A cellphone or tablet is a fanless device. So is the 12″ Macbook. That means you can do whatever is possible at any point in time within a TDP of approximately 5W.
Here is the power consumption of my cellphone over a 12h period. The scale on the left is mW, down is discharge, up is recharge (plugged in). It’s basically limited to 5W, and that only for short periods of time.
These devices also have batteries, and when they are running on batteries, they need to be sleeping most of the time and have their display off. Whenever they are not dark and/or sleeping, they drain the battery, fast.