The dollar number listed under the total solar heat output is the approximate value of the fuel saved.  It assumes propane fuel at $2 per gallon and the use of an 80% efficient heating system  --  "Your results may vary".

The columns in the table:

HDD -- heating degree days from the Bizzee site...

Heat Used -- This is the heating demand of our sample house... It is calculated using this Home Heat Loss calculator...

Andy Output -- This is the

Solar Heat Out -- This is the total heat output from the xx sqft collector array for this location in BTU.  It is the Andy Output column multipled by the array area, and converted to US units.

Solar Fraction -- This is the fraction of the space heating demand that is met by the solar collectors.  The total solar fraction for the year is the total solar heat output divided by the total heat demand (this is, its not the average of the months, but the true solar fraction for the year).

 

Santa Fe, NM

HDD Heat Used Andy Output(1) Solar Heat Out Month F-day BTU KWH/sqm-day BTU/mo Solar Fraction Jan 1029 6,272,784 2.14 4,134,208 0.66 Feb 786 4,791,456 2.04 3,943,642 0.82 Mar 702 4,279,392 1.93 3,735,014 0.87 Apr 473 2,883,408 1.64 2,883,408 1.00 May 0 0 1.46 0 0.00 Jun 0 0 1.33 0 0.00 Jul 0 0 1.48 0 0.00 Aug 0 0 1.79 0 0.00 Sep 0 0 2.35 0 0.00 Oct 423 2,578,608 2.46 2,578,608 1.00 Nov 676 4,120,896 2.33 4,120,896 1.00 Dec 1054 6,425,184 2.29 4,437,246 0.69 Total 5143 31,351,728   25,833,021 0.82         $702

How:

- Heating Degree Days are from ...

- The months that have the HDD and Solar Output zeroed out are considered to not be part of the heating season, and are basically not counted.

 

Notes:

- Good sun locations with warm climates (eg Las Vegas) achieve high solar fractions with the 240 sqft of collector, but do not save as much fuel as colder locations with pretty good sun (eg Denver).  

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Assumptions:

- The storage heat storage system is sufficient to make use of all of the heat supplied by the collectors.  That is, any heat collected by the collectors ends up heating the house, not being wasted.  For modest solar fractions, this does not require much storage, as the solar heat normally gets used within about a day. 

- No system losses are accounted for (eg heat loss from the storage tank that does not go to heat the house).  For a well designed system, these losses should be relatively small.

- The beneficial effect of solar radiation reflected from a snow field in front of the collectors is not accounted for.  This can be a significant bonus for vertical or steeply tilted collectors that have a snow field in front of them.