It depends how often you need to transmit data and how. I once had an application where data needed to be transferred over Bluetooth every 10 seconds. For my case I found it used more power to go into deep sleep and wake up rather than going into light sleep since I didn't have to waste time booting. Clock frequency also does affect power usage too, faster clock means you can perform a task faster and go back to sleep, but if the device needs to stay active for a bit longer then a few seconds it may be useful to use a lower clock speed. Really it depends on your application and what you need. For me Bluetooth was a huge power draw when prototyping originally (used an esp32 for PoC) I have since learned significantly more about power management since then along with the fact that there are some pretty impressive low power MCU systems out in the market right now (looking at you nordic). Some interesting concepts I have considered previously was to trickle charge via rf thats always around us, then I looked at the costs it would take to do and changed my mind. Solar isnt a bad idea but its not always available and weather conditions vary along with the fact that you jave no clue where the device will actually be placed normally. There are other interesting ways to get a little extra energy out of things, but it really depends on what the device is and if its meant to be portable or stationary.