It costs almost nothing to charge a smartphone for a year

How much does the electricity needed to charge a smartphone over the course of a year cost? Under a dollar? A few dollars? Tens of dollars? Hundreds of dollars? The figure will surprise you.

I'm on a mission, and that mission is to save on my power bill.

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I've already done the obvious stuff, like replace lightbulbs - especially the 500W halogens floodlights I have outdoors - with low-power LEDs, and I'm being more careful as to how I use heating and cooling. But along with making big changes, I've also been looking at just how much power all the random stuff I have plugged in uses.

I've already looked at how much power smartphone chargers consumed when there wasn't a smartphone attached to them. Now let's look at how much power it takes to charge a smartphone for a year.

Now, the proper way to do this test would be to measure the power consumption over a year. Well, I want results quicker than that, so I'd have to do shorter periods of real-world testing and extrapolate out the results, which shouldn't be a problem.

So, what I did rather than keep detailed charging note for a year, or find how much power it took just to charge the battery from 0 percent to 100 percent, and try to fudge that into some real-world figure, I replicated what most people do and put my smartphone on to charge overnight and measure the nightly power consumption.

I chose this method for two reasons:

• It's a usage pattern that matches how many people use their device
• It is more real-world, since when the device is on charge overnight, not only is power being used to charge the battery, but also to run the device (remember, your device is doing stuff in the background like checking email), so this goes beyond just measuring the power used to charge the battery

Power consumption was measured using a WattsUp? PRO power meter.

My test subject was the iPhone 6 Plus, which has one of the biggest batteries that Apple offers for the iPhone. I'm also a pretty heavy user, and this meant that going all day was sometimes tricky (the things I do for you). This means that my results are going to be at the high-end, and that more restrained smartphone users are going to have a smaller power bill than I do.

So here's what I found.

On average, during an overnight charge, the iPhone consumed 19.2 Wh.

According to figures published by the US Energy Information Administration for November 2016, the average cost per kWh for residential power in the US was close to 13 cents. There was considerable variation across the states, with Washington being the cheapest (9.51 cents per kWh) and Hawaii being the most expensive (28.48 cents per kWh).

Remember that 1 kWh equals 1,000 Wh.

So, take our average of 19.2 Wh per day, multiplying that by 365 days, we get 7 kWh, which works out at \$0.91 a year.

Given how much I get done on my iPhone over the course of a year, this running cost is surprisingly low, and much lower than running a desktop or laptop PC would be.

So if you guess under a dollar, well done.