[Intel-gfx] OT: Power metering [Was: Re: [PATCH] add LVDS downclocking support]

[Peter Clifton]

On Mon, 2009-03-23 at 00:30 +0000, Steven J Newbury wrote:
> > [1]
> > http://www.alamaison.com.au/p/361923/voltcraft-plus-energy-monitor-3000.html
> I'm not surprised you didn't see any change measuring power usage at the
> outlet.  Laptop power supplies are rarely switched mode, and typically
> provide their full rating while in use.  They aren't typically designed
> with energy efficiency in mind.

Actually, this isn't correct. Every laptop PSU is almost certainly a
Switched mode unit, as is every PC PSU since probably the AT days.

SMPS, as other linear types will provide as much energy as demanded of
them (within sensible bounds), and care is taken as to the efficiency of
the design since this determines the heat transfer requirements from the
unit. On laptop supplies, there isn't the luxury of using forced air
cooling.

I'd hazard a guesstimate of the main PSU unit's efficiency at 80-90%,
Part / no load efficiency will be worse. I don't know how efficient
laptop supplies tend to be when connected to AC but not charging the
laptop (laptop off / on standby), but I could probably find out if
anyone was really interested. (Our lab has a very precise calorimetric
power measuring system, which has been used to verify standby powers for
things such as mobile phone chargers and set top boxes).

Also remember that there are also several switching PSU units within the
laptop, including the one which steps the battery voltage down to the
required levels. Those are probably designed with efficiency in mind,
due to the thermal issues, and to increase battery life.

What is true though, is that you're looking for a fraction of a watt
power change in a device pulling about 20 watts, via a mains adaptor
which will draw harmonic currents well beyond 50Hz. The plug-top power
meters are designed to measure up to about 13A, (~3kW). The unit you
mentioned claims <1% accuracy of voltage and current readings, meaning
its current measurement might be accurate 130mA. I'm not going to get
into the statistics of whether the voltage / current measurement
inaccuracies will be correlated, but lets say at best 1% accuracy in
power measurement. That is corresponds to 30W.

In reality, the harmonics drawn by the switching PSU could easily throw
off such a power meter if it doesn't sample V and I at a high frequency,
with the V and I channel measurements well synchronised.

> If you want to measure the energy efficiency of a laptop, your best bet
> is powertop when running from battery.

Long story short... yes, I concur. These measurements, whilst of unknown
absolute accuracy and calibration, are almost certainly made on smooth
DC power rails inside the laptop, and with sensing components scaled to
match the expected range of powers drawn by the laptop.

Best regards,

-- 
Peter Clifton

Electrical Engineering Division,
Engineering Department,
University of Cambridge,
9, JJ Thomson Avenue,
Cambridge
CB3 0FA

Tel: +44 (0)7729 980173 - (No signal in the lab!)