Our power went out last May right after we got 5 chicks! That was NOT fun, but taught me to be creative. Fortunately I could light our gas stove, and heat water. I couldn't find a rubber hot water bottle anywhere, so we just filled jars and wrapped them in towels, and completely lined one half of the brooder with them. I also have a pizza steel (pizza-sized thick steel plate) which we heated over the stove and slid underneath the brooder, making sure there were lots of towels under the plate, and one between it and the brooder so we didn't burn holes in anything. This would stay warm for hours! I used a meat thermometer to make sure I wasn't making it too warm, and adjusted a lid to help maintain a good temperature. They had room to snuggle close to the heat, or get further away. It was super tedious, but they all survived the THREE DAYS without power! Microwaving bags of rice and using a heating pad sounds like it would be easier. When the power was on we also used a heating pad under the brooder. Just make sure yours doesn't have an auto shut-off as many of them do.
