Water Heater Energy Efficiency (EF/UEF) Savings Calculator

ANA›Life Services Authority›National Calculator Authority›Water Heater Energy Efficiency (EF/UEF) Savings Calculator

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Water Heater Energy Efficiency (EF/UEF) Savings Calculator

Compare your current and new water heater efficiency ratings (EF or UEF) to estimate annual energy consumption and cost savings.

Fuel / Energy Type

Natural Gas Electric (Resistance) Electric (Heat Pump / Hybrid) Propane (LP Gas) Fuel Oil

Current Water Heater EF / UEF

Typical gas storage: 0.55–0.70 | Electric resistance: 0.90–0.95 | Heat pump: 2.0–4.0

New Water Heater EF / UEF

ENERGY STAR gas tankless: 0.82+ | Heat pump: 2.0–4.0 | Condensing gas: 0.90+

Daily Hot Water Usage (gallons)

DOE test draw pattern: 64 gal/day for a family of 4

Energy Rate ($/therm)

U.S. avg natural gas ~$1.20/therm | Electric ~$0.16/kWh | Propane ~$2.50/gallon | Oil ~$4.00/gallon

Cold Water Inlet Temperature (°F)

U.S. average groundwater ~58°F; warmer climates 65–70°F

Hot Water Delivery Temperature (°F)

Recommended safe delivery temperature: 120°F

Calculate Savings

function watUpdateFuelLabels() { var fuel = document.getElementById('wat-fuel-type').value; var rateLabel = document.getElementById('wat-rate-unit-label'); var rateHint = document.getElementById('wat-rate-hint'); var rateInput = document.getElementById('wat-energy-rate'); var efHint = document.getElementById('wat-current-ef').nextElementSibling;

if (fuel === 'natural_gas') { rateLabel.textContent = '($/therm)'; rateHint.textContent = 'U.S. avg natural gas ~$1.20/therm'; rateInput.value = '1.20'; efHint.textContent = 'Typical gas storage EF: 0.55–0.70 | Condensing: 0.90+'; } else if (fuel === 'electric') { rateLabel.textContent = '($/kWh)'; rateHint.textContent = 'U.S. avg electricity ~$0.16/kWh'; rateInput.value = '0.16'; efHint.textContent = 'Electric resistance EF/UEF: 0.90–0.95'; } else if (fuel === 'heat_pump') { rateLabel.textContent = '($/kWh)'; rateHint.textContent = 'U.S. avg electricity ~$0.16/kWh'; rateInput.value = '0.16'; efHint.textContent = 'Heat pump water heater UEF: 2.0–4.0'; } else if (fuel === 'propane') { rateLabel.textContent = '($/gallon)'; rateHint.textContent = 'U.S. avg propane ~$2.50/gallon'; rateInput.value = '2.50'; efHint.textContent = 'Propane storage EF: 0.55–0.70'; } else if (fuel === 'oil') { rateLabel.textContent = '($/gallon)'; rateHint.textContent = 'U.S. avg fuel oil ~$4.00/gallon'; rateInput.value = '4.00'; efHint.textContent = 'Oil-fired storage EF: 0.53–0.68'; } }

function watCalc() { var fuel = document.getElementById('wat-fuel-type').value; var currentEF = parseFloat(document.getElementById('wat-current-ef').value); var newEF = parseFloat(document.getElementById('wat-new-ef').value); var dailyGal = parseFloat(document.getElementById('wat-daily-gallons').value); var rate = parseFloat(document.getElementById('wat-energy-rate').value); var inletTemp = parseFloat(document.getElementById('wat-inlet-temp').value); var outletTemp = parseFloat(document.getElementById('wat-outlet-temp').value); var resultDiv = document.getElementById('wat-result');

