I dont think you can accurately assess the capacity of bars in relation to the wattage consumption of 12volt systems such as fan blowers, radio etc ,aircon ,headlights etc. These take power from the the small 12volt battery. The power used by the 12v battery is topped up by the HV battery, so will indirectly deplete a bar in time, and require more petrol to top it up again. But the traction motor also shares the same 'Bar 'and will take the lions share of a 'bars worth' of power when active. And this amount of consumption tends to suggest the bars represent a large and relatively vague measurement subject to fluctuations in precise accuracy.
I have never yet been able to predict when EV power will recharge itself, or not. Sometimes the engine starts when there are still plenty of bars, other times it does not start even when there are only 2-3. bars
I dont think waiting to see how quickly 12v systems alone would drop a bar would give a meaningful or accurate measure. You could probably tell using more sophisticated measuring instruments, but the bottom line is that any unnecessary use of heating, aircon etc comes with a small fuel penalty. Which will reveal itself as a change in mpg.