Inspired by Kevin Darrahs video on Low Power Arduino! Deep Sleep Tutorial, I set out to see if I could apply some power saving to the basic blink program.
After the usual trial and error, I did get results. Normal current usage of lit/unlit Arduino nano running blink is 14/12mA. Replacing delay() with my own function going to sleep in power-down mode reduced the current to 6.6/4.6mA.
The power LED is always lit, consuming 2mA. Kevin was running his tests on a breadboard Atmega328P, obviously the Nano has a bunch more circuitry so thats where a bunch of current goes to.
And also, both the Arduino Nano and Arduino UNO will happily run off a 4 volt battery. Just connect + to the 5V rail (not VIN) and – to GND. I was using a battery from a mobile phone (nominal 3.7V but fully charged to 4V).
Source can be found here: PowerBlink.ino