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Water Heater BTU Calculator

Heat input a batch of water needs inside a time window, in nameplate BTU/hr, kW, amps and MBH.

Inputs

Batch to heat

The batch: a tank, a spa, a stock tank or any batch of water to heat by the full rise.

Groundwater temperature. Cold-season water runs 37-45 F in the northern states, near 50 F in the mid-Atlantic and 65-75 F in the Deep South.

The temperature the batch must reach.

The hours the heat has to land inside. A bigger window means a smaller burner or element.

How many of these batches run each day. Drives the daily heat-load figure that feeds the operating-cost calculator.

Heat source

Decides whether the answer reads in amps or in MBH.

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.

Nameplate input of the unit you already have, to screen whether it can heat this batch inside the window. Leave 0 to skip the check.

Result

Nameplate input required
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Heat the batch must absorb
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Net heat rate over the window
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Same input in electric terms
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Electric current at 240 V
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Same input for gas
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Fuel-specific checks
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Where this stops applying
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Calculation details
Daily heat absorbed
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Daily site energy
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Input headroom
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Available / required input
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Temperature rise
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What to check

  • Gallons to heat and the start and target temperatures
  • The time window the batch has to land inside
  • Rated input of the unit you already have, to screen it

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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