Hi everyone this is my first post so bear with me. I'm new to woodworking but HVAC is right up my alley.
You need to calculate the amount of heat it will take to heat the air coming into the space and not the air you are exhausting. the calculation for this is simple. BTU's= CFM x 1.08 x change in temperature. If the air is coming in from outside in the winter at 20 degrees and you want to heat it to 70 then your change in temperature is 50deg. From this point if you use electricity you divide the BTU's by 3413 to get the KW of heat required.
Hope this helps