← Back to context

Comment by 0xbadcafebee

13 hours ago

I think the Pi 3 range is a sweet spot for low cost, low power draw, decent-enough CPU. Newer models draw increasingly more power; going from 1.4W to 2.8W may not seem like much, but that's half your battery life. There's a few differences in Raspberry Pi 3 versions that may lead you to buy one or the other:

- The Pi 3B has 10/100 Ethernet, 802.11n (single-band) WiFi, Bluetooth 4.1. Power idled at 1.4W and peaked at 3.7W.

- The Pi 3B+ removed the 10/100 Ethernet in favor of USB Ethernet (~300Mbps w/USB2.0). CPU cores were overclocked from 1.2GHz to 1.4GHz (so a heatsink is more necessary), with ~15% increase in benchmark performance. It added 802.11ac (dual band) WiFi and Bluetooth 4.2 w/BLE. Power idled at 1.9W and peaked at 5.1W. This is also the only 3-model supporting PoE (w/ extra HAT).

- The Pi 3A+ removed Ethernet and reduced USB to a single port. The RAM was reduced from 1GB to 512MB. Power idled at 1.13W and peaked at 4.1W. The A+ form factor is more compact. Overall the 3A+ is smaller, cheaper, and less power draw than the 3B+ (but not as low as the 3B).

The lowest power draw with acceptable performance is the 3B. For slightly more power draw and more CPU performance, go with 3A+. For "everything" (including PoE) the 3B+ is it.

If you want the 3A+ but don't need the video, want a smaller form factor, and half the power draw, the Pi Zero 2 W is it. Though the Pi Zero 2 W is supposed to be cheapest, due to demand it's often sold out or more expensive. The 3A+ is still cheap (~$25) and available, with the downside of the higher power draw and larger form factor.

(disabling HDMI, LEDs, Wifi, Bluetooth, etc reduces power draw more. in testing, the 3A+ drew less power than the Zero 2 W with everything disabled. all of them draw ~0.1W when powered off)

6 comments

0xbadcafebee

Reply
tzs  12 hours ago

> I think the Pi 3 range is a sweet spot for low cost, low power draw, decent-enough CPU. Newer models draw increasingly more power; going from 1.4W to 2.8W may not seem like much, but that's half your battery life.

Is that with the same load? The chart in the article shows a Pi 3 and Pi 4 using the same idle power, with the 4 drawing more under full load. But the 4 can do more at full load, raising the question of what would be the 4's power usage running a load equal to the 3's full load?

  • 0xbadcafebee  8 hours ago

    The author's chart is incorrect. He has the idle power at "1" for the Pi 2, "2" for the Pi 1/3/4, and "3" for the Pi 5. No other published power draw numbers are whole numbers like this, they are floating points, like 1.2, 1.9, etc. Google around and you'll find several different power testing comparisons with more detail.

    Most reports show the Pi 4 drawing ~2.8 W in idle headless mode, and the Pi 3B+ drawing ~1.9-2.0 W in idle headless mode. With full load the Pi 4 draws more (6.4W, to the Pi 3B+'s 5.1W) with the same test procedure.

    But you do have to check the testing method; enabling/disabling hardware features changes the figure, and each additional USB peripheral draws more power. Otoh, to get a "max power draw" reading you have to enable everything and stress all CPUs at once, and then it will dip under thermal load.

    • tzs  7 hours ago

      I had Home Assistant on an RPi 3 with 1 GB RAM and a USB thumb drive until sometime maybe 7-10 days ago when I switched to an RPi 5 with 8 GB RAM and the 256 GB SSD and NVME had sold by the RPi company.

      The 3 was and the 5 is plugged into an energy monitoring smart outlet. Here's a graph of power for that outlet [1] for the last 30 days.

      HA uses long term statistics for older than 10 days, which is why the 2/3 to the left shows much less variation. The switch from the 3 to the 5 is somewhere a little way into the right 1/3, where it is using shorter term data which has a much higher sample rate.

      It does look like the average power goes up with the 5, but not nearly as much as I would have thought. The left 2/3 which was definitely all RPi 3, averages 2.48 W. From Jan 17 to the present, which was definitely all RPi 5, it averages 2.64 W.

      If those reports of 2.8 W for an idle 4 are accurate I'm curious why my HA 5 is lower. I've got an unused energy monitoring smart plug of the same kind my HA Pi is on. I'll have to put the 4 on that and see how it compares.

      [1] https://imgur.com/a/qa7SD2O

  • MBCook  7 hours ago

    Right. In CPU scheduling this is the “race to idle” idea.

    Sometimes it’s more efficient to run a task at full CPU power for 25ms than low (non-idle) CPU power 100ms.

    It wouldn’t surprise me too much if the 4 could run the 3’s full load in less power than the 3 does.