Which to trust the math or the meter?

[Shocked and confused]

I was trying to figure out how many watts an inverter should have to run a nebulizer, on the bottom of the nebulizer it says 1.1A and 120V so the math should put it at 132 watts but I recently bought a Kuman meter and it's only reading 51.7 watts at the max. Anyone know why the math and the meter are so far off?

 

[rjenkinsgb]

There can be a big difference between watts and "VA" ratings in some types of equipment; also power consumption labels may be worst case, rather than continuous consumption.

If the nebuliser has either a motor or a simple transformer rectifier power supply, the "power factor", how well the current taken lines up with the instantaneous voltage through each AC cycle, may be poor, meaning the current * voltage wattage does not equal the "VA" power it consumes.

Or it could just need less power than the label says..

Your meter appears to have a "Power factor" display - have a look at that?
A pure resistive load will (or should) have a power factor of 1, it behaves as you would expect with current proportional to voltage.

Anything reactive (capacitive or inductive) will have a lower power factor; the instantaneous current * voltage is somewhat higher than what you get by multiplying independently measured current and voltage as the current is not in step with the voltage.

More info: https://circuitdigest.com/tutorial/what-is-power-factor-and-how-it-affects-your-energy-bills


Also note that with anything that takes a switch-on surge like a motor or PSU that has to charge up, the inverter must be big enough to take that without overloading and shutting down.
Normal power systems and fuses can take quite high surges for such things without any noticable effect, but inverters will often trip far faster and not take a load that is apparently OK for them.

 

[Shocked and confused]

There can be a big difference between watts and "VA" ratings in some types of equipment; also power consumption labels may be worst case, rather than continuous consumption.

If the nebuliser has either a motor or a simple transformer rectifier power supply, the "power factor", how well the current taken lines up with the instantaneous voltage through each AC cycle, may be poor, meaning the current * voltage wattage does not equal the "VA" power it consumes.

Or it could just need less power than the label says..

Your meter appears to have a "Power factor" display - have a look at that?
A pure resistive load will (or should) have a power factor of 1, it behaves as you would expect with current proportional to voltage.

Anything reactive (capacitive or inductive) will have a lower power factor; the instantaneous current * voltage is somewhat higher than what you get by multiplying independently measured current and voltage as the current is not in step with the voltage.

More info: https://circuitdigest.com/tutorial/what-is-power-factor-and-how-it-affects-your-energy-bills


Also note that with anything that takes a switch-on surge like a motor or PSU that has to charge up, the inverter must be big enough to take that without overloading and shutting down.
Normal power systems and fuses can take quite high surges for such things without any noticable effect, but inverters will often trip far faster and not take a load that is apparently OK for them.

Thanks for the quick response
Not going to lie you kind blew my mind a bit there, I'll have to read about the power factor thx for the link. I did another test with the meter and got .43 as the power factor and a max of 55.5W and 1.002A

 

[rjenkinsgb]

That looks about right:
Going from the info plate rating of voltage current, then multiplying by the power factor you measured:
120 x 1.1 = 132VA, x 0.43 = 56.76W so pretty close.

The 0.43 ratio of VA to Watts means the current is quite a long way out of phase with the AC voltage.

you kind blew my mind a bit

It did the same to me, many years ago when I first discovered power factor & VA vs Watts!

 

[Nigel Goodwin]

I was trying to figure out how many watts an inverter should have to run a nebulizer, on the bottom of the nebulizer it says 1.1A and 120V so the math should put it at 132 watts but I recently bought a Kuman meter and it's only reading 51.7 watts at the max. Anyone know why the math and the meter are so far off?

As has already been said - the figure on the unit isn't it's consumption, it's usually the worst possible case, and often plus a considerable margin (for ass covering) as well. An exception would be something that's a purely resistive and perfectly predicable load, such as a kettle.

As for an inverter, aim for one HIGHER than the maximum figures on the unit.

 

[Shocked and confused]

Thanks for the help all
I bought the meter so I could find out how many watts I would need to run something when there wasn't any info on it, so would it be a safe practice to multiply the max tested wattage by 3 when the power factor is low to be on the safe side or is there a better way?

 

[rjenkinsgb]

More like the maximum rated current * 3 * voltage (so VA * 3 rather than watts * 3) as a minimum, for things that do not have large motors or are motor-based.

You may get away with less, but until you try a specific combination of inverter and load, you just don't know.

Something like a power tool or vacuum cleaner, that is pretty much just a motor in a casing, may need 6 * rating or more, unless it has a soft-start system.
edit - typo

 

[Shocked and confused]

Thanks again for all the help I think I can wing it from here, or at least not blow anything up

 

[KeepItSimpleStupid]

The motor has to "start". Physics says that the current in an inductor can't change instantaneously.

What that really means is that the instant the motor starts, it it "looks' like a low value resistor. The inverter has to start that load.
It could be up to 5x the steady state value. Generators are rated in VA, not Watts. For restive loads A and Watts are the same.