Smartphones have become such universal devices that they’ve replaced many of the other little gadgets we used to carry around -- even the Mp3 players from our gym bags. Maybe you’ve experienced the dread of showing up at the gym ready for an intense workout, only to discover that your smartphone battery is nearly dead. And if you think that’s bad, imagine what it would be like if you had to exercise just to fill the battery back up again.

How Much Electricity Does Phone Charging Use?

Today’s smartphones can juggle multiple processes all day long on a single charge, thanks to batteries that keep getting thinner even as they grow more powerful. But even the biggest batteries won’t put a strain on your electric bill.

When Forbes tested the electricity consumption of iPhone batteries in a 2013 test, they found that the iPhone’s 1,440 milliamp-hour (mAh) battery would only gobble up about two kilowatt hours (kWh) after an entire year of daily charging. At an average price per kWh of $.12, that means it costs less than a quarter per year to keep that phone going.

Today’s batteries are a bit bigger: the iPhone X clocks in with a 2,716 mAh battery, nearly twice as big as the battery from Forbes’ test. When you’re counting this energy in electricity cost, it’s simply the difference between one quarter and two. But when you’re counting the energy in push-ups, you’ll feel every last milliamp-hour the next morning.

The newer generation of phones also supports higher wattage chargers that can restore battery power faster than before despite the increase in battery size, but for our test, we’ll use the standard 5-watt charging adapter that ships with the iPhone X. At this wattage, it takes about two hours and fifteen minutes to fully charge the iPhone X, according to TechRadar.

How Many Calories Does It Take to Charge Your Phone?

Technology has yet to provide a way for us to plug our phone chargers directly into our calorie-burning bodies, so for now, we’re stuck with more traditional sources of electricity. But we can still calculate how much effort it would take to generate enough energy to fill that battery to the brim.

To burn just one calorie, a person must generate 4.1868 watt-seconds of energy (a watt-second is the amount of energy required to sustain one watt of electricity for one second). With this figure, we can see how much calorie burn it would take to power your 5-watt charger for one second:

5 watts ÷ 4.1868 watt-seconds = 1.19422948314 calories

How Much Exercise Does It Take to Charge a Phone?

To standardize our exercising efforts, we’ll choose one from the Compendium of Physical Activities, which lists MET values for a range of sports and exercises (metabolic equivalents, or METs for short, will help us crunch these calorie counts). Let’s try calisthenics -- push-ups, sit-ups, pull-ups, jumping jacks and the like. With vigorous effort, this exercise generates 8 METs.

We’ll also need to standardize the weight of our test subjects, for which we’ll turn to the Centers for Disease Control and Prevention for their average weights: 88.76 kg for men and 76.43 kg for women.

Let’s begin by seeing how many calories an hour of vigorous calisthenics will burn:

8 METs x 88.76 kg = 710.08 calories per hour for men

8 METs x 76.43 kg = 611.44 calories per hour for women

Since it takes just over a calorie per second to fire up that little 5-watt charger, this might not take too long. But there are a lot of seconds in two hours and fifteen minutes -- 8,100 to be exact.

8,100 seconds x 1.19422948314 calories = 9673.25881343 calories for a full battery charge.

So for the man burning 710.08 calories per hour, it would take 13 hours, 37 minutes and 22 seconds of calisthenics to get that battery to 100% charge. And for the woman churning up 611.44 calories per hour, it will take a little longer at 15 hours, 49 minutes and 13 seconds.

So while it’s nothing compared to running your refrigerator with a stationary bike, charging up your smartphone via exercise still makes for a very full day at the gym.

Check out how much exercise it would take to power your other appliances and electronics!