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.

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 $2.0\textrm{W}$.