Most of the new ones have safeties that shut them off of they tip over, etc.

Or get an oil filled electric one.

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What city/state?
If it doesn't have water in the shed, maybe let it ride.
If you're in a warm climate and you'll need ac too, maybe the smallest minisplit.
They also make electric heaters for garages, with a thermostat.
So depending on your loads, maybe electric heater & window/thru the wall AC, or a minisplit for better overall comfort.

 

Fires from electric heaters can occur at the plug. Be sure your duplex plug is tight. Use a radiator type heater that heats up a thermal mass and surface temperatures will be at safe low temperture.

 

LOW HEAT: 750 WATTS, 8.5 Amps
_________ ___ OPERATE on a 15 AMP BREAKER Circuit

__ Maintain Cleanliness

____ _ _ https://www.vornado.com/shop/heaters/avh2-plus-whole-room-heater-with-auto-climate
____ OR Similar

 

Far as I know, no portable space heater is listed to be left unattended for extended periods of time. in other words, must use a space heater that is permanently installed, like a basebd htr.

have also seen heaters where the plug nearly melted apart, was left unattended for several days before I unplugged it. the prongs pulled out the plastic plug it was so soft.