I have an oil filled electric space heater. I have been leaving it on 24/7 in the winter. It does a very good job. If I need more heat, I have a small bench top ceramic heater I can turn on for a quick heat up, but I don't usually have to run it more than about 15 minutes once I enter the shop. I feel the oil filled heater is fairly safe. It may cost me another $10-$20 per month in electric costs but I feel it is worth it to me to have a shop to go out to whenever I want. If I don't have any ongoing projects in the winter, then I won't have any heat on in the shop.