By far, air exchange with the outside is the biggest factor in keeping the inside a different temperature from the outside.
You can have a 10ft by 10ft (100 sq ft) wall of single pane glass with ice on the inside of it and there won't be as much difference as having a couple of 6 inch holes to the outside.
The reason is that cold air is denser than hot air. The pressure of cold outside air will cause it to come in the lowest air gaps in the house while the hot air escapes through the highest gaps.
If you live in a two story building, and there is an opening from the first to the second story, like a stairwell, there will be a great deal more pressure difference and the cold air will tend to fill up the bottom floor while the hot air will go upstairs and escape. This is one reason it's easier to heat higher floors and harder to cool them.
Of course, we don't usually have 100 sq ft glass panes or 6in holes to the outside. But if you go around and add up all the glass and all the gaps in, you might find a similar ratio.
The surfaces of the building, including the glass all have some conduction of heat energy. Walls and ceilings and even floors tend to be insulated at least a little. Newer construction has far more insulation than older construction. It's usually so much that adding more is almost a waste of energy to construct the extra material and increase the thickness of the walls etc.
Older buildings on the other hand were often built when sources of heat were cheaper and insulation materials either unknown, or simply too expensive. Many older buildings were built with no insulation at all. There is an exterior wall surface, an air gap between studs, and an interior wall surface. There is some insulation value here thanks to the materials of the walls and the air gap, but it's quite low, and there can be a significant loss of energy through the walls just by conduction.
The windows on newer buildings tend to be sealed against air exchange better, and are usually double pane with an air gap, or better in colder climates.
Older houses were often tighter against air exchange when built, but settling and opening and closing doors and windows creates a lot of gaps over time.
This is something you have no control over, but it makes a big difference in the calculation of savings.
If you live in a place like San Francisco, you get average daily high temperatures of about 72 F and average daily low temps of about 46 F.
Cooling in SF is mostly a matter of opening a window at night and staying in the shade in the day time.
Heating is almost as easy. Even on the coldest nights of the coldest times of the year, a warm blanket in a tent is good enough to keep you comfortable. And (most) people in San Francisco have better living situations than that.
In contrast, Northern Minnesota has an average daily high of about 76 F in July and an average daily low of -3 F in January.
Cooling is still not a problem, but there's a huge difference between comfortable temperatures and what it's like outside in Jan.
That temperature delta of about 70 degrees creates much denser air outside and so the pressure for it to get inside is increased. All of the surfaces of the building are also getting a large gradient of heat across their thickness so that the 67 degree air in the house is coming into contact with walls that are something above -3 F on the outside, and something below the 67 F air.
That creates a convection of air against the walls and other surfaces, further increasing the heat transfer just like a breeze makes any air feel colder.
Even in the hottest climates in the US, the average daily high temperatures are only around 100 F. That, while uncomfortable, is only about 30 degrees from a comfortable temperature. So, buildings are not protected much against heat by comparison to how much protection against cold in colder climates.
The last consideration for putting up plastic window coverings is Solar Gain. Using plastic coverings on windows has almost no impact on how much the sunlight will impact the temperature of a building.
Though this kind of plastic treatment won't make such difference, having sunlight hit the interior of a building can have a huge impact on the temperature.
Buildings can see significant heat gains in the winter by allowing the sunlight in. They should be shielded from sunlight in the summer to avoid the same.
There are plastic films that can be put on windows, (ideally the outermost panes) to reflect much of the energy of the sunlight away. Those can make some difference to reduce summer cooling needs.
Finding Air Leaks
You can use smoke from incense or some other very visible source to track where the air is escaping the building.
Simply wait until after dark, ready a flashlight, light a stick of incense, turn off all the lights, and walk around with it following the smoke as it leaves the building. Those will be your problem areas.
If the temperature difference is low, you can put a fan in a window blowing air into the building to create the pressure needed. But this might cause so much turbulence in the room with the fan that it's hard to see the smoke. In that case, simply hold the smoke source near suspected holes and test them to see if it exits there.
If the temp difference is high, you may be able to just use the natural pressure created by the existing holes. If that's not enough, open a door or window to the outside a little bit. The temporary loss of energy for those few minutes will be more than made up for by the thousands of hours of better sealing later.
The higher the temperature difference is for your climate, the more helpful it will be to reduce airflow and increase insulation.
Airflow reduction is usually easier, and cheaper, and more effective than adding insulation.
Plastic window coverings do add some insulation against thermal conduction by adding another barrier. But their primary benefit comes from closing air gaps.
How much savings is a factor of how inefficient your building envelope and insulation are and how much difference in temperature there is from outside to inside.
When doing weatherproofing like this, also seal doorways so air infiltration is reduced there.
Keep passages from one floor to the next closed off if possible to reduce the effect of air pressure between floors causing more infiltration.
The plastic should seal off the entire window frame, not just the inside sashes which hold the glass. The air infiltration is mostly from the gaps in the fit between sashes and the frame, and sometimes from the frame itself like in the case of frames that have ropes and pulleys.
Caulking around the frame against the wall can also help.
Keeping window film on all year round can make some difference in the warmer periods, mostly by stopping air exchange. But it's not as significant of an actual energy savings as it can be with larger temperature differences in colder periods.
That said, air cooling is usually powered by electricity at lower efficiencies than air heating which is usually powered by burning fuel at higher efficiencies. Electricity is also usually more expensive per unit of energy. So you may get a higher financial savings ratio while using air conditioning for every bit of energy you can save.
Overall, I'd suggest that if you are in a climate with high fluctuations of temperature, it's worth keeping plastic shrink wrap film up all year long until you find yourself in a building that has more energy efficient construction.
It's hard to give specific numbers on how much you might save without doing physical testing on your specific building. But I hope this information helps in understanding and deciding in your particular situation.
I know this is an old post. But I found it while looking for some specifics, so I'm sure others are ending up here as well. I wanted to clarify and correct some incorrect information.