Why window replacement needs a payback model
A window quote shows the cash cost, but it does not show how much purchased energy the project may save. Two homes with the same quote can have very different outcomes because window area, existing performance, climate, HVAC efficiency, energy price, and operating behaviour are different.
This calculator estimates the reduction in conductive heating and cooling load from the difference between the existing and replacement whole-window U-value. It then converts that load into purchased heating energy and cooling electricity, applies an editable realization rate, and values the result with the prices you enter. A second method anchors savings to current annual heating and cooling bills so that the physics result can be checked against the scale of actual spending.
You choose which method funds the economic cash flow. The calculator reports first-year savings, simple payback, discounted payback, 10-year and 20-year nominal savings, present-value savings, net present value, and low/base/high sensitivity scenarios. It does not apply one generic percentage to every building.
Useful for
- Owners comparing replacement-window quotes
- Residents planning an apartment, villa, or house retrofit
- Landlords prioritizing energy upgrades across properties
- Building managers preparing for an energy-audit discussion
Not determined here
- Code compliance, permits, or alteration classification
- Condensation, leakage, mould, structure, or ventilation safety
- Product quality, workmanship, or guaranteed savings
- Eligibility for a grant, loan, or local retrofit programme
Korea-specific 2026 performance context
Enter a whole-window U-value
Korea’s current Building Energy Conservation Design Standard is MOLIT Notice No. 2026-360, effective July 8, 2026. The National Law Information Center record is Administrative Rule serial 2100000282390. Article 5 defines the U-value for windows and doors as an average that includes the glazing and frame or door frame. A centre-of-glass value is therefore not an equivalent input.
Article 6 identifies a KS F 2278 test report, the standard table method, a registered window-set label under the efficiency-equipment rules, or an ISO 15099 calculation as possible performance evidence. Match the manufacturer, model, frame material, opening type, size scope, and glazing configuration in the quote to the evidence before entering a value.
Article 2 connects regulated building work to regional and assembly-specific thermal-transmittance requirements, but application depends on building use, work type, heating and cooling status, and exceptions. This calculator does not decide whether a project meets a statutory threshold. Confirm code applicability with the designer, supervisor, permitting authority, or another qualified Korean professional when the work is connected to construction, a major alteration, or a change of use.
Korea context: the default currency is KRW and the source guidance describes Korean window-set evidence. The heat-transfer equation itself is not country-specific. All prices, degree days, U-values, HVAC performance, project costs, and financial rates remain user inputs rather than Korean statutory defaults.
Prepare comparable inputs
First align the physical and commercial scope. Measure only the windows and glazed doors included in the quote. Confirm whether demolition, disposal, perimeter insulation, air sealing, waterproofing, finishing, lifting, protection, cleaning, and VAT are included. If one quote covers a different area or finishing scope, its payback cannot be compared directly with another quote.
Recommended source and scope checks for window payback inputs| Input | Preferred evidence | Scope check |
|---|
| Window area | Site measurement or quote take-off | Use the same windows, doors, and balcony sections as the quote |
| Existing and new U-values | Audit, KS F 2278 report, or registered model | Use glass-and-frame W/m²·K values on the same basis |
| HDD and CDD | Nearest suitable weather station | Keep base temperature, station, and period consistent |
| Energy prices | Recent usage and variable charges | Document whether fixed charges and taxes are included |
| Project cost and support | Written quote and confirmed award | Subtract only approved support from an aligned cash quote |
Heating and cooling degree days
Degree days accumulate the difference between outdoor temperature and a selected base temperature. The default 2,500 K·day HDD and 400 K·day CDD are editable planning examples, not official values for a city. Obtain one consistent dataset from the Korea Meteorological Administration data portal or another authoritative local source.
Effective energy prices
Convert gas, district heat, or another fuel to the purchased-energy kWh basis used in the model. A practical check is variable cost divided by usage over the same recent period. The default KRW 120/kWh and KRW 190/kWh only demonstrate the calculation; they are not current tariffs or national averages.
How the calculation works
Step 1: conductive load reduction
delta U = existing U-value - replacement U-value
annual thermal savings = delta U × area × degree days × 24 / 1,000
Multiplying W/m²·K by m² and K·day gives watt-days. Multiplication by 24 converts days to hours, and division by 1,000 converts Wh to kWh. Heating and cooling are calculated separately.
