multi-speed and fan power

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The fundamental fan power calculation resides in the fan object and is based on user inputs for total efficiency, pressure rise, maximum flow rate, motor efficiency and motor in air stream fraction. These inputs, along with the flow rate entering the fan, and the flow ratio based on fan maximum flow rate, are used to calculate fan power.

PartLoadRatio = MassFlow / MaxAirMassFlowRate
FanRuntimeFration = PartLoadRatio / OnOffFanPartLoadFraction
FanPower = MaxAirMassFlowRate * FanRuntimeFraction * DeltaPress / ( FanEff * RhoAir )

As noted in the previous answer, there is additional information available to the Fan:OnOff object that can be passed from specific parent objects (namely the AirloopHVAC:Unitary*, and AirloopHVAC:UnitarySystems) which can account for fan curves (power vs speed ratio).

When the speed ratio curves are used with Fan:OnOff then:

PartLoadRatio = MassFlow / MaxAirMassFlowRate
FanRuntimeFration = PartLoadRatio / OnOffFanPartLoadFraction

FanRuntimeFraction = FanRuntimeFraction / SpeedRatio
SpeedRaisedToPower = FanPowerRatioAtSpeedRatio = f(SpeedRatio)
EffRatioAtSpeedRatio =FanEfficiencyRatioAtSpeedRatio = f(SpeedRatio)

FanPower = MaxAirMassFlowRate * FanRuntimeFraction * DeltaPress / ( FanEff * RhoAir )
FanPower *= SpeedRaisedToPower / EffRatioAtSpeedRatio

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!