Water heater recovery rate & reheat time
This is the thermodynamics behind every sizing decision. It takes 8.33 BTU to raise a gallon of water one degree, so a heater’s recovery rate and reheat time fall straight out of its input and your temperature rise.
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This is straight physics — it takes 8.33 BTU to raise a gallon of water 1 °F. This heater recovers about 54.9 gph and reheats a 50-gallon tank in about 0.91 hours. A gas burner recovers far faster than a resistance element, which is why electric tanks need more storage.
“How long does it take a water heater to heat up?” is one of the most searched water-heater questions, and every answer online is a vague “about an hour.” It is not a guess — it is arithmetic. The energy to heat water is fixed at 8.33 BTU per gallon per degree Fahrenheit, so once you know the input power and the temperature rise, both the recovery rate (gallons reheated per hour) and the full-tank reheat time follow exactly.
This is also where gas and electric part ways. A 40,000-BTU/hr gas burner pours in heat far faster than a 4,500-watt element (which is only about 15,354 BTU/hr), so gas tanks recover in a fraction of the time. That single fact is why electric tanks need more storage to hit the same First-Hour Rating — they cannot lean on fast recovery.
Formula
Electric input first, then recovery and reheat:
input_BTU/hr (electric) = kW × 3412
recovery_gph = (input_BTU/hr × efficiency) ÷ (8.33 × ΔT)
reheat_hours = (gallons × 8.33 × ΔT) ÷ (input_BTU/hr × efficiency)
The 8.33 is the weight of a gallon of water in pounds; efficiency is a LABELED typical (~0.80 gas, ~0.98 electric).
Worked example
Gas — 40,000 BTU/hr, efficiency 0.80, rise 70 °F:
recovery = (40,000 × 0.80) ÷ (8.33 × 70) = 32,000 ÷ 583.1 ≈ 54.9 gph
Electric — 4.5 kW (×3412 = 15,354 BTU/hr), efficiency 0.98, rise 70 °F, 50-gallon tank:
recovery = (15,354 × 0.98) ÷ 583.1 ≈ 25.8 gph
reheat = (50 × 8.33 × 70) ÷ (15,354 × 0.98) = 29,155 ÷ 15,047 ≈ 1.94 h ≈ 1 h 56 min
So the gas burner recovers more than twice as fast, and the electric tank takes nearly two hours to reheat from cold — the concrete reason to size electric with extra gallons.
Measure first, avoid a wrong size
Recovery falls as the inlet drops. A colder winter inlet raises ΔT, which lowers gph and lengthens reheat — the same heater is measurably slower in January than in July.
- Gas input is BTU/hr; electric is kW. Convert electric with ×3412 first. A 5,500-watt element recovers a bit faster than a 4,500-watt one, but both trail almost any gas burner.
- Recovery feeds the First-Hour Rating. Take the gph here into the FHR tool: fast recovery lets a smaller tank post a big FHR, slow recovery forces more storage.
- Sediment slows real recovery. A scaled tank or a coated element heats slower than the label; an annual flush keeps recovery close to spec.
- Efficiency is a planning typical. Confirm the recovery figure and input rating on the unit’s spec sheet and EnergyGuide label.
Reference table
Illustrative recovery rate at a 70 °F rise — the gas-vs-electric gap in one view.
| Heat source | Recovery (gph) |
|---|---|
| Gas 40,000 BTU/hr @ 0.80 | 54.9 |
| Gas 50,000 BTU/hr @ 0.80 | 68.6 |
| Electric 4.5 kW @ 0.98 | 25.8 |
| Electric 5.5 kW @ 0.98 | 31.5 |