How to design a power circuit for a device that's been prototyped

I want to build a device that will run on batteries that is rechargeable via USB charger. I've never built a power circuit for any devices before and wanted to know the steps taken in its design.

Here are some additional details about my "device":

The device contains a uC chip (ATmega328P - DIP packaged), an OLed screen, and perhaps an RTC (real time clock) and Wifi-module.

Here's How I Would Approach Designing the Power Circuit:

I'm guessing the steps are as follows:

  1. Prototype the device (being sure to include all modules that will be connected to the embed system) and write the software that will run on the device.

  2. Measure the device's power consumption under typical usage, noting max/min current draw, and max/min voltage levels.

  3. Decide on the duration I want the device to run without resupplying new batteries (noting that in general, longer runtime = larger battery size). Find a battery and charging circuit (after deciding if I want the device to be rechargeable via USB connector or something) that meets the duration requirement.

  4. Find a linear voltage regulator (if it isn't included in the recharging circuit). Connect it between the power source (battery) and the uC or peripherals that needs it own separate power supply (assumed to have its own v. regulator).

  5. Prototype power circuit, connecting it to main device.

  6. Test main device & power circuit under typical software application/usage cases.

  7. Debug if problems related to power consumption/failures occur.

Does that sound reasonable?

Best,
MT

This Adafruit Feather board has a charger on the board : Adafruit Feather 32u4 Basic Proto : ID 2771 : $19.95 : Adafruit Industries, Unique & fun DIY electronics and kits
Or an other board of the Feather family.

Power consumption can be dramatically reduced (by a factor of up to a million) by having the devices "sleep" or powered down whenever possible. Here is a good summary for the processor.