I came across this question on another forum. I did some real-life tests in my own van. The picture shows my
caravan meter first with only the battery being charged. The second reading has the Ultraheat switched to 1KW. The next when 2KW is switched on and finally, the fridge is switched to mains electric.
The meter is a boon, especially when on a site with a limited electricity supply. Without a meter, the owner is not aware of when the water heater kicks in, or the fridge, or when the battery is charging.