Watt Meter Wattage Draw question

[mikego34]

I've been messing around with a watt meter at work by measuring various plug loads such as the fan I used at my desk. Well i took the voltage and the amperage that was drawn by the fan, which was measured by the watt meter and multiplied them to get a power use which ended up being 42 watts. Amperage was .357 and voltage was 118 which equals the 42 watts. My question is when i change the watt meter to measure the power draw it tells me 22 watts why am I off by a factor of 2 ?

 

[itsmoked]

Because, looking just at the amps and just at the voltage doesn't take into account the power factor or the lag between their peaks which the POWER setting does.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[mikego34]

Thank you sir that makes sense now. I wasn't accounting for PF.

 

[KuanYau]

To mikego34 (Mechanical)
(OP)

1. Your desk fan is connected to an [ac single-phase source].

2. For an [ac single-phase load], power = voltage x current x power-factor x efficiency ....(W).
Where the unit of power in W, voltage in V, current in A; power-factor and efficiency are [ratios (without unit) with values not
exceeding 1 (respectively)].

3. In your case, the power consumption is 22 Watt; where the value or the [product of power-factor x efficiency] is around 0.5, which is the reciprocal of 2.

I hope the above is helpful to you

 

[davidbeach]

The efficiency only matters if you're trying to figure out the work being done and is not something that a watt meter will include. If the fan is question is in fact 50% efficient, then the 22 electrical watts will only produce 11 watts worth of air movement.