The left hand side is on.
Stoves (hobs) and ovens (cookers) tend to not only have on/off switches on the sockets they're plugged into, but they are also individually fused. This is also the case for refrigerators, freezers, washing machines and dryers. The large red switch here is the fuse to the mircorwave. The smaller red switch is the on/off switch for the outlet. And the microwave can (and should) be turned on and off on its own.
If you've known me for any length of time, you're aware that I am a magnet for unintentional fires. I'm a bit obsessive now about making sure appliances are turned off before I leave the flat. I say this is because I don't want to waste energy, but it's really about not coming home to a burned out flat. I really like being able to tell at a glance that everything is safely off. I did once flip the switch for the fridge by mistake. Oops! But no fire.