Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
The EnergyPlus Input-Output Reference says for several curves "This curve is typically a biquadratic but any curve or table with two independent variables can be used:", so I think you can probably use a cubic curve. If that causes a problem, you should consider plotting a cubic curve, then using those values to derive a bi-quadratic curve with similar shape. I use Excel's Linest function for this regularly with good success.