Step 2: purchased energy and cost
heating energy = heating thermal savings / efficiency × realization
cooling electricity = cooling thermal savings / COP × realization
physics savings = heating energy × price + cooling electricity × price
The realization rate is an editable haircut for operating schedules, installation conditions, and model omissions. It is neither a statutory rate nor a product warranty. For a heat-pump heating system, use an appropriately derived seasonal equivalent or rely more heavily on the bill method because a boiler-style efficiency percentage is not the same as heating COP.
Step 3: bill-anchored cross-check
bill savings = current HVAC cost × window share × U-value improvement × realization
The bill method respects the scale of current spending but requires an assumed share attributable to window conduction. Walls, roof, floor, infiltration, ventilation, solar gains, internal gains, hot water, cooking, and other loads may be embedded in a bill. Treat the window share as a scenario input, preferably informed by an audit.
Step 4: payback and net present value
net project cost = gross quote - confirmed subsidy
simple payback = net project cost / first-year savings
PV in year t = first-year savings × (1 + escalation)^(t-1) / (1 + discount)^t
NPV = sum of discounted savings - net project cost
Discounted payback is the first point where cumulative discounted savings reaches net project cost. The tool linearly interpolates within that year. A null result means that payback is not reached within the selected horizon, not that recovery can never occur.
A reliable step-by-step workflow
- Align the replacement scope. Measure the same windows and doors covered by each quote and record inclusions such as removal, sealing, finishing, and VAT.
- Verify whole-window performance. Use a comparable glass-and-frame value for both existing and new windows. Run a range when the existing value is uncertain.
- Use one climate convention. Select HDD and CDD from an appropriate station and keep base temperatures and periods consistent.
- Normalize energy spending. Separate heating and cooling where practical, calculate effective per-kWh prices, and avoid mixing a full-year bill with a partial-year usage figure.
- Compare both savings methods. A large gap is a reason to recheck area, U-values, degree days, bill scope, prices, and window-share assumptions.
- Stress the economics. Test support at zero until confirmed, adjust discount and escalation rates, and use the uncertainty band as a planning range rather than a confidence interval.
Worked example: 20 m² of replacement windows
Consider 20 m² of windows improving from 3.0 to 1.2 W/m²·K. Use 2,500 K·day HDD, 400 K·day CDD, 90% heating efficiency, cooling COP 3.5, and an 85% realization rate. Heating and cooling prices are KRW 120/kWh and KRW 190/kWh. The project costs KRW 6,000,000 with no confirmed subsidy. The analysis uses 30 years, a 3% discount rate, and 2% annual energy-price escalation.
Worked window replacement energy and payback results| Result | Calculated value | Interpretation |
|---|
| U-value improvement | 1.8 W/m²·K · 60% | Reduction relative to the existing whole-window value |
| Heating thermal savings | 2,160 kWh thermal | 2,040 kWh purchased-energy savings after efficiency and realization |
| Cooling thermal savings | 345.6 kWh thermal | About 83.9314 kWh electricity after COP and realization |
| Physics first-year savings | KRW 260,747 | KRW 244,800 heating plus KRW 15,947 cooling |
| Bill-anchored savings | KRW 275,400 | Uses 35% heating and 20% cooling window-share assumptions |
| Simple payback | 23.0108 years | Net cost divided by physics first-year savings |
| Discounted payback | 26.8048 years | Includes 2% escalation and a 3% discount rate |
| 20-year nominal savings | KRW 6,335,466 | Nominal total with the escalation assumption |
| 30-year NPV | KRW 616,272 | KRW 6,616,272 discounted savings less project cost |
What the example does and does not say
Energy-only payback can be long. That finding should be kept separate from acoustic comfort, reduced draughts, ease of operation, water-leak repairs, scheduled maintenance, and asset-condition benefits. A positive NPV does not certify workmanship or solve condensation, while a negative NPV does not prove that every non-energy benefit is worthless.
