When I am drawing less than 15 watts from the AC inverter, the display shows 0, so I have no idea how much power I’m using.
I’ve tried a few different combinations of devices and the lowest number I’ve seen is 15, but even then it quickly jumps between 15, 16, and 0. Is this normal?
Because the power is too small, the current is not well sampled, AC output below 15W will not be displayed. But it still will charge the device normally, do not worry.
Most likely The power Watts displayed on the AC180 is the Apparent Power, VA Watts. It may not display the Active, True Power, Watts, consumed from the battery in the AC180.
NOTE: It is quite possible that all the Bluetti Power Stations, display Apparent Power, VA Watts, and not the Active, True Power, Watts.
On AC circuits understanding the interaction between Power Loads and Power Sources is a bit complex, loads on a Power Source can have Reactivate Power, VAR Watts.
Reactive Power, VAR Watts, does NOT consume battery power from the power source, but it is a property / quality of the load devices / appliances related to the Resistance and Impedance of the loads on the power source. This can cause a phase difference between the AC sinusoidal Volts, and the AC sinusoidal Amps.
There is a relationship between Apparent Power, VA Watts, and Active, True Power, Watts, and Reactive Power, VAR Watts. The relationship can be described by the Power Triangle below.
You may measure the Active, True Power, Watts, of a load (devices / appliances) using a Kill-A-Watt meter plugged into one of the 120 Volt outlets of the AC180. See example below for the AC200MAX.