You might build a plastic tent enclosing the over winter plants in the metal building. I would buy a portable electric heater that way you have a temp control. Same with the potting shed. Just be sure to have plenty of clearance for the heater. No need to burn things down.

A 100 watt bulb will keep a small well-insulated room, like a well-house from reaching freezing temps, if you don't open the door but probably not a shed like you're talking about. Better go with the above suggestions to heat either the whole shed with an elec heater or build a box within the shed and use the bulb in that. Put in a thermometer and see if you're getting the desired temps before you put in the plants.

Thank you for your answers. I am on my way to Home depot..the cold came so fast, I have to prioritize my work. Thanks again.

Strings of Christmas lights. The old type kind.

Chere