The short answer
Storage water heaters, heat pump models included, are sized by first-hour rating: the hot water a full tank delivers in its first hour. Count your busiest hour with AHRI's average gallons per use (shower 20, bath 20, shave 2, hand and face washing 4, shampoo 4, hand dishwashing 4, automatic dishwasher 14, food preparation 5, automatic clothes washer 32) and choose a rating at least that large. The heat-pump planner adds it up.
Worked example
| Use | Gallons each | Times | Gallons |
|---|---|---|---|
| Shower | 20 | 3 | 60 |
| Shave | 2 | 1 | 2 |
| Shampoo | 4 | 1 | 4 |
| Hand dishwashing | 4 | 1 | 4 |
| Total | 70 |
Read the label
The first-hour rating is on the yellow EnergyGuide label and listed for each ENERGY STAR model in its product finder. ENERGY STAR's draft criteria set a 45-gallon minimum first-hour rating for heat pump models.
Note
AHRI's worksheet assumes no water-saving fixtures, so homes with WaterSense showerheads use less.
Questions readers ask
What size heat pump water heater for a family of four?
Count the busiest hour: AHRI's example of three showers, a shave, a shampoo and hand dishwashing comes to 70 gallons, so look for a first-hour rating of about 70 gallons or more.