Window films to prevent heat losses in cold climate

In a cold region, large windows may contribute to short-term overheating in summer, but glazing with a low solar heat gain coefficient must be used with caution because of the energy penalty it causes over the rest of the year.
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What can films do?

Technical data for 3M window films

Heat loss through single pane 1/4" clear glass can be reduced by 30% with a film LE35AMARL that only transmits 31% of visible light, or by 23% with a film LE20SIAR that transmits only 17% of visible light

However conductive losses through aluminium frames and physical transport of heat through air changes are also significant.

What are the potential savings?

Here are some calculations for Edinburgh:

The "U-value" quoted for 1/4" single-pane clear glass is 1.06. The Imperial (BTU/hour foot² fahrenheit) U-value is equivalent to 5.6782633373 USI (W/m²K), based on SI units of Watts, degrees Kelvin, and metres.

For my office the glazed area is approx 15m² and the maximum monthly average temperature difference is 17K (20°C internal, external Jan & Feb average 3°C) - so the loss through glass in Jan and Feb is approx 1.5 kW

On an annual basis, average external temperature is 8.5°C so average conductive loss through single pane clear glass is 50W/m². The potential saving is thus around 15W/m².

Cost of film including installation was approx £40/m². Heating costs should be substantially less than £0.05 per kWh - so, even at this high energy price, payback through fuel savings will take around 6 years.