// --- Validation --- var errors = []; if (isNaN(currentEF) || currentEF 5) errors.push('Current EF/UEF must be between 0.01 and 5.0.'); if (isNaN(newEF) || newEF 5) errors.push('New EF/UEF must be between 0.01 and 5.0.'); if (isNaN(dailyGal) || dailyGal 500) errors.push('Daily hot water usage must be between 1 and 500 gallons.'); if (isNaN(rate) || rate = outletTemp) errors.push('Inlet temperature must be between 32°F and less than outlet temperature.'); if (isNaN(outletTemp) || outletTemp 160) errors.push('Outlet temperature must be between 90°F and 160°F.'); if (newEF 0 && !(errors.length === 1 && errors[0].startsWith('New EF/UEF should'))) { resultDiv.innerHTML = '' + errors.map(function(e){ return ''; }).join('') + ''; return; }

// --- Core Physics --- // BTU needed per day to heat water // Q_btu_day = gallons/day × 8.3454 lb/gal × 1 BTU/(lb·°F) × ΔT var deltaT = outletTemp - inletTemp; // °F var btuPerDay = dailyGal * 8.3454 * deltaT; // BTU/day (heat content of water) var btuPerYear = btuPerDay * 365; // BTU/year

// --- Fuel unit conversions to BTU --- // Natural gas: 1 therm = 100,000 BTU // Electric: 1 kWh = 3,412 BTU // Propane: 1 gal = 91,452 BTU // Oil: 1 gal = 138,500 BTU var btuPerUnit = { natural_gas: 100000, electric: 3412, heat_pump: 3412, propane: 91452, oil: 138500 }; var unitName = { natural_gas: 'therms', electric: 'kWh', heat_pump: 'kWh', propane: 'gallons', oil: 'gallons' };

var btu_unit = btuPerUnit[fuel]; var unit = unitName[fuel];

// --- Annual Energy Consumption --- // Annual energy input = BTU needed / EF // EF (Energy Factor) = useful energy output / total energy input // => Energy input = useful output / EF var currentAnnualBTU = btuPerYear / currentEF; var newAnnualBTU = btuPerYear / newEF;

var currentAnnualUnits = currentAnnualBTU / btu_unit; var newAnnualUnits = newAnnualBTU / btu_unit;

var currentAnnualCost = currentAnnualUnits * rate; var newAnnualCost = newAnnualUnits * rate;

var annualUnitSavings = currentAnnualUnits - newAnnualUnits; var annualCostSavings = currentAnnualCost - newAnnualCost; var annualBTUSavings = currentAnnualBTU - newAnnualBTU;

// Efficiency improvement % var effImprovement = ((newEF - currentEF) / currentEF) * 100; // Energy reduction % var energyReduction = (annualBTUSavings / currentAnnualBTU) * 100;

// 10-year and 15-year savings var savings10yr = annualCostSavings * 10; var savings15yr = annualCostSavings * 15;

// CO2 emissions (approximate) // Natural gas: 11.7 lbs CO2/therm | Electric: 0.386 kg/kWh = 0.851 lbs/kWh (U.S. avg grid) | Propane: 12.68 lbs/gal | Oil: 22.4 lbs/gal var co2PerUnit = { natural_gas: 11.7, electric: 0.851, heat_pump: 0.851, propane: 12.68, oil: 22.4 }; var co2Unit = { natural_gas: 'lbs CO₂/therm', electric: 'lbs CO₂/kWh', heat_pump: 'lbs CO₂/kWh', propane: 'lbs CO₂/gal', oil: 'lbs CO₂/gal' }; var currentCO2 = currentAnnualUnits * co2PerUnit[fuel]; var newCO2 = newAnnualUnits * co2PerUnit[fuel]; var co2Savings = currentCO2 - newCO2;

// Format helpers function fmt(n, d) { return n.toLocaleString('en-US', {minimumFractionDigits: d||0, maximumFractionDigits: d||0}); } function fmtC(n) { return '$' + Math.abs(n).toLocaleString('en-US', {minimumFractionDigits:2, maximumFractionDigits:2}); }

var savingsColor = annualCostSavings >= 0 ? '#2e7d32' : '#c62828'; var savingsSign = annualCostSavings >= 0 ? '' : '-';

var warningHtml = ''; if (newEF ⚠️ The new EF/UEF is not higher than the current — the new unit uses more energy.'; }

