[From the last episode: We saw how frequencyThe rate of change of a signal, specified in hertz, or cycles per second. has an important impact on powerThe rate of energy consumption. For electricity, it’s measured in watts (W)..]
We’ve been talking a lot about power, even though what we really care about is energy. To be clear, power is still important – if power is too high, then things can get too hot. So how fast we gobble energy matters, but our focus here is on how much energy we end up gobbling net net.
Temperature and other reliability considerations aside, if we’re going to use X amount of energy to do a job, it doesn’t really matter how long we take to do it. We still used that much energy. The question, then, is, “When does lower power mean lower energy?”
We know that energy is power multiplied by time. Double the time and you double the energy for a fixed power level. Double the power and you double the energy used for a fixed time. But here’s the thing: changing power usually also means changing time.
Lay Off the Gas
Let’s use a car as an example. If you want to cut down on how fast you’re guzzling gas, then you should let up on the accelerator. That clearly lowers your power. But it also slows you down, so that means that, for a given trip, yeah, your power is lower, but it took you longer to get there, so the engine was running for longer.
If that’s the case, then how does the whole energy thing stack up? Well, let’s say you cut power in half – but it takes twice as long to get there. That means you’re multiplying half power by twice time, and that “half” and “twice” cancel out – and you’ve used the same energy in either case.
But that’s actually not likely. In the case of a vehicle, you have to deal with wind resistance. And that goes up really fast as you speed up – it goes as the square of the speed. So double your speed and you have four times the wind resistance – you’re having to work much harder to complete the trip. Looked at another way, by going faster, some of that extra energy isn’t going into getting you there quicker – it’s going into battling the wind (which many would consider a waste). Said yet another way, you use more gas to go the same distance in a faster time.
So, in that case, doubling your power doesn’t get you there in half the time. Flipping that around, cutting your power in half gets you there in less than twice the time. “Half” times “less than twice” no longer cancels: in fact, you end up with a lower energy number for going slower because you get such a big break from the wind.
What About Electronics?
The same effect can happen with electronics. If you cut power, circuits will run slower. If the effect is linear – as in, half the power gives half the speed (or twice the delay), then everything balances out and you didn’t save any energy. What needs to happen is that, if you cut power by X amount, then the speed needs to slow down by less than a factor of X. Then you get real energy savings.
But, in some cases, cutting power in half may more than double the time it takes to do the job. In that case, the lower-power setting uses more energy because it has to keep running for so long.
So why all the hoo-hah about power then? Are we playing a losing game by focusing on lower power? Well, first of all, remember that engineers often confuse power for energy. Everyone says “power” when they often mean “energy.” So there’s that.
But what it really means is that designers constantly try to find ways to do the same job more efficiently. Ideally, that means doing the same job in the same time, but with lower power required to do it. That yields real energy savings, since power goes down and time doesn’t go up as much or at all. It’s not just about turning a power dial; it’s about having great, creative new ideas about how to do things better.
So the point here isn’t that reducing power is a waste of effort. The point is simply that, just because you lower power doesn’t mean you lower energy consumption. You have to run analysis to see if it’s effort well spent.
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