How much power does a Raspberry Pi consume?


Posted by Diego Assencio on 2014.01.04 under Technology (Raspberry Pi)

I have recently purchased a USB current and voltage tester and decided to check how much power my Raspberry Pi typically consumes. The tester device works simultaneously as a voltmeter and as an ammeter: it goes in series with the USB device and has an LED display which shows the current and voltage being used (see figure 1).

The Raspberry Pi website claims the model B uses between 700-1000mA (depending on the connected peripherals) and needs 5V to operate. I have measured, however, significantly smaller values for the current even when the Raspberry Pi was under heavy load.

Fig. 1: Measuring the power consumption of the Raspberry Pi. The pictures show examples of the measured current ($I = 0.36\textrm{A}$) and voltage ($V = 4.96\textrm{V}$) respectively.

With the measured current $I$ and the voltage $V$, one can compute the power consumption using: $$ P = VI $$ For $V = 5\textrm{V}$ and $I = 700\textrm{mA} = 0.7\textrm{A}$, the power consumption would then be: $$ P = 5\textrm{V} \times 0.7\textrm{A} = 3.5\textrm{W} $$ However, the highest current I registered was $I_{\max} = 0.43\textrm{A}$ and the highest voltage was $V_{\max} = 4.95\textrm{V}$ (these maximum values were seen during boot time and under heavy load). The maximum power consumption I saw was then: $$ P_{\max} = V_{\max} I_{\max} \approx 2.13\textrm{W} $$ which is significantly less than the $3.5\textrm{W}$ computed above. Under normal conditions, my Raspberry Pi consumed between $1.8\textrm{W}$ and $2.0\textrm{W}$.

Comments

Emmanuel on Jan 18, 2014:
It looks like your power meter is connected at the output of the USB charger.
Such chargers are usually a switching power supply with a voltage regulator. They have around 80% efficiency at best.
More info here: http://www.righto.com/2012/10/a-dozen-usb-chargers-in-lab-apple-is.html
Diego Assencio on Jan 18, 2014:
@Emannuel: Thank you very much for sharing this post with me! I will definitely take this information into account when purchasing my next USB chargers.

Indeed the charger wastes some energy to perform the voltage conversion and regulation. This is the reason why I did not measure the voltage and current directly at the pins of the charger as this would give me an inaccurate value for the power consumption of the Raspberry Pi.
shahzad.iqbal on Jul 31, 2014:
USB standard only allows USB ports to provide a max of 500 milliamp current.Anything which trys to pull more then this amount causes the port to shutdown.Try that by shorting all wires of USB on your PC. However this doesnt blow up the port.

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