These are good calculations also. I do like the electric idea if you have 220. Check Northern Industrial too.
Size Your Heater!
Which model is right for you? Start by determining the following:
How large is the area being heated in cubic feet? Calculate area dimensions (Length x Width x Height) = Total Cubic Feet
What is the desired temperature increase to be gained by the heater? Calculate (Desired temperature(with Heater) - Area temperature(without heater)) = Desired Temperature Increase
Calculate the BTU's required to achieve desired temperature. Calculate ((Total Cubic Feet x .133) x Desired Temperature Increase) = BTU's Required