If it has a concave bottom there is not enough physical contact to warm the cup properly.

This one certainly, defiantly not 24W.I had a brand-less cup warmer with 24W power and had the cup nearly boiling-hot

Fighting with this warmer takes the fun out of enjoying a cup of java.

On the positive side, providing you have more FLAT bottom mugs than the traditional ceramic mugs that have a depressed inner round, this warmer we heat up your cup and maintain the temperature really well

How many cups of tea are interrupted by ‘stuff’ resulting in wasted tea.

I don't know how many cups of coffee have gone to waste because I had to get up from my desk for various things in and around the office.

It keeps my cup of tea very warm for much longer than other products I have tried

They want you to buy their stupid cup

But this loss in temperature might lend credence to the argument of buying a cheaper cup warmer without the bells and whistles (I paid $26US for Cosori which is not outrageous).

I've never used a coffee cup warmer before.