The 40-gallon water heater is the volume size for 2–3 person households — second only to 50-gallon in residential install volume. The 40G saves $30–$80 upfront vs the 50G and fits the same standard 22"-diameter footprint with slightly less height. For 1-bathroom homes and smaller households, 40G is often the right sizing.
The marginal cost of stepping up from 40G to 50G is small enough that most buyers default to 50G as insurance against future demand growth. The 40G is the right pick when capacity needs are genuinely modest.
For 40G-replacement applications considering tankless:
The 40-gallon water heater is the right pick for 2–3 person households where capacity needs are modest. For long-horizon owners, step up to 50G as cheap demand insurance. For all-electric service, evaluate heat pump at the same capacity for 3x lower operating cost. For sizing-up applications, see 50-gallon or 75-gallon categories.