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Why am I not seeing an energy reduction when installing a solar screen on east facing skylights for the summer months only? I have tried modeling this by lowering the U-factor and changing the interior shading factor. In either case, I get an increase in energy use.
The solar screens cause an increase in heat load and a decrease in cooling load. The cooling load decrease is actually larger than the heating load increase. However, the cooling system has a much higher efficiency than the heating system. Heat pumps convert energy into met load at a much higher rate than combustion systems. Thus you end up expending more energy in terms of overall MBtu. However because there are such significant transmission losses for electricity, you probably get a net savings in energy at the point of generation. You also get a net savings in energy cost, due to the higher cost of electricity.
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