Question about Power use monitors-inconsistent readings

[Sparkswillfly]

Have any of you used appliance power use monitors to measure how much power a device is using? I recently bought one that plugs into a wall socket and then you plug the appliance into the device and it tells you the voltage, amp draw, wattage and Kwh the appliance uses.

If I plug a fan heater into it with no other devices running, it says the heater is using 3300-3400 watts and 14 and a bit amps (we use 220-240volts here) However, if I plug a multiboard surge protector into the power monitor and plug the heater into the multiboard along with a computer, the heater is now only using about 2200 watts on the same setting-about 2300-2400 overall including the computer equipment (the heater still feels just as effective too). Then if I plug the power monitor into one of the multiboard sockets and the heater into the monitor,(so just the heater on the monitor without the computer) the monitor has it back at 3300-3400 watts. I don't know a huge amount about electricity but what could account for this difference in power use? Poor quality power monitor?

I have had this heater run before on 15amp circuits for more than half an hour plenty of times in the past with a PC running on the same circuit and no issues. It causes me to have a hard time believing the heater could really be 3300 watts.

Apologies if this isn't the right area to post this

 

[pc1966]

There should be a rating plate on the heater giving the approximate power use.

I would suspect it is a fault of the monitor. If you flex it slightly when in use does the reading change?

Do you see similar changes in power when testing on a lower power accessory (i.e. stand light)?

 

[Lucien Nunes]

Agree that the power monitor is probably defective. Many have known flaws such as measuring the apparent power in kVA but displaying is as the real power in kW. The heater load is an easy one for it to measure, since it is a pure resistance and has unity power factor. You might like to test it with other heaters of known power rating, to see if it's even in the ballpark.

Note that any disparity between a heater's rated working voltage and the actual voltage of the supply, can cause a significant change in power consumption since the power depends on the square of the voltage. E.g. an appliance rated to use 1kW on 240V will reduce to 918W if the supply voltage drops to 230V.