This is fairly unsafe...but it will work in a pinch.
This was suggested by an electrician to keep the house from freezing about 5 Christmases ago.
Turned off all of the breakers and the main...it would be bad to fry the lineman.
Picked up a 5500 watt generator, put it under the outside porch roof. Got heavy gauge wire, got the one end plugged into the 220v output of the generator. (Here is where it gets dangerous) got a male electric dryer plug and hooked it to the other end of the cable. Plugged that end into the dryer outlet. Tie-wrapped the heck out of this plug to the wall box.
Fired up the generator and turned on a few breakers to the furnace and water pump and a few outlets.
Yes, I know it would be bad if the main were turned on, yes I know it would be bad if it came unplugged and the male plug were exposed. This was only in a pinch and two weeks later got the same electrician back to hook up a break out box and do it the right way.
Top Ten Things Overheard At The ANSI C Draft Committee Meetings: (2) Thank you for your generous donation, Mr. Wirth.