The Raspberry Pi 3, launched this week, is more powerful than the Pi2. It also requires a bit more electrical power if you’re going to be hammering those four A53 CPU cores. How much more? The recommended PSU is 5.1V, 2.5A, which is 0.5A higher than for previous Pis.
That doesn’t mean the Pi itself needs that much, but with four USB ports onboard, the PSU has to cater for power-hungry USB devices as well. I usually publish some power measurements each time a new Pi is released. But this time I learnt something new about the way I do my measurements, and I felt it was important to share it. All measurements have their limitations and it’s important to understand them.
I’m well aware that I do not know everything and have learnt an awful lot through the Raspberry Pi. Not knowing something is not shameful or shocking – as long as you don’t pretend to be what you are not. I’m here to learn and, hopefully, to pass on what I learn to others.
To make my Pi power measurements, I use my Emeter (essentially a multimeter with a calibrated 20A shunt) and a bench PSU.
It’s useful to do this because I can see if the readout on the PSU matches that of the eMeter. This gives me a useful sanity check of the measurements. But I’ll let Gordon Hollingworth explain the limitations of my measurement approach…
The problem with those measurements is that they don’t measure instantaneous peak power consumption which is extremely important. When your processor kicks off to do something important it can easily triple the power consumption for a few micro seconds. You’ll not notice this on a DVM…
What Does This Mean?
It means that my Emeter measurements are…
- fine for looking at overall power consumption, for example if you want to work out how long your battery will last for
- not so good for choosing the rating of your PSU, which needs to take instantaneous peak power consumption into consideration
So, when specifying your PSU, follow the official guidelines. But there is one further point to bear in mind – the “USB budget” (also from Gordon)…
But we do assume 1.2A continuous for the USB supplies, so if you’ve got nothing plugged in there then you’re looking at 1.3A min instead… But considering HDDs can pull 2A from a USB connector (when the spec says a maximum of 500mA you have to wonder why PC manufacturers allow this)
This explains the difference between what is specified and what I measure. It’s important to understand that the 2.5A is specified to work in pretty much all circumstances you can think of. But in many cases, a bit less will be OK, depending on what you’ve got plugged into your Pi.
OK. With the preliminaries over, let’s get on with the measurements…
These tests are done as a comparison of all consumer Pi models. The current consumed is measured while the Pi is…
- Loading LXDE
- Watching 1080p video
- Shooting 1080p video (without saving it)
As you can see clearly in the chart and table, these were all exactly the same as the Pi2B. I was surprised, but probably shouldn’t have been because the above tests don’t really test the differences due to the more powerful CPU cores.
The current draw when in shutdown state was 0.06A
Multi-threaded CPU Tests
I ran the same tests I created for Pi2. Other people do “proper official” benchmarks. I wanted to do something fun. (We’re sorting the Scrabble word list again.)
The Python script and data file is on github if you want to do your own…
git clone https://github.com/raspitv/pi2test/
Here’s where differences start to become apparent. Hammering all four Pi3 cores takes 480 mA more than idling, whereas the corresponding Pi2 figure was 190 mA. And don’t forget these measurements are blind to peak instantaneous current.
Also don’t forget that the Pi3 “does the job” in a little over half the time using a little under twice the power. Quantifying this…
7.9s * 420mA = 3318 mAs for Pi2
4.5s * 720mA = 3240 mAs for Pi3
So, for a given “piece of work”, you might argue that the overall power budget favours the Pi3, but the difference is not large and a lot more exhaustive work on this would be needed to draw any proper conclusions.
But you can safely say that, for these tests, the Pi3B is nearly twice as fast as the Pi2B. I’m sure other people will publish other benchmark tests (if they haven’t already).
Since the Pi 3 has built-in WiFi I thought I’d try to test the amount of power this consumed as well. The cleanest way I could think of testing this was just to download a file in the command line. So I downloaded the RasPiO Duino manual which is about 17 MegaBytes…
This was done several times. The idle current was 0.24 Amps, with occasional fluctuations to 0.28A. The first time, the file downloaded at 1.7 MB/s and the current peaked at 0.36 Amps. Trying several more times, I was unable to repeat this. Further attempts were all around 500 kB/s and peaked at 0.31 Amps with a reading of 0.28-0.29 Amps most of the time.
So, not very conclusive, but we can deduce that hard use of the onboard WiFi adds 40-100 mA (but it’s ~40 mA most of the time).
The Pi3B uses about the same power as the Pi2B except when the A53 CPU cores are being pushed. Then it can use ~300 mA more than Pi2B, but will get the job done nearly twice as fast.
Stressing the WiFi adds 40 mA with occasional peaks up to 100 mA. But when not in constant use, the wifi adds no noticeable difference to the overall consumption.