Water Heater Efficiency (EF/UEF) Calculator

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Water Heater Efficiency (EF/UEF) Calculator

Calculate your water heater's Energy Factor (EF) or Uniform Energy Factor (UEF), estimate annual operating costs, and compare savings between models.

Water Heater Type

Gas / Propane (Storage) Electric Resistance (Storage) Heat Pump Water Heater (HPWH) Tankless Gas / Propane Tankless Electric Solar Water Heater

UEF / EF Rating (0.50 – 0.70 typical for gas storage)

Draw Pattern / Usage Profile

Very Small (10 gal/day) Low (38 gal/day) Medium (55 gal/day) High (84 gal/day)

Fuel / Energy Rate

$/kWh (electricity)

Tank Size (gallons) (0 for tankless)

Compare with a Second Model (optional)

Comparison UEF / EF

Comparison Model Price ($)

Current Model Price ($)

Expected Lifespan (years)

Calculate

(function(){

/ ── fuel defaults ── / const FUEL_DEFAULTS = { gas: { rate: 1.20, unit: "$/therm", btu_per_unit: 100000, label: "Natural Gas" }, electric: { rate: 0.13, unit: "$/kWh", btu_per_unit: 3412, label: "Electricity" }, hpwh: { rate: 0.13, unit: "$/kWh", btu_per_unit: 3412, label: "Electricity" }, tankless_gas: { rate: 1.20, unit: "$/therm", btu_per_unit: 100000, label: "Natural Gas" }, tankless_elec: { rate: 0.13, unit: "$/kWh", btu_per_unit: 3412, label: "Electricity" }, solar: { rate: 0.13, unit: "$/kWh", btu_per_unit: 3412, label: "Electricity (backup)" } };

/ ── UEF hint ranges (DOE 10 CFR Part 430) ── / const UEF_HINTS = { gas: "0.50 – 0.70 typical; ≥0.67 ENERGY STAR", electric: "0.90 – 0.95 typical; ≥0.93 ENERGY STAR", hpwh: "2.00 – 4.00 typical; ≥2.20 ENERGY STAR", tankless_gas: "0.82 – 0.96 typical; ≥0.87 ENERGY STAR", tankless_elec: "0.96 – 0.99 typical", solar: "1.00 – 3.00 typical (combined system)" };

/ ── DOE annual energy (Btu) by draw pattern (10 CFR Part 430 Appendix E) ── / / Q_annual = (64.3 * draw_gal * 8.33 * ΔT) / UEF where ΔT=67°F, but DOE uses fixed reference Btu loads per draw bin: Very Small=3078, Low=12266, Medium=18719, High=28518 (in kBtu/yr) / const DRAW_BTU = { very_small: 3078000, low: 12266000, medium: 18719000, high: 28518000 }; const DRAW_LABELS = { very_small: "Very Small (10 gal/day)", low: "Low (38 gal/day)", medium: "Medium (55 gal/day)", high: "High (84 gal/day)" };

function watUpdateFields(){ const type = document.getElementById('wat_type').value; const fd = FUEL_DEFAULTS[type]; document.getElementById('wat_fuel_rate').value = fd.rate; document.getElementById('wat_fuel_unit').textContent = fd.unit; document.getElementById('wat_uef_hint').textContent = UEF_HINTS[type]; }

window.watUpdateFields = watUpdateFields;

/ initialise / watUpdateFields();

function fmt(n, dec=2){ return n.toLocaleString('en-US',{minimumFractionDigits:dec,maximumFractionDigits:dec}); } function fmtD(n){ return '$'+fmt(n,2); }

window.watCalc = function(){ const res = document.getElementById('wat_result'); const type = document.getElementById('wat_type').value; const fd = FUEL_DEFAULTS[type];

/ ── inputs ── / const uef = parseFloat(document.getElementById('wat_uef').value); const draw = document.getElementById('wat_draw').value; const fuelRate = parseFloat(document.getElementById('wat_fuel_rate').value); const tankSize = parseFloat(document.getElementById('wat_tank_size').value) || 0; const lifespan = parseFloat(document.getElementById('wat_lifespan').value) || 13; const uef2 = parseFloat(document.getElementById('wat_uef2').value); const cost1 = parseFloat(document.getElementById('wat_cost1').value); const cost2 = parseFloat(document.getElementById('wat_cost2').value);

/ ── validation ── / const errs = []; if(isNaN(uef) || uef 5) errs.push("UEF/EF must be between 0.01 and 5.00."); if(isNaN(fuelRate) || fuelRate 30) errs.push("Lifespan must be 1–30 years."); if(!isNaN(uef2) && (uef2 5)) errs.push("Comparison UEF must be between 0.01 and 5.00.");

if(errs.length){ res.innerHTML = 'Please fix:' + errs.map(e=>'').join('') + ''; return; }

