I think 20 W might be a little too low, but would probably work. Perhaps 100W is a little high, but it's better to have extra power if you need it. Probably about 40 W is what is needed to keep a drink near 5 deg C with ambient at 25 deg. C, assuming good heat sinking and and good insulation. But, on a hot day, you might need that 100 W if you want the drink to be very cold. (desktop implies room temp and maybe 5 deg C is too cold for your drink)
These are just estimates, of course, and the details matter.