edm3rd wrote:
Purchased a "Kill A Watt" 3-4 years ago. Tested on a lamp that had a 100 watt bulb. Device read 80 watts. Returned it the next day.
Question - did I give it a fair test? How would you determine it's accuracy for wattage?
For a load with a power factor of 1.0 (or nearly so) like an incandescent light bulb, you can independently measure the voltage and current with known (or presumed) accurate meters and multiply the two in order to get power. For devices with lower power factors, this simple approach doesn't work; you're only computing apparent power, not actual power.
The power of a resistive load varies with the square of the voltage, so if the voltage is a bit lower than the rated voltage of the bulb, you could easily get a rather different wattage. If we assume the resistance of the bulb is constant, a 100W 120V bulb would dissipate only about 84W at 110V. (The resistance is not perfectly constant, however; it varies depending on the temperature of the filament.)
