Energy Savings Calculator (Tank vs Tankless)

ANA›Life Services Authority›National Calculator Authority›Energy Savings Calculator (Tank vs Tankless)

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Energy Savings Calculator (Tank vs Tankless)

Compare the annual energy costs and long-term savings between a traditional tank water heater and a tankless (on-demand) water heater.

Daily Hot Water Usage (gallons)

Average U.S. household uses ~64 gallons/day

Fuel Type

Natural Gas Electric Propane

Energy Cost

$/therm

Natural gas: ~$1.17/therm (U.S. avg 2024)

Tank Heater Energy Factor (UEF)

Typical tank UEF: 0.58–0.70 (gas), 0.90–0.95 (electric)

Tankless Heater Energy Factor (UEF)

Typical tankless UEF: 0.82–0.96 (gas), 0.96–1.0 (electric)

Analysis Period (years)

Tank Heater Installed Cost ($)

Typical range: $500–$1,500

Tankless Heater Installed Cost ($)

Typical range: $1,000–$3,500

Calculate Savings

(function() { // Fuel defaults: [cost, unit, hint, tankUEF, tanklessUEF, btuPerUnit] const fuelDefaults = { natural_gas: { cost: 1.17, unit: "$/therm", hint: "Natural gas: ~$1.17/therm (U.S. avg 2024)", tankUEF: 0.67, tanklessUEF: 0.94, btuPerUnit: 100000, // 1 therm = 100,000 BTU unitLabel: "therms" }, electric: { cost: 0.16, unit: "$/kWh", hint: "Electricity: ~$0.16/kWh (U.S. avg 2024)", tankUEF: 0.92, tanklessUEF: 0.98, btuPerUnit: 3412, // 1 kWh = 3,412 BTU unitLabel: "kWh" }, propane: { cost: 2.50, unit: "$/gallon", hint: "Propane: ~$2.50/gallon (U.S. avg 2024)", tankUEF: 0.62, tanklessUEF: 0.90, btuPerUnit: 91500, // 1 gallon propane = 91,500 BTU unitLabel: "gallons" } };

window.eneUpdateFuelDefaults = function() { const fuel = document.getElementById("ene-fuel-type").value; const d = fuelDefaults[fuel]; document.getElementById("ene-energy-cost").value = d.cost; document.getElementById("ene-cost-unit").textContent = d.unit; document.getElementById("ene-cost-hint").textContent = d.hint; document.getElementById("ene-tank-ef").value = d.tankUEF; document.getElementById("ene-tankless-ef").value = d.tanklessUEF; };

window.eneCalc = function() { const resultDiv = document.getElementById("ene-result"); resultDiv.style.display = "block";

// --- Read inputs --- const dailyGal = parseFloat(document.getElementById("ene-daily-gallons").value); const fuel = document.getElementById("ene-fuel-type").value; const energyCost = parseFloat(document.getElementById("ene-energy-cost").value); const tankUEF = parseFloat(document.getElementById("ene-tank-ef").value); const tanklessUEF = parseFloat(document.getElementById("ene-tankless-ef").value); const years = parseFloat(document.getElementById("ene-years").value); const tankInstall = parseFloat(document.getElementById("ene-tank-cost").value); const tanklessInstall = parseFloat(document.getElementById("ene-tankless-cost").value);

// --- Validation --- const errors = []; if (isNaN(dailyGal) || dailyGal 0."); if (isNaN(energyCost) || energyCost 0."); if (isNaN(tankUEF) || tankUEF 1.5) errors.push("Tank UEF must be between 0.1 and 1.5."); if (isNaN(tanklessUEF) || tanklessUEF 1.5) errors.push("Tankless UEF must be between 0.1 and 1.5."); if (isNaN(years) || years 30) errors.push("Analysis period must be 1–30 years."); if (isNaN(tankInstall) || tankInstall 0) { resultDiv.innerHTML = "Please fix the following:" + errors.map(e => "").join("") + ""; return; }

const d = fuelDefaults[fuel];

