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I would like to know, how power consumption works.

I have a windows 7 host and a ubuntu desktop guest. When running, does it consume power for both systems? Or just for windows? Does reducing virtual pc's cpu and ram reduce battery consumption?

Please explain briefly, how does it work.

EDIT: Can virtualization be more effective (when it comes to power) than dual-booting? (assuming ubuntu battery-life is about half of windows)

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Turning the laptop on consumes battery power. If you never left the BIOS screen, and you did not boot into Windows at all, meaning you neither ran Windows nor Ubuntu in a VM, your battery would still drain... and actually at probably a high rate (since the fan would be spinning at max, the screen illumination would be up high, there would be no power saving features utilized, etc).

The harder you make the laptop work, the faster you will drain the batter. So, yes, in that sense, running a second OS in VM would make a CPU work harder and generate more heat, to make the BIOS turn the fan up, and drain more battery power.

Now, just because you are using two operating systems, you would NOT drain the equivalent of two laptops worth of power at the same time. You would not, for example, be able to drain the Windows's battery and leave the Virtual PC's battery still running. There is only one battery.

The Host computers monitoring of the system is all that is important, regarding the amount of charge you have. The Virtual PC could be showing minimal CPU usage, or it could be showing excessive CPU usage... but how hard the ACTUAL (not virtual) processor is working is what is going to drain the battery. The Virtual PC's reports of CPU activity are a simulation.

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Thanks for this great answer! So assuming windows has better power-management, the laptop should last longer while virtualizing ubuntu? (instead of running pure ubuntu) –  mreq May 6 '12 at 14:32
    
Just remember what "power management" entails. It means underclocking and undervolting a processor so that it runs slower and uses less power. It means dimming a screen. It means turning off hard drives when not in use. It means turning off screens after a short duration. There is no magic formula. Longer battery life means lowered performance. –  Bon Gart May 6 '12 at 14:39
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