Q:

RPi 3 - Reduce power consumption

I am looking to reduce the power consumption to a minimum on my RPi 3. 
For my project, the Raspberry Pi Zero is not an option.
for now, I disabled Bluetooth and WiFi and made sure to kill some process when the PI is running on battery until the power comes back. however, I am having issue disabling USB port individually as I need to keep one running do you guys have an idea how I can do that?
I am also looking to throttle the CPU as the PI, will be just checking sensors and sending payloads to DB. what is a reasonable arm_freq?
I would appreciate any tips on how I can reduce my consumption. and how I disable HDMI, Audio and USB from config.txt instead of using a command like /usr/bin/tvservice -o

A:

As far as I know, you can not disable only one USB. When you turn USBs power off, also the Ethernet port turned off. I don't know if there is a way to disable USB/Ethernet in config.txt, but in cmd line you can do it with:
echo '1-1' |sudo tee /sys/bus/usb/drivers/usb/unbind

To enable the ports use:
echo '1-1' | sudo tee /sys/bus/usb/drivers/usb/bind

What device do you use connected to the usb port? Maybe it could be possible to use GPIO to communicate with it (Serial, SPI, I2C).
--
About the config.txt, you can disable HDMI adding:
hdmi_blanking=2

I think that you can save a little bit of power disabling the LEDs on board (PWR and ACT). Add the following lines to config.txt:
dtparam=act_led_trigger=none
dtparam=act_led_activelow=off
dtparam=pwr_led_trigger=none
dtparam=pwr_led_activelow=off

About disabling audio, I'm not sure if it will help with power consumption, but you can try. I think that the line in config.txt is:
dtparam=audio=off

A:

This is an exercise in futility!
Disabling devices will make very little difference in power usage, which is mainly determined by processor load.
About the ONLY device which will have any affect is WiFi. 
You should consider a Pi3A+ which has lower consumption and no  USB hub or Ethernet. 

