Is this first-hour rating?

No. It’s a recovery estimate. First-hour rating includes stored hot water plus recovery during that hour.

Does efficiency matter?

Yes in reality; this uses nameplate input. Actual delivered BTU can be lower due to efficiency losses.

They have different effective input. Use the effective BTU/kW or expect slower recovery on heat pump mode.

Yes—adjust delta-T. Warmer inlet water reduces recovery time.

This models full tank reheat. Partial draws reheat faster; run multiple scenarios if needed.

energy calculator

Estimate how long a water heater takes to reheat based on tank size, input (BTU/kW), and temperature rise.

Fuel type: Choose gas (BTU/hr) or electric (kW).
Temperature rise: Setpoint minus incoming water temp; common: 70°F rise (50°F to 120°F).
Input: Use your heater’s rated input (BTU/hr for gas, kW for electric).

BTUs needed = Gallons × 8.34 × Temperature rise (°F).

Gas uses burner input BTU/hr; electric uses kW converted to BTU/hr (×3412).

Recovery time = BTUs needed ÷ Input BTU/hr.

BTUs needed = Gallons × 8.34 × ΔT
Effective input = BTU/hr (gas) or kW × 3412 (electric)
Recovery time (hr) = BTUs needed ÷ Effective input

Electric recovery is generally slower than gas; larger elements or heat pump hybrids change the input.
Lowering delta-T (incoming water warmer) speeds recovery; colder climates slow it down.
Heat pump water heaters have different profiles; this is for resistive electric or standard gas input.
Simplified; does not include efficiency losses, standby losses, or first-hour rating nuance.
Assumes full tank reheating; partial draws reheat faster.
Heat pump water heaters not modeled explicitly; use their effective input for a closer estimate.

Estimate water heater recovery time by entering tank size, gas BTU or electric kW, and temperature rise to get hours/minutes to reheat.

Use it to compare gas vs electric recovery and plan for large hot water demands.

Is this first-hour rating?: No. It’s a recovery estimate. First-hour rating includes stored hot water plus recovery during that hour.
Does efficiency matter?: Yes in reality; this uses nameplate input. Actual delivered BTU can be lower due to efficiency losses.
What about heat pump water heaters?: They have different effective input. Use the effective BTU/kW or expect slower recovery on heat pump mode.
Can I change incoming water temp?: Yes—adjust delta-T. Warmer inlet water reduces recovery time.
Does this handle mixed draws?: This models full tank reheat. Partial draws reheat faster; run multiple scenarios if needed.

construction

HVAC BTU Calculator

Estimate cooling BTU and tonnage based on room size, ceiling height, insulation, sun exposure, and occupancy.

energy

Heat Pump Sizing Calculator

Estimate heat pump capacity (BTU and tons) based on home size, climate, and insulation quality.

Approximate recovery only. Does not include efficiency or first-hour rating nuances. Verify with manufacturer specs for purchase decisions.