The short answer
The first-hour rating (FHR) estimates the maximum volume of hot water a storage water heater can deliver in one hour starting with a fully heated tank. It depends on tank size and how fast the heater recovers. It is printed on the EnergyGuide label; ENERGY STAR's draft criteria set a 45-gallon minimum for heat pump models.
Use it
Add up your busiest hour with AHRI's per-use gallons and choose a first-hour rating at least that large; the planner does it. Tankless heaters don't have an FHR: they're sized by flow at your rise.
Questions readers ask
Is first-hour rating the same as tank size?
No. Two 50-gallon tanks can have different first-hour ratings because they recover at different speeds.