What is the minimum voltage for general loads and peltier units
The voltage (not power) rating on an appliance or electronic device is the voltage that the device is designed to operate on.  If you have a 12 volt car radio, and connect it to a 6 volt battery, the radio is very unlikely to operate, but will not be damaged.  If instead you connect it to a 24 volt battery, the radio will very likely be destroyed because it is not designed to handle that voltage.

If you connect a device rated for 12 volts to your 230 volt mains, you will likely get smoke, flames, much excitement, and a destroyed device.

Many electronic devices these days are rated for 12 volts or other low voltage, but come with a power supply that can plug into 230 volt mains, and reduce the voltage to the required voltage.  Many such power supplies are built into the mains plug (we often call them "wall warts"), so to many people, they may just appear to be a mains plug.  If you look at the lable on one of these "wall warts", you should see input and output voltages listed.