Do you ever pour yourself a cup of coffee or a hot drink and then set it down and do something else and come back to it and it is lukewarm or cold? I have the answer for you.
A friend gave me a candle warmer along with the candle in a heavy glass container. I kept it going for many weeks...but my coffee was cold!
I tried the candle warmer, which you can probably purchase wherever candles are sold and it keeps it hot! It has to be 18 watts at least to keep the liquid hot enough. But don't forget to turn it off when you are through drinking your hot liquids! Savings? You don't throw the coffee away when cool.