Use sensitivity before making a decision
The default uncertainty band is ±20%. In the worked example, the low scenario uses 80% of base savings and produces simple payback of about 28.7635 years. It does not reach discounted payback within 30 years and has an NPV of approximately negative KRW 706,982. The high scenario uses 120%, gives simple payback of about 19.1757 years, discounted payback of about 21.8220 years, and NPV of approximately KRW 1,939,527.
Conservative
- Lower realization rate
- Higher net project cost
- Lower energy-price escalation
- Higher discount rate
Base
- Verified quote and model
- Consistent weather data
- Recent effective energy prices
- Documented assumptions
Upside
- Strong installed performance
- Lower confirmed net cost
- Long heating operation
- Higher effective energy price
Compare quotes without changing the question
Keep area, climate, HVAC, bills, and financial assumptions fixed while changing only the new U-value and net cost for each quote. This isolates the trade-off between incremental performance and incremental price. If a premium window has only a small U-value advantage but a large cost premium, its energy-only NPV may be weaker even though it could offer other benefits.
Common quote omissions
- Removal, loading, transport, and disposal of existing frames
- Perimeter insulation, packing, air-sealing tape, silicone, and waterproofing
- Wallpaper, paint, tile, trim, exterior finish, and cleaning reinstatement
- Access equipment, lifting, protection, furniture movement, and resident coordination
- VAT, defect warranty period, glass coverage, and hardware coverage
Important limits of the model
The model covers conduction only
Whole-window U-value describes heat transfer through the assembly under a test or calculation convention. The model does not directly calculate infiltration through gaps, solar heat gain coefficient, orientation, shading, thermal bridges around installation joints, thermal mass, ventilation, humidity, curtains, or occupant schedules. These effects can make actual savings higher or lower.
- Better interior surface temperature can reduce some condensation risk, but humidity, ventilation, perimeter bridges, and installation defects can preserve or relocate condensation.
- Cooling savings can be dominated by solar gain and shading. A lower U-value alone is not a complete glazing-selection analysis.
- Bills may mix space heating with hot water, cooking, appliances, fixed charges, or common-area costs. Separate the relevant variable cost where practical.
- Escalation and discount rates are scenarios rather than forecasts. Test more than one combination and record why it was chosen.
- Grants can depend on programme year, building type, ownership, region, budget availability, application timing, and approved specifications. Keep subsidy at zero until confirmed.
Frequently asked questions
Can I enter the glass-only U-value?
No. Use a whole-window value that includes glazing and frame. The current Korean design standard explicitly uses that combined average for windows and doors, and a glass-only value can overstate installed performance.
What if the existing U-value is unknown?
Check completion documents, product labels, building-management records, or an energy audit. If no verified value exists, do not present one estimate as fact. Run low, central, and high existing-U scenarios and treat the resulting payback spread as decision uncertainty.
Is the physics result more accurate than the bill result?
Not automatically. The physics method makes performance and climate assumptions visible, while the bill method respects actual spending but depends on an estimated window share. Use disagreement as a diagnostic signal rather than choosing the larger value without investigation.
Does a payback longer than window life mean I should not replace them?
It is a caution if energy savings are the only objective. Existing failure, leakage, draughts, noise, safety, operation, scheduled maintenance, and asset condition may still justify work. Keep those benefits separate rather than inflating the energy-saving input.
Will replacement windows eliminate condensation?
Not necessarily. Warmer glass and frames can help, but condensation also depends on indoor humidity, ventilation, outdoor conditions, perimeter thermal bridges, and workmanship. A building-envelope or moisture assessment is a separate task.
How much subsidy should I enter?
Enter only an amount confirmed for the project. Programme rules, specifications, ownership, use, region, timing, and remaining budget can change. Compare a zero-support case with the approved-support case rather than treating an application amount as certain cash.
Official sources and update cycle
Official context was checked on July 27, 2026. Recheck the design standard and window-label rules at least annually, energy prices and bills when a tariff changes or at least annually, and project cost and support whenever a quote or award changes.
Recalculate with the values in your quote
Replace every example with measured area, comparable whole-window U-values, suitable local degree days, recent effective energy prices, current annual bills, an aligned cash quote, and confirmed support. Compare physics and bill results, then read the low/base/high sensitivity table before relying on one payback number. Ask a qualified professional to assess waterproofing, airtightness, ventilation, thermal bridges, condensation, and permit or code questions separately.