Solar air saved help needed

Hi I have a solar air heater on my roof and need some help in determining the amount of energy I am saving.
Cold air from the basement enters 2 sq ft of duct and goes up to roof mounted then gets heated and blown back down another 2 sq ft duct to a room in basement that has barrels of water to absorb and store heat.
I have thermistors in both ducts and am recording temps to an excel file every 5 minutes...we measured airflow with a handheld anometer and have determined that we are putting just over 1500 cfm of air through system when blowwer is operating. Blower is turned on when air reaches 75 F inside heater.
Q : How close will our cfm calc be compared to the propper "Hood" measuring procedure and would our measurment typically be higher or lower than actual?....I found a formula...Kw = cfm x delt t/ 2650....delt t in C......soooo if I had a 10C delta t for one hour woul I have produced the heat equivilant of 5.6 Kwh?...since I am measuring every 5 minutes do I average the 12 temp readings over an hour or should i multiply each reading by 5/60...(min/hr)...
Thank youi Rob
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