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
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
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.