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About 65% of a home's heating needs is at night when the sun can not help.  So it is important to be able to store up some of the day time heat.  This is why thermal mass is so important in a solar home.  Thermal mass is the ability of a home to hold a temperature.   A home with a large amount of thermal mass takes a long time to warm up and a long time to cool off.  This helps in two ways, first in the spring and fall when you have cold nights and mornings then hot afternoons, many homes will run the air conditioner in the afternoon and the heater late at night and in the morning.  Because of the thermal mass in our house we do not need to run either the air of heater as the house temperature changes so slowly neither is needed.   By the hot afternoon the walls and floor are still cool enough from the previous evening to keep the house comfortable.  Then late at night the walls and floor give off the heat they absorbed in the afternoon, giving us free heat late into the night.

The reason is this especially important in a solar home is in the winter you have a lot of glass area absorbing the heat from the winter sun.  Without enough mass the house will overheat in the afternoon forcing you to open windows.  Then in the cold night when you most need the heat, there is none available causing you to run the heater.  With thermal mass the house heats up slowly in the afternoon sun as the floor and walls soak up the heat.  Then at night the floors and walls give off the heat. 

As a general rule you want the south facing window area to be about 10% of your floor area.  This has worked well for us with a slab floor for thermal mass.  

Our second home has about 48 tons of additional thermal mass to moderate it's temperatures.  We have about 18 tons in our floor or slab as we built a slab on grade home, and a ton of tile.  Then inside the ICF walls we have another 18 tons of concrete.  Then as most homes of our size we have about 10 tons of sheetrock.  Since a pound of concrete stores .2 BTU/^oF the walls and floor store 18,900 BTU's per ^oF.  With this amount of mass and low-e, argon filled windows our home rarely saw a temperature swing of more than 6 degrees.  So while the house would be 70 in the early morning it would be 76 by late afternoon.  This means nearly 113,000 additional BTU's have been absorbed into the home to be released through the night.  So all of our daily heating needs are met and on the average more than half of our nighttime heating needs.

Our first home had about 67 tons of thermal mass due mainly to it's block construction.  But there we saw temperature swings of nearly 10 degrees.  We attribute this mainly to the fact the old house had standard double pane windows which allow more more heat, about 15%, to come in during the day, but they lost nearly double the heat at night as the newer windows.  We also had 12% of our floor area in south facing glass versus 9% in the new home.