How to Measure and Calculate airflow in CFM (cubic feet per minute) output from a fan

CFM = FPM * Outlet Area (in Square Feet)

FPM = Feet per Minute (wind speed). This can be measured from the outlet with a handheld Anemometer. If possible, measure at the end of a straight 2' long duct temporarily sealed to the output fan. Try not to obstruct the air flow as this will decrease the area and cause errors in the measurement. The anemometer will obstruct the airflow, so try to take measurements near the sides of the duct.

How to calculate Thermal Power ("Heat") output from a Solar Air Heater, in BTUs/hour:

BTUs/hour = CFM * 1.08 * Td

Td = (Outlet Temperature °F) - ( Inlet Temperature °F
1.08 = See section, below, "Calculate Heat Capacity of Mass Airflow based on Elevation at location of a Solar Heater"

For more information and more advanced equations, see:

Convert from BTUs to Watts:

P(watts) = P(BTUs) / 3.412141633
P(BTUs) = P(watts)  * 3.412141633
In this context, P is "Thermal Power" which is harvested from collector elements that have been irradiated with Solar Energy. The phrase "Thermal Power" is used to describe Heating over Time.

Energy Potential of a Solar Collector:

1,000 Watts/Square Meter of Surface Area of Solar Collector*
317 BTUs/hr/Square Foot of Surface Area of Solar Collector*

* these are "rule of thumb" values, but in reality are variable and depend on location and weather conditions

Calculating or Measuring Sun intensity

Solar irradiance = power / area (e.g. watts per square meter)
Insolation = power * time / area (e.g. watt hours per square meter)

How to Estimate Fan requirements:


Eff%=75% (This value relates to Air Flow impedance inside the collector, heat loss through glazing and other components, as well as the capability of the Fan to overcome air flow impedance via a high Static Pressure rating). If you suspect there is a lot of internal resistance inside the collector, or heat loss, or the fan doesn't tolerate resistance well, it might be prudent to use a lower value, such as 60%.
BTUs=317*Collector Surface area in Square Feet

BTUs=317*27.5 Square Feet=8717 BTUs

In this example, a fan rated 108CFM is estimated to output 6,537BTUs (from Potential solar energy of 8717 BTUs) with actual output more like 80.7CFM. For this size of fan, we recommend a static pressure rating of at least 0.6" or 150Pa).

Convert between Fahrenheit and Celsius:

TF = TC* 1.8 + 32
TC = (TF - 32) / 1.8

For converting temperature delta values (e.g. Δ=T1-T2):

ΔΔF / 1.8
ΔΔC * 1.8