A house with no windows has walls which
consists of thermal insulation with resistance 
Km
/W
surrounded by hard wall with total resistance 
Km/W. The area of the wall
is A=80 m how much power is required to heat the house to 60 F on a day when it is
32 F outside.
The total effective thermal resistance is
The power consumption is thus
Now keeping the same overall area the same we put in windows with thermal resistance
Km/W and area 
m. The new effective thermal resistance is
obtained from the formula for parallel connection of thermal resistors:
Note the large reduction in thermal resistance resulting from the small thermal
resistance from the windows. This is the typical result of a parallel connection. The effective
resistance is smaller than the smallest resistance involved. The corresponding
rate of power consumption is now a factor 
larger:
