Yes, you need to keep the efficiency of the PSU in mind when measuring the input Watts, and the efficiency varies with the load.
For measuring the input Wattage I love the Kill-A-Watt. It does volts, hz, amps, VA, Watts, kWh etc.
It is an excellent tool. Check the power consumption of that old fridge/freezer in the garage. You will be surprised!
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