Nameplate Power Won't Reveal Actual Power Consumption
The horsepower, wattage, current, and/or voltage will not tell you how much power your fan is actually consuming. That is because the amount of power consumed depends on the load that the fan and your duct system imposes upon the motor. In other words, the actual power consumed depends very much on your particular installation.
Power Consumption Follows an Affinity Law
The relationship between fan RPM and power consumption follows an affinity law. Specifically power is proportionate to the cube of the RPM. So cutting your fan speed in half reduces your power consumption by 8x.
This relationship makes clear that reducing fan speed as much as possible is critical to reducing power consumption. However, affinity law alone won't help you determine power consumption in absolute quantities.
Reducing Your Furnace Fan Speed
Many household furnace fan motors are designed to operate at a variety of discrete speeds. This is usually accomplished by applying power to one of a variety of winding configurations built into the motor. Where this is the case, the furnace installation manual usually provides a table of connection wire colors and corresponding speeds for your fan.
The furnace fan speed is meant to be set to an appropriate value for the particular installation. In my experience, however, the speed installers typically set the fan to is much higher than optimal.
Fan Power Consumption Increases With Constricted Air Flow
A fan pushing air through a short length of oversized duct uses much less power than the same fan pushing the same rate of air through small, extremely long, or damped ducts. Reducing constrictions like filters, grills, and closed dampers on the entire air circuit including the return air path will reduce the fan power consumption for a given amount of air flow.
Measuring Actual Power Consumption Requires a Particular Type of Clamp Meter
Furnace fans typically get their power from a controller or relay within the furnace. It is at this location that electrical measurements can be made to determine fan power consumption. In order to measure the actual power consumed by a furnace fan, you need to do be able to do the following:
- on a common timebase,
- measure the waveform of voltage applied to the motor, and
- measure the waveform of current through the motor
- calculate the product of the above two waveforms
- find the time average of the product waveform over several cycles
The above is effectively what a real power meter does. The result will be in watts (W) which is what your utility power meter integrates over time to determine a watt-hour (Wh) reading which they use to calculate your bill.
The above is pretty complicated to do. Their are fairly common, specialized test instruments designed exactly for this purpose. The following are two examples:
There are some conceivable other ways of measuring real fan power consumption, but they are more complicated, error prone, and dangerous.
It is not possible to measure AC power consumption of a motor with a multimeter. Nor is it possible with an ammeter-only. In order to measure power consumption, a single piece of test equipment must measure both voltage and current over time (think thousands of samples per power cycle) and process that data to determine power consumption.