Window Heat Gain Loss Comparator Calculator

Purpose and model boundary

Compares user-entered window U-factors, solar-gain coefficients, and installed costs across heating, cooling, or combined scopes, then selects by annual energy cost, positive payback, or peak load.

Do not treat this result as an energy-code determination, Manual J, HVAC or ventilation design, moisture-control guarantee, structural or concrete design, permit approval, equipment selection, cost quote, or endorsement by a cited source. Site climate, assemblies, room use, ventilation, windows, material properties, geometry, manufacturer instructions, locally adopted codes, and approval thresholds remain the user's responsibility.

Inputs and units

• climate scope: a listed route or text value.
• selection objective: a listed route or text value.
• solar treatment: a listed route or text value.
• window count: entered in windows.
• area per window: entered in ft^2/window.
• existing u factor: entered in Btu/(h ft^2 deg F).
• existing shgc: entered in fraction.
• heating indoor temperature: entered in deg F.
• heating outdoor temperature: entered in deg F.
• cooling indoor temperature: entered in deg F.
• cooling outdoor temperature: entered in deg F.
• heating degree days: entered in deg F day/year.
• cooling degree days: entered in deg F day/year.
• heating season solar: entered in Btu/ft^2-year.
• cooling season solar: entered in Btu/ft^2-year.
• peak solar irradiance: entered in Btu/(h ft^2).
• heating solar utilization: entered in fraction.
• heating energy cost: entered in $/kBtu.
• cooling energy cost: entered in $/kBtu.

The option grid contains 8 fixed data rows plus one header row with these columns: Include, Option label, U-factor (Btu/(h ft^2 deg F)), SHGC (fraction), Installed cost/window (user currency/window), Service life (years), Source note. Numeric columns retain the units shown here; text, route, and include columns are intentionally unitless.

Governing relationships

Total window area is:

A = N_window A_each

Annual heating conduction, useful heating solar, and heating load are:

E_h,cond = U A 24 HDD/1000

E_h,solar = SHGC A H_solar,h f_use/1000

E_h,load = max(0,E_h,cond-E_h,solar)

Annual cooling conduction, cooling solar, and cooling load are:

E_c,cond = U A 24 CDD/1000

E_c,solar = SHGC A H_solar,c/1000

E_c,load = E_c,cond+E_c,solar

Annual energy terms are in kBtu/year. U is Btu/(h-ft^2-deg F), HDD and CDD are degree-days/year, SHGC and f_use are fractions, and seasonal solar H_solar is Btu/ft^2.

Annual cost is heating load times heating energy cost plus cooling load times cooling energy cost. Winter and summer peaks are:

Q_h,peak = U A(T_in,h-T_out,h)

Q_c,peak = U A(T_out,c-T_in,c)+SHGC A I_peak

The solar term is active only when selected. The active-scope peak is:

Q_peak = max(Q_h,peak,Q_c,peak)

so combined scope does not sum noncoincident winter and summer peaks.

For option j, installed cost is per-window cost times count, annual savings are baseline annual cost minus option annual cost, and simple payback is:

PB_j = Cost_j/Savings_j

PB_j is in years and is computed only for positive savings. The shortest-positive-payback objective treats a row as eligible only when both conditions hold:

Savings_j > 0

PB_j <= Life_j

Life_j is its entered service life (years); the other objectives rank annual cost or the noncoincident peak.

Calculation sequence

1. Validate every active selector, scalar bound, type, and every fixed-grid entry.
2. Apply the selected route and ignore values from inactive fields.
3. Normalize unit-dependent inputs into the workbook's calculation basis.
4. Evaluate every included row or candidate and aggregate the active schedule.
5. Calculate primary and supporting outputs, populate the authored chart series, and apply derived numeric checks.
6. Return the workbook status with the computed results; invalid input is never converted into a plausible engineering answer.

Outputs and interpretation

• existing annual cost ($/year): Modeled annual delivered-energy cost for the entered existing-window properties.
• selected option: Active option selected by the entered objective, or a clear no-positive-payback result.
• selected annual load (kBtu/year): Modeled active-scope annual load for the selected option.
• selected annual cost ($/year): Modeled active-scope annual delivered-energy cost for the selected option.
• annual cost savings ($/year): Existing-window annual cost minus selected-option annual cost; negative means higher modeled cost.
• simple payback (years): Selected installed cost divided by positive annual cost savings; zero means unavailable.

The chart is an explanatory view of the workbook's named chart ranges. It does not add a separate calculation or replace the numeric outputs.

Validation and status logic

The workbook evaluates Model_Status in the order shown. The first matching condition wins.

Assumptions and limitations

• Every shipped window option, cost, rating, and exposure is synthetic and illustrative until replaced.

• All windows use the same entered area and exposure.

• Heating solar credit is limited by an entered utilization fraction.

• Air leakage, installation quality, frame-edge effects, condensation, comfort asymmetry, incentives, financing, degradation, and resale are not modeled.

• The workbook is not a product recommendation or energy audit.

Restrictions and non-computing states

Each numeric control offers a working range chosen for usable entry, which is not always the limit this calculator enforces; a value outside that limit is rejected with a message rather than quietly accepted. The calculator additionally enforces allowed choices, active-field types, complete fixed-grid rows, and conditional or cross-field rules before computing.

Unsupported route values, incomplete rows, out-of-domain inputs, relational failures, nonfinite derived arithmetic, and workbook error tokens are non-computing states. Hidden inputs remain in the values sent to the calculator but must not affect results while those fields are inactive.

Errors and warnings

A rejected entry means the submitted state failed the published input rules. Workbook NOT VALID identifies an invalid active domain or unsupported derived arithmetic. Workbook CHECK retains a finite result while flagging a planning condition that needs review. OK means only that the delivered workbook logic closed within the entered assumptions; it is not code compliance, professional approval, or a safety guarantee.

This version is checked against 124 saved test cases, covering both calculated results and entries that should be refused, run against this exact workbook.

• U.S. Department of Energy window update and replacement guidance - Primary public context for evaluating repair, retrofit, and replacement options. Official public U.S. government source; no copied product table, text, figure, or recommendation is embedded.
• U.S. Department of Energy window energy-performance ratings - Primary public definitions for U-factor and solar heat-gain coefficient. Official public U.S. government source; no copied product rating, table, text, or figure is embedded.
• NIST energy unit guidance - Primary public metrology context for energy-unit conversion. Official public U.S. government metrology source; unit arithmetic is independently implemented without copied text, tables, or figures.

Licensing conclusion: No licensed, proprietary, manufacturer, utility, product-price, code-table, climate table, material table, forecast, or external dataset is embedded. The calculator independently implements general heat-transfer, moisture, airflow, energy, geometry, and quantity arithmetic from cited public and official sources; all defaults, labels, property rows, equipment assumptions, diagrams, and test values are synthetic illustrations. No product rating, climate, incentive, cost, or manufacturer table is reproduced.