Start with the shortfall
Describe when hot water runs short
Note when hot water runs out or turns lukewarm: back-to-back showers in the morning, laundry running alongside a bath or the dishwasher during the evening. The times and the combinations of fixtures say more about demand than the size of the old tank.
Mention whether the household has changed since the current heater went in. More people or different routines can mean the old capacity no longer fits, while a heater that always kept up may point to a like-for-like replacement.
