heaters get put on timmers. if you both work heater off 9pm-5am on 5am-8am off8am-4pm something like that the ages of your children also have a big factor in all of this. teen agers stay in the bathroom or on the phone i never thought i would be telling people to go with elec water heaters. but my fuel price went up ti $4.a gallon a few mths ago. went down..but i think it will go back up

"When you think about it 40 gallons isn't a lot of water." I disagree. That is a lot of HOT water at 140°. If you were to draw it all down at 2.5 GPM, it would take 8 minutes. ThatÂs theoretical, because the way you draw it is to push it with COLD water. But thatÂs OK, because you arenÂt going to take too many showers at 140°  one, maybe. So to make it comfortable, you mix in 40 gallons of water at say, room temperature, and now you have 16 minutes at 102°. All the time, the fire is burning in the heater. If you have not been drawing a quart of water from the bottom of your heater once a month (And I know you havenÂt.) then you may have a scale at the bottom. This fools the thermostat into thinking you have many gallons of hot water, when all you have is a thin scale of hot deposits. Pinoke

I have a Triangle Tube Series 50 (45 gal.) in a 2,000 sf house with 2 bathrooms. My plumber said I should put in a smaller unit and I wanted a bigger one. We compromised on this unit, because he said a bigger one would consume a lot more energy. I take a 25 min shower with a 5 gpm shower head, and I get enough hot water from this unit, but it took some doing. The first thing I would suggest is to turn the temperature on the water heater all the way up. We have an L-shaped ranch with the water heater on one end of the house, and the master bath on the other (over an unheated crawl space). So we need to keep the water very hot to get enough hot water at the bathroom. The hotter the water is, the more cold water you'll mix in, and the longer your hot water will last. Despite this, I was still running out of hot water after about 15 min. As my shower valve doesn't have a volume control, I put a small valve on the shower arm which allowed me to slow down or shut off the water when I was soaping myself. This allowed the hot water to recover a bit before I finished my shower. The real breakthrough came last summer though, when I had a new boiler installed. The installer told me that the circulator pipes between my boiler and water heater were only 3/4", when they should be 1 1/4". He also set up the new boiler so that the water heater gets priority over heat to the house, and the boiler automatically heats up to 180 degrees whenever the unit calls for more hot water. Now I have plenty of hot water for a 25 min shower followed by a shave, even without turning the volume of hot water down during my shower. I think you should be able to get enough hot water out of this unit provided you use it to its full capacity. You may need to have someone do some work on your boiler though. One rule of thumb I live by, is to never let a contractor tell me how much water or hot water I need. Aside from this, do you have separate shower valves for your 2 shower heads, and are you satisfied with the water flow? Many newer shower valves limit the flow to 2.5 gpm or less. You can check your actual water flow with a 2.5 or 5 gal bucket and a stopwatch. If you're getting enough flow out of the shower arms, but not out of the shower heads, there are some places that sell shower heads with easily removable flow restrIctors.