resultDiv.innerHTML = warningHtml + '' + '### Annual Energy & Cost Comparison ' + '' + '' + 'Metric' + 'Current Unit(EF/UEF ' + currentEF.toFixed(2) + ')' + 'New Unit(EF/UEF ' + newEF.toFixed(2) + ')' + 'Annual Savings' + '' + 'Energy Used (' + unit + '/yr)' + '' + fmt(currentAnnualUnits,1) + '' + '' + fmt(newAnnualUnits,1) + '' + '' + savingsSign + fmt(Math.abs(annualUnitSavings),1) + ' ' + unit + '' + 'Annual Energy Cost' + '' + fmtC(currentAnnualCost) + '' + '' + fmtC(newAnnualCost) + '' + '' + savingsSign + fmtC(annualCostSavings) + '' + 'CO₂ Emissions (lbs/yr)' + '' + fmt(currentCO2,0) + '' + '' + fmt(newCO2,0) + '' + '' + savingsSign + fmt(Math.abs(co2Savings),0) + ' lbs' + '' +

'' + '' + 'Efficiency Improvement' + '' + (effImprovement >= 0 ? '+' : '') + fmt(effImprovement,1) + '%' + 'EF/UEF increase' +

'' + 'Energy Reduction' + '' + (energyReduction >= 0 ? '' : '-') + fmt(Math.abs(energyReduction),1) + '%' + 'less energy used' +

'' + '10-Year Savings' + '' + savingsSign + fmtC(savings10yr) + '' + 'cumulative cost' +

'' + '15-Year Savings' + '' + savingsSign + fmtC(savings15yr) + '' + 'cumulative cost' + '' +

'' + 'Inputs Summary: ' + fmt(dailyGal,0) + ' gal/day | ΔT = ' + fmt(deltaT,0) + '°F (' + fmt(inletTemp,0) + '→' + fmt(outletTemp,0) + '°F) | ' + fmt(btuPerYear/1e6,2) + ' MMBtu/yr heat load | Rate: $' + rate.toFixed(3) + '/' + unit.replace('/yr','').replace('s','') + '' + ''; }

#### Formulas Used

1. Annual Hot Water Heat Load (BTU/year):

Qload = Gallons/day × 365 × 8.3454 lb/gal × 1 BTU/(lb·°F) × (Tout − Tin)

2. Annual Energy Input (fuel units/year):

Einput = Qload / EF → convert BTU to fuel units (÷ BTU/unit)

Where EF (Energy Factor) or UEF (Uniform Energy Factor) = useful energy output ÷ total energy input. A higher EF/UEF means less fuel consumed for the same hot water output.

3. Annual Cost:

Cost = Einput (units/yr) × Rate ($/unit)

4. Annual Savings:

Savings = Costcurrent − Costnew

Fuel Unit Conversions: Natural gas: 100,000 BTU/therm | Electric: 3,412 BTU/kWh | Propane: 91,452 BTU/gal | Fuel Oil: 138,500 BTU/gal

#### Assumptions & References

Water density used: 8.3454 lb/gallon (at ~60°F); specific heat of water: 1 BTU/(lb·°F).
Default daily draw of 64 gallons/day matches the DOE/FTC standardized test draw pattern used to rate EF/UEF on EnergyGuide labels.
EF (Energy Factor) is the legacy DOE metric (pre-2015); UEF (Uniform Energy Factor) is the current DOE metric (post-April 2015) per 10 CFR Part 430. Both represent useful energy output ÷ total energy input over a test cycle.
Default cold water inlet temperature of 58°F is the U.S. national average groundwater temperature used in DOE test procedures.
Recommended hot water delivery temperature of 120°F per ASHRAE and CDC guidelines to balance scalding risk and Legionella prevention.
This calculator estimates water heating energy only; standby losses, pipe losses, and distribution system efficiency are captured within the EF/UEF rating itself.

Water Heater Energy Efficiency (EF/UEF) Savings Calculator

Water Heater Energy Efficiency (EF/UEF) Savings Calculator

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Water Heater Energy Efficiency (EF/UEF) Savings Calculator

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