/ ── core calculation ── / / DOE formula: Annual Energy Consumption (AEC) in fuel units AEC = Q_ref / (UEF × BTU_per_fuel_unit) Annual Cost = AEC × fuel_rate / const Q_ref = DRAW_BTU[draw]; // Btu/yr const aec1 = Q_ref / (uef * fd.btu_per_unit); // fuel units/yr const annCost1 = aec1 * fuelRate; // $/yr const lifeCost1 = annCost1 * lifespan;

/ standby loss estimate for storage tanks (ASHRAE): Standby_loss_Btu/hr ≈ 0.84 × tank_vol(gal) × 0.645 (simplified) We derive it from UEF vs. EF relationship instead: For storage: UEF ≈ EF − 0.0005 × tank_vol (DOE approximation) / let efEquiv = uef; if((type==='gas'||type==='electric') && tankSize > 0){ efEquiv = uef + 0.0005 * tankSize; // approximate EF from UEF }

/ standby heat loss fraction: (1 - UEF) portion is losses / const standbyFrac = Math.max(0, 1 - uef / efEquiv);

/ ── ENERGY STAR threshold check ── / const ES_THRESHOLDS = { gas: 0.67, electric: 0.93, hpwh: 2.20, tankless_gas: 0.87, tankless_elec: 0.96, solar: 1.80 }; const esPass = uef >= (ES_THRESHOLDS[type] || 0);

/ ── comparison model ── / let compHTML = ''; if(!isNaN(uef2) && uef2 > 0){ const aec2 = Q_ref / (uef2 * fd.btu_per_unit); const annCost2 = aec2 * fuelRate; const lifeCost2 = annCost2 * lifespan; const annSave = annCost1 - annCost2; // positive = model2 saves money const lifeSave = lifeCost1 - lifeCost2;

let paybackHTML = ''; if(!isNaN(cost1) && !isNaN(cost2) && annSave !== 0){ const extraCost = cost2 - cost1; if(annSave > 0 && extraCost > 0){ const payback = extraCost / annSave; paybackHTML = Payback Period**${fmt(payback,1)} years**; } else if(annSave > 0 && extraCost Payback PeriodImmediate (lower upfront cost); } else { paybackHTML =Payback PeriodComparison model costs more to operate`; } }

compHTML = ` ### Model Comparison

Metric Current (UEF ${fmt(uef,2)}) Comparison (UEF ${fmt(uef2,2)})

Annual Energy Use ${fmt(aec1,1)} ${fd.unit.replace('$/','')}s ${fmt(aec2,1)} ${fd.unit.replace('$/','')}s Annual Operating Cost ${fmtD(annCost1)} ${fmtD(annCost2)} Lifetime Cost (${lifespan} yr) ${fmtD(lifeCost1)} ${fmtD(lifeCost2)} Annual Savings (vs current) — ${annSave>=0?'+':'-'}${fmtD(Math.abs(annSave))} Lifetime Savings — ${lifeSave>=0?'+':'-'}${fmtD(Math.abs(lifeSave))} ${paybackHTML}

`; }

/ ── efficiency rating label ── / let effLabel = ''; if(uef ⚠ Below Average'; else if(uef ◑ Average'; else if(uef ✓ Good'; else effLabel = '★ Excellent';

res.innerHTML = ` ### Results

${fd.label} ${type.includes('tankless')?'Tankless':'Storage'} Water Heater

UEF / EF Rating
${fmt(uef,2)} — ${effLabel}
ENERGY STAR Qualified
${esPass
? '✓ Yes'
'✗ No (threshold: '+fmt(ES_THRESHOLDS[type]||0,2)+')'} Draw Pattern ${DRAW_LABELS[draw]} Reference Heat Load ${fmt(Q_ref/1000,0)} kBtu/yr Annual Energy Consumption ${fmt(aec1,2)} ${fd.unit.replace('$/','')}s/yr (${fmt(aec1 * fd.btu_per_unit / 1000000, 2)} MMBtu/yr) Annual Operating Cost ${fmtD(annCost1)}/yr @ ${fd.unit} = ${fmtD(fuelRate)} Monthly Operating Cost ${fmtD(annCost1/12)}/mo Lifetime Cost (${lifespan} yr) ${fmtD(lifeCost1)} ${(type==='gas'||type==='electric') && tankSize>0 ? Approx. EF (from UEF) ${fmt(efEquiv,3)} : ''}

${compHTML}`; };

})();

#### Formulas Used

Annual Energy Consumption (AEC):

AEC (fuel units/yr) = Q_ref / (UEF × BTU_per_fuel_unit)

Annual Cost ($) = AEC × fuel_rate

Lifetime Cost ($) = Annual Cost × lifespan_years

Annual Savings = Annual_Cost_Model_A − Annual_Cost_Model_B

Payback Period (yr) = (Cost_B − Cost_A) / Annual_Savings

EF ≈ UEF + 0.0005 × tank_volume_gal [storage tanks, DOE approximation]

Reference Heat Loads (Q_ref) per DOE 10 CFR Part 430 Appendix E:

Fuel Energy Content: Natural Gas = 100,000 Btu/therm; Electricity = 3,412 Btu/kWh

#### Assumptions & References

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References

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