Based on my experience

If it's something along the lines of a Candlelier then it will do a good job of heating a cup of water, as long as it's a metal cup. It'll keep a porcelain cup warm I think it would take a quite a while for it to heat up to a significant degree (YMMV, I tend to be impatient at times)

Note - I've never tried to boil water with it - "hot" is good enough for me when it comes to melting snow and making coffee, tea, cocoa and instant oatmeal. I've meant to try lining the inside with a sheet of tin foil to see if it gets the top hot enough to boil water but haven't ever gotten around to it.