Cool! Wrong formula... You're modeling the N900 as a resistor (voltage goes up, current goes up), when the main load on the battery is a switching DC-DC power-supply (voltage goes up, current goes down). I'd go with P=I*V, and since the battery voltage shouldn't change much, and in the opposite direction of the current (increased voltage drop due to internal resistance at the heavier current), I'd neglect it: P ~ I is conservative. So by my math, you've got increase by 2.5 only (but actually less). I'd figure: Voltage ratio = ((1.35+1.2)/2)/1.075=1.186 Frequency ratio = 600/250 = 2.4 Predicted power ratio = 1.186^2*2.4 = 3.37 So, actually, it's less than V^2*F.