Nothing beats the MB block heater. Your 1000w coolant heater draws more than twice the power to do the same job and loses heat out through the radiator. The MB block heater delivers heat directly where you need it and does an AWESOME job. 1 hour on the MB block heater gets my '83 240D started at -20F no problem. 2 hours and it starts like its July. Back when I was an apartment dweller I had a 12v marine deepcycle battery I used to power an inverter and run the heater. After one hour my battery was still at ~60% capacity. I used this setup over 3 winters with no issues except at one point the cord to the heater frayed apart right at the block. Replaced it and all was well again. If you needed more capacity you could get 2 batteries. If you were really slick you could install the battery so the car charged it as you drove. If you were really really slick you could set it up so the cord to the block heater was already connected and a remote car start system put power to the inverter with the push of a button from inside the house!

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Sadly Benz-less