We all know the following two Universal Truths about smartphones:

• We need our phones (and they aren’t even really phones).
• Our phones drain their batteries like vampires sucking blood.

Having your phone die on you before the end of the day is not fun. In an effort to fix this problem, Apple iOS 9 has a “Low Power” mode that should help increase battery life by decreasing background activity. What effect does this have on my iPhone 6? Let’s run some tests.

I’ve looked at power consumption in smartphones before to see how screen brightness effects battery life. For that case, I charged the phone to 100 percent battery level, then plugged it into the Watts Up Pro power meter. My assumption was that on a fully charged battery, all of the energy would be going to run the phone and not charge it. Yes, there would still be some power loss in the AC to DC converter but I assumed that would be small.

For the Low Power mode on the iPhone, I will do the same thing with my own phone. The Watts Up Pro can connect to Vernier’s Logger Pro software so you can record the power usage every second. Here is the plan.

• Leave the phone in standby in the normal power setting. Record data for 400 seconds. I will send myself an email, to simulate normal activity.
• Use the phone as I would in a typical “on” situation. Check some apps, surf. Record data for 400 seconds.
• Repeat the same two measurements, but in “Low Power” mode.

Here is the data from those four runs.

Yes, that looks a little messy. Maybe it would be better to just find the average power for each run. I am going to include the standard deviation of the power as error bars in this bar graph.

It looks like the “Low Power” setting uses less power—so I guess that’s encouraging. Based on this small sample, Low Power mode uses 80 percent of the power in normal mode and 90 percent of the power in regular use. Really, I should have collected data for a longer period—but I’m impatient. OK, fine. Here is another run for both standard and Low Power mode for 30 minutes (but only in standby mode).

From this longer data run, the Low Power mode uses about 70 percent of standard power. Now for some of your comments.  Yes, I know your comments even before you do.

• But doesn’t the power savings depend on what apps you have running in the background? Yes. That’s why you might have a different power savings. This is all about my power savings, not yours. But if you must know, according to the battery manager my biggest power-eating apps were Facebook, Google, and Chrome.
• But what if you get an email or text message during the test?  Wouldn’t that throw off the results? Absolutely—but not by that much.  You can see during the last (and longer) run that there were some emails during that time (the spikes in the power). Also, at the end of the Low Power run, I received a text message. Still, most of the time the phone was idle—I think the test shows (for my iPhone) that it used less power in Low Power mode.
• Don’t you think the total power consumption depends on the type of USB charger? Yes. However, looking at the readings with nothing plugged into it I get a power of around 0.2-0.4 watts.  That’s not too bad. Also, both runs used the same charger.

But how much longer battery life will this give me? Let’s say that in normal mode, my iPhone runs for 14 hours (a rough estimate). I can use the following definition for power and solve for change in energy of the battery (since the energy stored is constant in either setting):

Since the stored energy is the same in either case, I can compare the product of power and time for both modes (the s subscript is for standard mode and the l is for Low Power):

This means that if the Low Power mode uses 70 percent of the power in standard mode, the battery should last 1.43 times as long. A 14 hour battery life would be 20 hours. That’s not too bad. I can live with a 20-hour battery life. Of course, the iPhone only suggests you turn on Low Power mode when you get to 20 percent battery life. In that case, the extra time is about an hour. Again, not too bad. Every little bit helps.