We have a 50 gallon hot water heater that came with our house. It fills my oversized tub halfway before the hot water runs out. Two people can't take a shower one after the other for any length of time, either. The water takes about 20 minutes to heat up again. Is there a size/type/brand recommendation to share?

I've heard mixed reviews on the "on demand" heating - inconsistent heating, etc...??

Thanks!