Storage-tank capacity: Tank volume matters, but so does first-hour rating: how much hot water a fully heated unit can deliver over an hour. Compare that rating with peak household demand.
Tankless output: Selection depends on the simultaneous flow required and the temperature rise needed to heat incoming water. Colder incoming water in winter can affect the usable flow rate.
Fuel source and connections: A change in equipment may require plumbing, electrical, fuel-supply or venting alterations. Those changes should be part of the installation assessment.
Installation location: Space, access and the requirements of the selected appliance affect where the unit can be installed. A heat pump water heater, for example, needs suitable surrounding-air conditions and space.