Window Film Solar Heat Gain Reduction and Payback

Solar heat gain coefficient is the fraction of incident solar energy that ends up as heat inside; film works by lowering it, and the saving is the DIFFERENCE, multiplied by the glass area and by how hard the sun is hitting it. That last factor is why orientation dominates -- a west exposure at 4 pm in summer is the worst case in almost every building, both because the irradiance is high and because it coincides with the peak of everything else. The two outputs answer different questions. PEAK reduction in tons is a capacity argument: a building whose cooling plant is short may avoid an equipment replacement, and a ton avoided at peak is worth far more than a ton avoided on average. ANNUAL kilowatt-hours is the operating argument, and it converts through the equipment's EER, because dividing cooling BTU by EER gives watt-hours of electrical input -- that is what EER means. Two hundred square feet of west glass going from 0.70 to 0.35 SHGC at 230 BTU/hr per sq ft removes 16,100 BTU/hr, or 1.34 tons at peak, and over 1,200 equivalent full-sun hours saves about 1,610 kWh and $225 a year against $1,800 of installed film -- eight years, which is a poor investment if energy is the only reason. But 1.34 tons of peak capacity freed in a building short of cooling, plus the glare and fading control that motivated most film jobs in the first place, is a different calculation and usually the real one. COOLING ONLY. Film lowers SHGC year-round, so in a heating-dominated climate it takes away useful winter solar gain and the annual figure here does not net that out; the true annual saving there can be near zero or negative. Equivalent full-sun hours and peak irradiance are orientation, latitude, climate, and shading dependent and must come from local data. It does not address visible light transmittance, glare, or fading, and it does not evaluate the thermal stress risk that applying absorbing film to annealed or already-stressed glass creates, which can break the glass and voids many glass warranties. The film manufacturer, the glass manufacturer's warranty terms, and a qualified installer govern.

peak gain = area x SHGC x peak irradiance, before and after; peak reduction = the difference, and tons = that / 12000; annual = peak reduction x equivalent full-sun hours; kWh = annual BTU / EER / 1000; dollars = kWh x price; payback = area x installed cost per sq ft / dollars.

The solar heat gain relation, area x SHGC x incident irradiance, and the EER definition -- cooling BTU per watt-hour of electrical input -- used to convert avoided cooling into kilowatt-hours, by name. Cooling only: the winter solar gain the film also removes is not netted out. The film manufacturer, the glass manufacturer's warranty terms, and a qualified installer govern.

Every term is arithmetic on user-entered values; the SHGC pair comes from the film and glazing manufacturers' published data and the irradiance and full-sun hours from local climate data.

Estimate. AHJ and licensed professional govern.

Field names used by the API: area_sqft, shgc_before, shgc_after, peak_irradiance_btuh_sqft, full_sun_hours, eer, price_per_kwh, cost_per_sqft, peak_before_btuh, peak_after_btuh, peak_reduction_btuh, peak_reduction_tons, kwh_saved, dollars_saved, payback_years

• EER is the conversion cooling BTU divided by EER is watt-hours of electrical input, by definitionequipment rating practice
• Cooling only the winter solar gain the film also removes is not netted out; in a cold climate the annual saving can be negativebuilding energy practice
• Thermal stress not evaluated absorbing film on annealed or already-stressed glass can break it and voids many glass warrantiesfilm and glass manufacturers