Sizing a water heater starts with the people in the house, not the catalog. Use this to work out what the household asks of the heater on a busy morning, and what the space and hookups allow.
Count the peak hours
Write down the times hot water gets used most. Showers, laundry, the dishwasher, and hand washing at the sink often overlap in the morning or the evening. Note which uses overlap and which run alone. A household that stacks showers and a load of laundry into the same stretch of time needs more hot water on hand than one that spreads them out.
Ask about first-hour delivery
Installers and manufacturers often describe a heater's output by how much hot water it can deliver in the first hour of use and how quickly it recovers after. Ask for those figures in writing for each model you consider. Compare them against your peak hours, not against the largest figure on the sheet.
Plan for change in the house
Think about who will live in the house over the coming years. A new baby, an older parent moving in, or a home office that adds daytime use all change the peak. Size for the household you expect to have when the heater nears the end of its typical life, not only the household you have on install day.
Check the space and hookups
Measure the space for the unit, the clearances around it, and the path for the drain pan. Confirm the fuel type matches the hookup you already have. Check the venting for a gas unit and the power supply for an electric one. Ask who will install it and whether permits are needed where you live.
Set the planning date
Record the install date and model in your home file. The install date is your starting point for planning the next change. The ranking page gives context for planning windows across the major appliances: The major appliances, ranked by typical service life.