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The documentation for the Fan:OnOff object in E+ shows that it models a constant volume fan that cycles on and off and would calculate power using efficiency, air flow rate, and pressure drop. However, if this object is used as part of multi-speed compound objects (like the AirLoopHVAC:Unitary:Furnace:HeatCool object that combines a fan, cooling coil, and heating coil), then E+ calculates fan power slightly differently. Instead, the ratio of the compound (Unitary) object air flow rate to the fan’s (Fan:OnOff) maximum air flow rate is used to determine the power at alternate fan speeds. Below the last minimum input field for the Fan:OnOff object (Air Outlet Node Name) is an optional input field for Fan Power Ratio Function of Speed Ratio Curve Name. This field must be used to model multi-speed fan operation. An optional fan total efficiency ratio curve is also available to model efficiency differences at alternate fan speeds.

In the MultiSpeedACFurnace.idf example file, the fan's max flow rate is 1.7 m3/s. If you look at the last 8 input fields of the unitary object, they list air flow rates for the four stages of heating and cooling (illustrated below). Heating has just the same 1.7 m3/s for single-stage operation, while cooling has four separate air flow rates (one for each stage).

AirLoopHVAC:UnitaryHeatPump:AirToAir:MultiSpeed,
  DXAC Heat Pump 1,        !- Name
  .... 
  1,                       !- Number of Speeds for Heating
  4,                       !- Number of Speeds for Cooling
  1.7,                     !- Speed 1 Supply Air Flow Rate During Heating Operation {m3/s}
  ,                        !- Speed 2 Supply Air Flow Rate During Heating Operation {m3/s}
  ,                        !- Speed 3 Supply Air Flow Rate During Heating Operation {m3/s}
  ,                        !- Speed 4 Supply Air Flow Rate During Heating Operation {m3/s}
  0.4,                     !- Speed 1 Supply Air Flow Rate During Cooling Operation {m3/s}
  0.8,                     !- Speed 2 Supply Air Flow Rate During Cooling Operation {m3/s}
  1.2,                     !- Speed 3 Supply Air Flow Rate During Cooling Operation {m3/s}
  1.7;                     !- Speed 4 Supply Air Flow Rate During Cooling Operation {m3/s}

I hope this helps!