If you have a heat-load analysis performed on your property, the results are likely to come back in a very odd measurement: Btu per heating degree day per square foot . How can you use this to work out your solar thermal space heating requirements?

A heating degree day is a one-degree average difference in temperature at your location for one day, below 65°F. In other words, if the outside temperature averages 64°F for a 24-hour period, that's one heating degree day; if the outside temperature was a constant 60°F for a week, that would be 5° x 7 days = 35 heating degree days. In a cold month, you'll probably have over 1,000 heating degree days.

A heat-load analysis is usually done by an architect, though many solar and heating contractors can perform the same tests. The results are just a starting point to work out your actual heat requirements. First you need to work out your total Btu per heating degree day:

(Btus per heating degree day per sq. ft.) x (Total sq. ft. in home) = Total Btu per heating degree day

Next, you need the number of heating degree days across the entire heating period. You can normally get this information from bills or from your local weather service. Some utility companies will also provide the data if you ask nicely. Use these figures to work out the average number of heating degree days per (actual) day over the entire period:

(Total # of heating degree days in whole heating period) ÷ (# of days in period)  = Average # of heating degree days per actual day

Take this average figure and multiply it by the Btu per heating degree day for your whole home to get the average daily load you need to cover with your solar thermal space heating system:

(Average # of heating degree days per actual day) x (Total Btu per heating degree day)

For the mathematically-minded, here's the whole thing in one. Note that there's no need to worry about operator precedence, since they're all divisions and multiplications:

(Total # of heating degree days in whole heating period) ÷ (# of days in period) x (Btus per heating degree day per sq. ft.) x (Total sq. ft. in home) = Average daily load in Btus

If you have utility bills, life is a lot simpler: you can work out your average daily fuel consumption by dividing the amount of fuel used by the number of heating degree days and converting the answer to Btus - there's a fuel conversion table on this page !