How much energy does a laptop / notebook PC use under the bottom line?

       

This long-distance-travellers’ model comes with an optional attachable supplementary battery and two chargers. Let us attach the extra battery and see how far we get. It has a rated energy capacity of 95 Wh and, when fixed, operates together with the internal standard battery that adds another 55 Wh to it.

When fully charged, the total 150 Wh from these two batteries will run the PC for a minimum of 5:15 hours if a program or procedure is started which places full load on the CPU and the screen is set to full brightness. If the screen is dimmed down as low as possible and the CPU is idle this total amount of energy suffices for a maximum of no less than 18 hours of operation! Note that when you are only reading a PDF on-screen, typing a text or entering figures into an Excel sheet this is practically idle for the CPU. All usual applications will come to lie somewhere between these two points, but much closer to the lower load, so with commonplace usage the PC will work for about 15 hours, yielding a net DC power demand being between 8 W and close to 30 W, with a typical average consumption of 10 W. This even includes the screen, whereas with a common desktop PC the screen alone already uses more than that!

Now let us take the smaller one of the two power supplies, rated 65 W, and charge the batteries up again (while the PC is on hibernation). It takes only 4:15 hours, and both of them are full again (whereas, since the external one can easily be detached again any time, it is given preference to discharge, and the internal one is first when it comes to recharging). One may also wonder why a bigger power unit (90 W) is supplied with this PC. Two small ones would have been even more useful.

The gross energy consumption to get back the 150 Wh consumed before is 200 Wh. Note that this latter figure includes all losses incurred by both the charger and the charge-discharge cycle of the batteries! So we are faced with a pretty good overall efficiency of the process being as high as 75%, and we can say that the gross power consumption of this portable PC when used off the line, including all charge-discharge and conversion losses, is between 11 W and barely 40 W.

When battery charging is finished the remaining power intake from the socket is 1.8 W. Without the second battery this value is only 1.2 W, and when you turn off the power once and turn it on again, strangely enough it even drops to 0.6 W in either case.

Now let us unplug the PC and view the charger alone: The no-load consumption is as low as 0.07 W! While the price of this AC adapter is nothing much higher than any common branded model on the market, you may really wonder why so many other electronic devices have such high stand-by / idle / no-load losses. Remember how happy we were and what a progress it then was – only a few weeks ago – when we discovered how you can reduce the “off” consumption of a desktop PC from 6 W to 2 W!

Morals:

• Reducing stand-by losses next to 0 is technically feasible and economically viable.
• Energy efficient data processing equipment is also feasible but is done only when energy is short for some reasons other than costs (confines of weighty storage capacity, in this case), not because energy is allegedly oh so very expensive. Energy is still much too cheap. Its price reflects its value in no way at all.
• If governments consider supporting the energy efficiency of IT equipment they should simply give a subsidy for replacing mains operated with battery operated PCs and cross-finance this with an extra tax upon the mains operated units. One could lay a benchmark of, for example, 1 kWh/day, and those above pay for those which lie below this limit.

Let us look back upon the shameless power consumption of a desktop PC with a comparable performance during the coming weeks.