How to gauge wattage, from input or output

[Rolomoto]

I'm trying to figure out how many watts a power supply for a router is pulling. It says:
input 120VAC 0.58a
output 12 VDC 1.8a

Which one would you base a wattage calculation on?
Thanks

[TheAmmoniacal]

Try to measure it, don't trust the markings.

What device do you want to know the power consumption of?

It should mean that your PSU can draw 0.58 A max from the 120 VAC supply (of course it will most definitely always draw a lot less), and can supply 1.8 A max at the 12 VDC output.

The power usage of your PSU will depend on the power usage of your router. The power consumption of your router will be changing constantly, depending on what it's doing.

You can expect your PSU to always draw about 15% more power than your router, assuming it's 85% efficient (which is a reasonable figure if it's an SMPS).

[michaeliv]

As the previous poster pointed out, the difference in power is due to inefficiency.
However in your case it says it can draw up to 70W from the wall but output max 22W.
I'm guessing the difference is because it can draw 70W when plugging it into the wall. There will be a 70W draw while the caps are charging for micro/mili seconds.
So depending on what you need the power calculation for, use the corresponding figure.

[Augustus]

Usually somewhere in the range of 5 ... 12 W for a typical consumer router (drawn from the wall).   

[uncle_bob]

Hi

Simple answer for most small gizmos:

The output is spec'd as DC. There is no power factor, amps X volts gives you power.

The input is rated simply in terms of current. That matters for fusing so it's a relevant spec. As mentioned above, the power factor on a lightweight transformer is likely pretty terrible. Maybe half of the current goes to power, the rest is just wasted energy. The efficiency of the device is on top of that. If you have a 70% efficiency on top of the power factor it all sums up.

Next up you have the somewhat odd fact that even on resistive load stuff, they can't seem to get the numbers right. If you spend some time checking appliances, they pretty much never pull to much current. They often pull 20 or 30% less current than they should. Why should? If I bought a 500W toaster oven ... it should be 500W. In this case it's both tolerances and marketing.

Bob