Absolute Power

# Peak, or just Average?

This blog originally posted on the Low Power Engineering Community 12/10/09.

Over the years, one of the most common questions I’ve gotten regarding low power is, “what’s more important, peak power, or average power?”  Technically speaking, they are important for different reasons – reliability, thermal concerns, and battery life, amongst others.  Power is simply the rate at which energy is transferred.  Average power seems intuitive to most people (it determines energy consumption and battery life), but the idea of peak power (especially “instantaneous” peak power) is somewhat more confusing, since the idea of “rate” doesn’t easily apply to an instantaneous situation.

One simple example might be to relate power (energy per unit time) to speed (distance per unit time).  If it takes me a total of six hours to drive between San Francisco and Los Angeles (say 360 miles), then my “average” speed comes out to be 60 mph.  However, that doesn’t mean that I drove any significant part of the trip at 60 mph.  I might have been driving 90 mph on I-5, 35 mph through the city, and 5 mph sitting in traffic near LA.  The CHP cares about “peak” speed, and if I were to approach the maximum 155 mph speed of my car, it would start to complain as well, but none of these necessarily has any appreciable impact on my 6-hour, 60 mph journey.

So a cell phone that draws 1 watt of power on average will run for 5 hours on a 5 watt-hour battery.  But actual power consumption could easily be 3 watts while viewing streaming video over the internet, 0.5 watts while talking on the phone, or just a few tens of milliwatts in standby mode.  To truly optimize for energy efficiency, we need to analyze the total energy consumed in each of these modes, including the time spent in each mode.

Oh, yeah – and my best real example to date of how “peak” and “average” are really independent: driving home from LA on the Sunday after Thanksgiving in a giant traffic jam on I-5 – “peak” speed 97 mph, total trip time 10.5 hours – you do the math!