Window Film Energy Savings Calculator

Related Calculators

Table of contents

• How to use
• Common mistakes
• Definitions
• Methodology
• Limitations & assumptions
• Sources

Total glass area: The square feet of glass that will receive film, not the frame, wall, or trim.

SHGC: Solar heat gain coefficient. It is the share of sun heat that passes through glass. A lower SHGC means less sun heat gets in.

SHGC drop from the film: Before-film SHGC minus after-film SHGC. A positive drop means the film blocks some solar heat by the entered values.

Glass that gets direct sun: The percent of the filmed glass that receives direct sun during the cooling season.

SEER: Seasonal energy efficiency ratio. In this calculator, 1 SEER means 1 Btu of cooling per watt-hour of electricity.

Simple payback time: Installed film cost divided by estimated yearly cooling bill savings. It is shown as N/A when savings are zero or negative.

Film area to buy with trimming extra: The covered glass area plus extra film for trimming, mistakes, and small overlaps.

Mistake: Using the full wall opening or frame size for Total glass area.
Fix: Enter only the glass area that will actually receive film.

Mistake: Mixing different window sizes under One window width, One window height, and Number of same-size windows.
Fix: Use those fields only for matching windows, or enter the combined Total glass area instead.

Mistake: Entering a percent in Electricity price instead of a dollar price per kWh.
Fix: Use a bill rate such as 0.18 for Electricity price, not 18%.

Mistake: Picking Glass before film that does not match your actual glass, then entering a precise Film SHGC after installation.
Fix: Use Custom before-film SHGC when you have product data, and keep both SHGC values between 0 and 1.

Mistake: Leaving Central AC SEER rating at a guess when your AC is much newer or older.
Fix: Check the AC label or paperwork and update Central AC SEER rating, because it changes avoided kWh and savings.

Mistake: Setting Extra film for trimming to 0 when planning a real purchase.
Fix: Use a trimming extra such as 10 percent unless an installer or product instructions give a better number.

• This is a screening estimate for cooling-season solar heat through glass. It is not a utility bill guarantee, installer quote, or product warranty.
• Actual savings can change with climate, window direction, shade from trees or buildings, indoor temperature settings, occupant behavior, and how often the AC runs.
• The single-pane clear and double-pane clear SHGC presets are rough defaults. Use Custom before-film SHGC when you have a window label, product sheet, or energy report.
• The calculator does not estimate winter heating penalties or benefits, glare, fading, comfort changes, glass stress risk, or manufacturer warranty limits.
• Material results assume total film area can be converted directly into roll length from Film roll width. Real orders may need more because of cut layout, seams, roll minimums, defects, or installer methods.
• If Film SHGC after installation is higher than the before-film SHGC, savings can be negative. The calculator keeps that sign and shows payback as N/A.

How the estimate is calculated

The calculator first finds the covered glass area. If you choose total area, it reads Total glass area and ignores the width, height, and count fields. If you choose window size and count, it reads One window width, One window height, and Number of same-size windows, then converts square inches to square feet.

glass_area_ft2 = total_glass_area_ft2

glass_area_ft2 = window_width_in * window_height_in * window_count / 144

The before-film SHGC comes from the selected glass preset, or from Custom before-film SHGC when Custom SHGC is selected. The preset values are screening defaults for clear glass, not a substitute for product data.

before_shgc = preset_shgc or custom_before_shgc

shgc_drop = before_shgc - film_shgc

Cooling-season solar heat blocked is estimated from glass area, the direct-sun share, daily sun heat, cooling-season length, and the SHGC drop. The month length uses 30.4375 days, which is 365.25 divided by 12.

blocked_solar_heat_btu_per_year = glass_area_ft2 * (sun_struck_percent / 100) * solar_heat_btu_per_ft2_day * (cooling_months * 30.4375) * shgc_drop

SEER is treated as Btu of cooling per watt-hour. Dividing by SEER converts blocked cooling load to watt-hours, and dividing by 1000 converts watt-hours to kWh.

avoided_ac_kwh_per_year = blocked_solar_heat_btu_per_year / (ac_seer * 1000)

annual_cooling_bill_savings = avoided_ac_kwh_per_year * electricity_rate

Simple payback uses positive yearly savings only. If savings are zero or negative, the payback card shows N/A instead of dividing by zero or showing an impossible payback.

simple_payback_years = installed_cost / annual_cooling_bill_savings

Film material is calculated from the same glass area, with the trimming extra added. Roll length is the film area divided by roll width in feet.

film_area_needed_ft2 = glass_area_ft2 * (1 + waste_percent / 100)

film_roll_length_ft = film_area_needed_ft2 / (roll_width_in / 12)

Worked mini-example

For 200 sq ft of double-pane clear glass, before-film SHGC 0.70, film SHGC 0.35, 75 percent direct sun, 1200 Btu per sq ft per day, 5 cooling months, SEER 14, and $0.18 per kWh, the SHGC drop is 0.35. The blocked heat is 9,587,812.5 Btu per year, avoided AC electricity is 684.84375 kWh per year, and estimated cooling bill savings are $123.27 per year. With an installed cost of $1,800, simple payback is about 14.6 years. With 10 percent trimming extra and a 60 inch roll, film area to buy is 220 sq ft and estimated roll length is 44 ft.

Calculation checks

The calculator strips commas, spaces, and matching unit suffixes from numeric entries, rejects ranges and formulas, checks SHGC values from 0 to 1, requires positive SEER and roll width, and keeps negative SHGC drop and negative savings when the entered film lets in more solar heat than the before-film glass.