Manipulating Heat Source characteristics

I'm developing a measure allows the user to set characteristics of the heaters in the DHW system loop. I'm allowing them to set the fuel type, efficiency of heater, Tank type (instantaneous or tank), r-value of tank, and if it's in a conditioned or unconditioned area. I'm now trying to set the loss coefficient based on these parameters. I will need the surface area of the tank but am unsure of how to obtain this. Any ideas? Also how will the r-value of the tank affect the loss coefficients?

If you're using WaterHeater:Mixed there is no surface area of the tank as it doesn't have a defined shape, just a volume. If you're using WaterHeater:Stratified then Tank Height and Volume are input parameters and you can calculate a surface area from that. I'm not entirely sure what use you have for it though since the loss-coefficients are independent of surface area.