Styrofoam is the best cup material for the best insulator and for keeping drinks warm and hot.
Stainless Steel insulated cups are also great insulated cups for keeping drinks warm and hot.
To keep a drink warm without electricity you can use a thermos, good insulated cup, insulated stainless steel cup.
You can wrap your cup with a cup sleeve or mug cozy.
Depending on the type of thermos boiling water will stay hot in a thermos for 6 to 12 hours.
You can safely put boiling water in a Thermos without any issues.
You do need to be careful with glass lined Thermoses as the quick change in temperature can cause it to shatter, but stainless steel Thermoses are fine.
The boiling water will stay hot for 6-12 hours.
As a general rule of thumb, you can keep food hot in a thermos for up to 5 hours, although some made of lower-quality materials can only heat the food for up to 2 hours.
The bigger the thermos, the longer it'll stay hot.
You can preheat your thermos by following the below instructions.
Take some hopefully boiling, but at least very hot, water and dump it into your Thermos.
Put the lid on it.
Let it sit for a few minutes.
Dump out the water.
Fill your Thermos with coffee/soup/hot chocolate/tea.
Enjoy super hot beverages all day long.