Tank vs. Tankless Cost Comparison Calculator
ANA›Life Services Authority›National Calculator Authority›Tank vs. Tankless Cost Comparison Calculator
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Tank vs. Tankless Cost Comparison Calculator
Compare the total cost of ownership between a traditional tank water heater and a tankless (on-demand) water heater, including purchase, installation, operating, and maintenance costs over your chosen time horizon.
### Tank Water Heater
Tank Unit Purchase Price ($)
Tank Installation Cost ($)
Tank Lifespan (years)
Tank Energy Factor (UEF, 0.50–0.95)
Tank Annual Maintenance Cost ($/yr)
### Tankless Water Heater
Tankless Unit Purchase Price ($)
Tankless Installation Cost ($)
Tankless Lifespan (years)
Tankless Energy Factor (UEF, 0.80–0.99)
Tankless Annual Maintenance Cost ($/yr)
### Shared Inputs
Daily Hot Water Usage (gallons/day)
Fuel Type
Natural Gas Electric Propane
Energy Rate ($/therm)
Analysis Period (years)
Annual Discount Rate / Inflation Rate (%) (for NPV)
Calculate
function tanUpdateFuelLabel() { var fuel = document.getElementById("tan-fuel-type").value; var label = document.getElementById("tan-energy-rate-label"); var input = document.getElementById("tan-energy-rate"); if (fuel === "gas") { label.textContent = "Natural Gas Rate ($/therm)"; input.value = "1.20"; input.step = "0.01"; } else if (fuel === "electric") { label.textContent = "Electricity Rate ($/kWh)"; input.value = "0.13"; input.step = "0.001"; } else { label.textContent = "Propane Rate ($/gallon)"; input.value = "2.50"; input.step = "0.01"; } }
function tanGetVal(id) { return parseFloat(document.getElementById(id).value); }
function tanValidate(fields) { for (var i = 0; i fields[i].max)) { return fields[i].msg; } } return null; }
// Annual BTU needed = gallons/day * 365 * 8.34 lb/gal * 1 BTU/(lb·°F) * deltaT°F // Standard deltaT = 70°F (groundwater ~50°F to delivery ~120°F) // For gas/propane: BTU content per therm = 100,000 BTU; per gallon propane = 91,500 BTU // For electric: 1 kWh = 3,412 BTU // Annual energy cost = (Annual BTU needed / UEF) / BTU_per_unit * rate_per_unit
function tanAnnualEnergyCost(gallonsPerDay, uef, fuel, rate) { var deltaT = 70; // °F var annualBTU = gallonsPerDay * 365 * 8.34 * deltaT; // BTU/year needed var inputBTU = annualBTU / uef; // BTU input required accounting for efficiency var cost = 0; if (fuel === "gas") { cost = (inputBTU / 100000) * rate; // therms * $/therm } else if (fuel === "electric") { cost = (inputBTU / 3412) * rate; // kWh * $/kWh } else { // propane cost = (inputBTU / 91500) * rate; // gallons * $/gallon } return cost; }
// NPV of a series of annual costs over N years at discount rate r // NPV = sum_{t=1}^{N} cost / (1+r)^t function tanNPVAnnual(annualCost, years, r) { if (r === 0) return annualCost * years; return annualCost * (1 - Math.pow(1 + r, -years)) / r; }
// Number of replacements needed within analysis period (beyond the first unit) // replacements = floor((analysisYears - 1) / lifespan) // Each replacement cost discounted to present value at time of replacement function tanReplacementNPV(purchaseCost, installCost, lifespan, analysisYears, r) { var totalNPV = 0; var t = lifespan; while (t 0."}, {id:"tan-analysis-years", min:1, max:50, msg:"Analysis period must be 1–50 years."}, {id:"tan-discount-rate", min:0, max:20, msg:"Discount rate must be 0–20%."} ];
var err = tanValidate(fields); var resultDiv = document.getElementById("tan-result"); if (err) { resultDiv.innerHTML = "⚠ " + err + ""; return; }
var tankPurchase = tanGetVal("tan-tank-purchase"); var tankInstall = tanGetVal("tan-tank-install"); var tankLifespan = tanGetVal("tan-tank-lifespan"); var tankUEF = tanGetVal("tan-tank-ef"); var tankMaint = tanGetVal("tan-tank-annual-maint");
var tlPurchase = tanGetVal("tan-tl-purchase"); var tlInstall = tanGetVal("tan-tl-install"); var tlLifespan = tanGetVal("tan-tl-lifespan"); var tlUEF = tanGetVal("tan-tl-ef"); var tlMaint = tanGetVal("tan-tl-annual-maint");
