Hey I saw this video on YouTube , judging by the comments it seems to work.
Incase they shut off our electricity
Focus on one room instead of the home. Make a "base of operations" in your warmest room. Get a nice set of windows that are large for passive solar heating and cover them when the sun is down with space blankets then a large, warm blanket. Get propane and use the propane to make some warming food and heat water for hot water bottles, which are to be placed under warm blankets. Better to just boil water a bit and use the bottle to heat you for 8+ hours than have to deal with running out of propane all of the time using it for heating. Wear a merino wool base layer (store in ziploc when you aren't using or moths are an issue) under your clothes and you are good to -5F (Got to this temp last March actually in a non-insulated room with just 2 layers of OSB). Look around at camping/survival forums for good products as far as blankets and clothing layers go. Make sure you have a lot of books to read :) If the grid is down I'd just a defensible room over one with large windows though, not sure if that is your primary consideration.
Hot water bottles are amazing, if you have family members make sure you are all sleeping in the same bed. With just my wife and I we get hot even when camping in the low teens (Fahrenheit) and I will only wear a base layer.