The wattage on the label is the max power it can draw.
Nothing to do with the actual power it draws.
A monitor will say 150w but, when measured only 75w.
The power may not include starting currents... just to make it more confusing.
A PC power supply will have 400 watts or 500 watts on the label. The actual power it draws is dependent on the load(s) attached to it.
The little brick power modules draw power without anything connected. Normally just a watt or two, but may be more. The load connected determines the actual power drawn.
The battery in a computer should last 3-5 years. Need more time? Batt cost 75 cents.
You can't use a regular DVM (digital volt meter) to measure power. The meter can measure volts and amps, but the “real” power is not just the V*A or V*I the phase relationship determines the “real” power, number is 80% to 100% correct, you just can't tell. If resistive load, then is correct. Most loads are not only resistive.
Is it confusing yet?
Have fun.