Battery Powered

To power portable IoT modules, we need to regulate battery voltage to a stable voltage, usually 3.3V.   A LiPo pack is attractive: it can be very small and still pack a considerable power and it holds power well.  A 150mAh LiPo with integrated protection circuit is 1"x2/3"x1/3" and 0.2oz.  The LiPo is nominally 3.7V and can start from 4.2V and  falls off rapidly after 3.3V.  So use an LDO is still very efficient, > 80%.  The LDO should have low ground current and very low dropoff voltage.  Here is a simple small board that can supply 250mA.

Alkaline battery or NiMH rechargeable is another option.  A single cell has a voltage from 1.5V to 1.0V, so a boost circuit is necessary.  Fortunately, IC's are available to operate to this voltage.  Here is a board that supplies 3.3V.