Watts and Amps

You could use I=P/V every time you plug some something in to work out if you are going to trip the electric, but that would cause brain-ache fast.  Here is a table that lists how much amperage  is needed to satisfy a particular power requirement, assuming you have 240V in UK and 220V in France:

How many amps needed for given power in UK ad France

So if your microwave is 800 watts and your heating is 900 watts, and you are on 6 amp French electric, that will be enough to trip the electric supply.