Sometimes called a block heater it's a small mains powered immersion heater that heats the cooling water before the engine is started, so effectively you are starting on a cold morning with a warm engine.
Usually fitted on a circuit with a time clock so it switches on an hour or so before you need the car.
Heating and demisting are there straight away and fuel economy is improved.
The only downside is having to remember to switch off/and unplug the mains supply to it before you drive off - so no use if you are forced to park in the street.
An example here - www.enginepreheater.co.uk/
I have no idea how fitting one of these would affect the car maker's warranty however.
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