How the math works
Tanklessly mathPeak-hour demand = the uses in your busiest hour × AHRI's average gallons per use: shower 20, bath 20, shave 2, hand and face wash 4, shampoo 4, hand dishwashing 4, automatic dishwasher 14, food preparation 5, automatic clothes washer 32. A heat pump water heater fits when its first-hour rating, from the maker or the yellow EnergyGuide label, is at least that figure.
LimitsAHRI's per-use figures assume no water-saving fixtures, so they run high for homes with low-flow showerheads and newer appliances. 120-volt plug-in models recover more slowly than 240-volt units.
Questions
What size heat pump water heater do I need for four people?
Count the busiest hour. Two showers, a shave, two hand washes, a dishwasher load and some cooking come to 69 gallons on AHRI's figures, so look for a first-hour rating of about 70 gallons or more.
Is a 50-gallon heat pump water heater the same as a 50-gallon electric tank?
Not necessarily in delivery. A heat pump recovers more slowly than a resistance element, so two 50-gallon tanks can have different first-hour ratings; compare those, not tank sizes.