Sorry to clickbait with that title... but it's actually true. I can help you improve power use by 140x—for power off power consumption, at least.
By default, the Raspberry Pi 5 (like the Pi 4 before it) leaves the SoC powered up (just in a shutdown state) when you shut down the Pi.
Because of this, a Pi 5 will still sit there consuming 1.2-1.6W when completely shut down, even without anything plugged in except power.
That's a lot—even compared to a modern desktop PC!
Why is this?
Apparently some HATs have trouble if the 3v3 power rail is off, but 5v is still active—which would be the case if you completely power off the SoC, but still have your 5V power supply plugged in.
Because of that, the Pi ships by default with the setting
POWER_OFF_ON_HALT=0, and the Pi eats up precious watts all the time.
Fixing the Pi's power consumption
The fix is, thankfully, easy.
Edit your EEPROM config by running
sudo rpi-eeprom-config -e, and make sure the following settings are configured:
[all] BOOT_UART=1 WAKE_ON_GPIO=0 POWER_OFF_ON_HALT=1
The first setting is irrelevant here, but I'm including it for completeness. Also,
WAKE_ON_GPIO doesn't seem to do anything on Pi 5 (since there's a power button and pads on the board for power switching, instead of GPIO-pin-based power control), but it's still there for now. I'm mostly including it because you can set these options on the Pi 4 too, and get it to reduce it's powered-off consumption too!
Save that configuration and reboot, then next time you shut down, you should see power consumption go down from 1-2W to 0.01W or even less:
Why can't we change the default?
Well... if it were up to me, it would be changed. But it's not :)
We could either name and shame the HATs that don't work correctly with 5V but without 3v3, or we could push for the Pi team to figure out some solution that allows everyone to default to
Can you still boot the Pi 5 with
Yes! The power button still works just as normal, and the red LED is still illuminated when it's shut down.
The RTC still keeps time, too, so watchdog-related functions (like booting during some interval, or at a certain time of day) should also work. I haven't personally tested this (yet), but here's a forum thread with a little more info.