Water Heater Recovery Rate Calculator

Inputs

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Nameplate input — BTU/h on gas, element watts on electric (4,500 W ≈ 15,354 BTU/h).
kW
Recovery/thermal efficiency: ~80% atmospheric gas, 90%+ condensing, ~98% electric resistance.
%
Outlet setpoint minus inlet water temperature. 50°C ≈ 90°F is the classic rating rise.
°C
Storage volume, for the recovery-time and first-hour outputs. 150 L ≈ 40 gal.
L
Share of the tank deliverable before outlet temperature sags — commonly taken near 70%. Used only for the first-hour estimate.
%

Results

Default result
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Recovery rate
161.3L/h
Pass

Hot water produced per hour at the entered rise.

Also computed

Full-tank recovery time55.8min

Time to lift the whole tank through ΔT from cold.

First-hour estimate266.3L

Usable draw + one hour of recovery — the listed FHR is a tested rating, this is the arithmetic estimate.

Method notes 4 notes
  • Pure heat balance: recovery = P·η / (c_p·ΔT) with c_p = 4186.8 J/kg·K and 1 kg/L water — identical to the imperial gph = BTU/h·η/(8.33·ΔT) rule.
  • Gas heaters enter the BTU/h input with recovery/thermal efficiency (~80% atmospheric, 90%+ condensing); electric elements run η ≈ 98%, and heat pumps deliver MORE heat than input power (COP > 1) — enter heat output, not compressor draw.
  • The first-hour figure here is arithmetic (usable draw + one hour of recovery); the FHR printed on the EnergyGuide label is a standardized test result that includes draw patterns and mixing.
  • Standby losses, scale on elements/flues and inlet-temperature seasonality all shave the real-world numbers.

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