When I have had power outages I just leave the incubator alone and I do not open it. I have never known how much the temp. dropped because I take my thermomter out once I get it set, that way I don't keep trying to adjust it.
leave the bator closed and wrap it in dry towels, blankets, quilts, jackets, ect. any insulation will do. kinda like wraping insulation around your hot water heater to save energy. be careful and remove this when the power comes back on because it will start to produce heat and will lose it much slower than normal if you don't remove it.
i use this method to regulate a homemade incubator. it works very well but can lead to temp spikes when something changes and heat is still being produced internally.