// --- Core Formula --- // BTU needed to heat water per year: // Q_annual = gallons/day × 365 × 8.33 lb/gal × 1 BTU/(lb·°F) × ΔT // ΔT = 120°F (target) − 58°F (avg groundwater) = 62°F (DOE standard) const deltaT = 62; // °F temperature rise const btuPerGalPerDegF = 8.33; // BTU per gallon per °F const annualBTU = dailyGal * 365 * btuPerGalPerDegF * deltaT;

// Energy units consumed per year = BTU_needed / (UEF × BTU_per_unit) const tankUnits = annualBTU / (tankUEF * d.btuPerUnit); const tanklessUnits = annualBTU / (tanklessUEF * d.btuPerUnit);

// Annual operating cost const tankAnnualCost = tankUnits * energyCost; const tanklessAnnualCost = tanklessUnits * energyCost; const annualSavings = tankAnnualCost - tanklessAnnualCost;

// Lifetime operating costs const tankLifetimeCost = tankAnnualCost * years; const tanklessLifetimeCost = tanklessAnnualCost * years;

// Total cost of ownership (installed + operating) const tankTCO = tankInstall + tankLifetimeCost; const tanklessTCO = tanklessInstall + tanklessLifetimeCost; const tcoSavings = tankTCO - tanklessTCO;

// Simple payback period (years) const extraUpfront = tanklessInstall - tankInstall; const payback = annualSavings > 0 ? extraUpfront / annualSavings : Infinity;

// Efficiency improvement % const effImprovement = ((tanklessUEF - tankUEF) / tankUEF) * 100;

// --- Format helpers --- const fmt = (n, d=2) => n.toLocaleString("en-US", {minimumFractionDigits:d, maximumFractionDigits:d}); const fmtC = (n) => "$" + fmt(Math.abs(n)); const sign = (n) => n >= 0 ? "+" : "−";

const paybackStr = isFinite(payback)
? (payback = 0 ? "#2e7d32" : "#c62828";
const tcoLabel = tcoSavings >= 0
? "Tankless saves " + fmtC(tcoSavings) + " over " + years + " years (total cost of ownership)"
"Tank saves " + fmtC(tcoSavings) + " over " + years + " years (total cost of ownership)";

resultDiv.innerHTML = ` ### Results

Metric Tank Tankless

Energy Factor (UEF) ${fmt(tankUEF,2)} ${fmt(tanklessUEF,2)}

Annual Energy Used (${d.unitLabel}) ${fmt(tankUnits,1)} ${fmt(tanklessUnits,1)}

Annual Operating Cost ${fmtC(tankAnnualCost)} ${fmtC(tanklessAnnualCost)}

Installed Cost ${fmtC(tankInstall)} ${fmtC(tanklessInstall)}

${years}-Year Operating Cost ${fmtC(tankLifetimeCost)} ${fmtC(tanklessLifetimeCost)}

${years}-Year Total Cost of Ownership ${fmtC(tankTCO)} ${fmtC(tanklessTCO)}

Annual Savings with Tankless: =0?'#2e7d32':'#c62828'};"> ${sign(annualSavings)}${fmtC(annualSavings)}/year

Efficiency Improvement: ${fmt(effImprovement,1)}% more efficient

Simple Payback Period: ${paybackStr}

${tcoLabel}

`; }; })();

#### Formula

Annual BTU Required:

Q = Gallons/day × 365 × 8.33 BTU/(gal·°F) × ΔT

Where ΔT = 120°F (target) − 58°F (avg groundwater) = 62°F (DOE standard)

Annual Energy Consumed (in fuel units):

E = Q / (UEF × BTU_per_unit)

Annual Operating Cost:

Cost = E × Price_per_unit

Simple Payback Period:

Payback = (Tankless_installed − Tank_installed) / Annual_savings

Total Cost of Ownership (TCO):

TCO = Installed_cost + (Annual_cost × Years)

BTU conversion factors used:

#### Assumptions & References

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References

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