How much electricity does a phone consume when left on charger overnight?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

26 January 2025

The Kenya Power and Lighting Company, KPLC, recently alerted electricity consumers about leaving phones on the charger overnight. In a post on X, the company stated that “once your phone is fully charged, it continues to draw small amounts of power, wasting energy over time.” That got me wondering: how much electricity does the phone consume while plugged in while fully charged?

To find out, we need to know how much a phone consumes when left on but not in use, that is, on standby mode. This is the charge that needs to be replenished when the phone is left plugged in while fully charged. I did some digging and found that smartphones consume about one percent of full-charge while in standby mode. That surprised me because it means that in about 8 hours it would drain 8pc; from 100pc to 92pc. It’s too much!

So, I tested with my phone. One evening I charged it full and then unplugged it when going to bed. When I woke up about seven hours later, the phone was at 98pc charge. In other words, it had lost just 2pc.

The phone has a 5,000mAh battery that operates at about 4 volts. To get the total electric energy stored, we simply multiply the mAh value by the voltage. This comes to 20,000mVAh, or 20,000 milliwatt-hours (mWh).

Now, one watt has 1,000mW, so 20,000mWh = 20Wh. Further, electricity is sold in kilowatt-hours (kWh) and one kWh is 1,000Wh. