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Water Heater Heat-Up Time Calculator

How long a tank takes to reach its thermostat setting from the temperature the water is at now, and the input needed to hit a target time.

Inputs

Rating plate

Decides whether the input reads in BTU/hr or kW.

Nameplate input for gas, element wattage for electric. Switch the unit to kW for an electric element (4,500 W = about 15,400 BTU/hr).

Steady-state efficiency in percent, not the UEF. The DOE test method credits electric resistance heating with 98 percent (10 CFR 430 Subpart B Appendix E); atmospheric gas units run about 80 percent; condensing gas units 90-96 percent.

Water being heated

Nominal tank gallons, or the gallons actually drawn if you are recovering part of a tank.

Where the water actually is. A tank that has sat cold overnight is at groundwater temperature, roughly 37-50 F in the northern states in winter and 65-75 F in the Deep South.

The temperature the tank is trying to reach.

Enter the time you want the heat-up to take and the tool returns the input that would do it. Leave 0 to skip the inverse sizing.

Result

Time to reach the thermostat setting
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Same heat-up in hours
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Water temperature after 30 minutes
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Input needed for the target time
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Heat the water must absorb
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Energy content of that heat
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Same input in electric terms
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—
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Fuel-specific checks
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Where this stops applying
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Calculation details
Rated input / input needed
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Heat delivered to the water
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Net output needed for the target
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Temperature rise being recovered
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Temperature after 15 minutes
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Temperature after 60 minutes
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What to check

  • Tank volume and the water temperature right now
  • Rated input from the rating plate and the steady-state efficiency
  • Any target recovery time you need to hit

Standards referenced

  • DOE Uniform Energy Factor test procedure — 10 CFR Part 430, Subpart B, Appendix E · Uniform test method for measuring energy consumption of water heaters; UEF ratings on nameplates and EnergyGuide labels come from this procedure.

Related calculators

Screening estimate only. The result panel lists the limits that apply to this calculation. Verify nameplate data, rates, groundwater temperatures and installer quotes before deciding, and have gas, venting and electrical installations checked against the local code by a licensed contractor.

Saved calculations

Keep the assumptions with the result.

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