You will get 12V only off the battery. Your lights, water pump, furnace, stove, refrig, and water heater will all work off 12V. Your Air Conditioner, microwave and outlets all require 120V, which won't work off the battery unless you install an inverter. In 12V mode your water heater is actually running off of propane and your refrig is running off of propane but needs 12V for the controls.
And also note, one cold night running the furnace will probably deplete your battery. We can get about a week running off our battery providing we don't run the furnace, and any lights or anything unnecessarily (and that is using lanterns and portable battery operated lights mostly and saving the house battery as much as possible).