var gallons = tanGetVal("tan-daily-gallons"); var fuel = document.getElementById("tan-fuel-type").value; var rate = tanGetVal("tan-energy-rate"); var years = tanGetVal("tan-analysis-years"); var discountPct = tanGetVal("tan-discount-rate"); var r = discountPct / 100;
// Annual energy costs var tankAnnualEnergy = tanAnnualEnergyCost(gallons, tankUEF, fuel, rate); var tlAnnualEnergy = tanAnnualEnergyCost(gallons, tlUEF, fuel, rate); var annualEnergySavings = tankAnnualEnergy - tlAnnualEnergy;
// NPV of energy costs var tankEnergyNPV = tanNPVAnnual(tankAnnualEnergy, years, r); var tlEnergyNPV = tanNPVAnnual(tlAnnualEnergy, years, r);
// NPV of maintenance costs var tankMaintNPV = tanNPVAnnual(tankMaint, years, r); var tlMaintNPV = tanNPVAnnual(tlMaint, years, r);
// Initial capital costs (Year 0, not discounted) var tankInitial = tankPurchase + tankInstall; var tlInitial = tlPurchase + tlInstall;
// NPV of replacement costs var tankReplNPV = tanReplacementNPV(tankPurchase, tankInstall, tankLifespan, years, r); var tlReplNPV = tanReplacementNPV(tlPurchase, tlInstall, tlLifespan, years, r);
// NPV of residual values (subtract — it's a benefit) var tankResidual = tanResidualNPV(tankPurchase, tankInstall, tankLifespan, years, r); var tlResidual = tanResidualNPV(tlPurchase, tlInstall, tlLifespan, years, r);
// Total NPV of cost of ownership var tankTotalNPV = tankInitial + tankReplNPV + tankEnergyNPV + tankMaintNPV - tankResidual; var tlTotalNPV = tlInitial + tlReplNPV + tlEnergyNPV + tlMaintNPV - tlResidual;
var npvSavings = tankTotalNPV - tlTotalNPV; // positive = tankless saves money
// Simple payback (undiscounted): extra upfront cost / annual savings (energy + maint diff) var extraUpfront = (tlInitial - tankInitial); var annualSavingsSimple = (tankAnnualEnergy + tankMaint) - (tlAnnualEnergy + tlMaint); var simplePayback = (annualSavingsSimple > 0) ? (extraUpfront / annualSavingsSimple) : Infinity;
// Break-even year (discounted, cumulative) var breakEvenYear = null; var cumulativeTankCost = tankInitial; var cumulativeTlCost = tlInitial; for (var y = 1; y " + "" + "Cost Component" + "Tank (NPV)" + "Tankless (NPV)" + "" +
"Initial Purchase + Install" + "" + fmt(tankInitial) + "" + "" + fmt(tlInitial) + "" +
"Replacement Units (NPV)" + "" + fmt(tankReplNPV) + "" + "" + fmt(tlReplNPV) + "" +
"Energy Cost (NPV)" + tankUnits + " vs " + tlUnits + " " + fuelLabel + " annually" + "" + fmt(tankEnergyNPV) + "" + fmt(tankAnnualEnergy) + "/yr" + "" + fmt(tlEnergyNPV) + "" + fmt(tlAnnualEnergy) + "/yr" +
"Maintenance (NPV)" + "" + fmt(tankMaintNPV) + "" + "" + fmt(tlMaintNPV) + "" +
"Residual Value (NPV, credit)" + "−" + fmt(tankResidual) + "" + "−" + fmt(tlResidual) + "" +
"Total NPV Cost of Ownership" + "" + fmt(tankTotalNPV) + "" + "" + fmt(tlTotalNPV) + "" + "" +
"" + "NPV Savings (Tankless vs. Tank): " + "= 0 ? "#1a7a4a" : "#c0392b") + ";font-size:1.1em;'>" + (npvSavings >= 0 ? "+" : "") + fmt(npvSavings) + " in favor of " + (npvSavings >= 0 ? "tankless" : "tank") + "" + "Annual Energy Savings (Tankless): " + fmt(annualEnergySavings) + "/yr" + "Simple Payback Period: " + fmtY(simplePayback) + "" + "Discounted Break-Even Year: " + (breakEvenYear ? "Year " + breakEvenYear : "Not reached within " + years + " years") + ""; }
#### Formulas Used
Annual Hot Water Energy Needed (BTU/yr): BTUneeded = Gallons/day × 365 × 8.34 lb/gal × ΔT°F where ΔT = 70°F (groundwater 50°F → delivery 120°F)
Annual Energy Input Required: BTUinput = BTUneeded ÷ UEF
Annual Energy Cost: Gas: Cost = (BTUinput ÷ 100,000 BTU/therm) × $/therm Electric: Cost = (BTUinput ÷ 3,412 BTU/kWh) × $/kWh Propane: Cost = (BTUinput ÷ 91,500 BTU/gal) × $/gal
Net Present Value of Annual Costs: NPV = AnnualCost × [1 − (1 + r)−N] ÷ r
Replacement Cost NPV: NPVrepl = Σ (Purchase + Install) ÷ (1 + r)t, for t = lifespan, 2×lifespan, … < N
Residual Value NPV (credit): Residual = (Purchase + Install) × (RemainingLife ÷ Lifespan) ÷ (1 + r)N
Total NPV Cost of Ownership: Total = Initial + NPVrepl + NPVenergy + NPVmaint − NPVresidual
Simple Payback: Payback = (Tankless Initial − Tank Initial) ÷ Annual Savings (energy + maintenance)
#### Assumptions & References
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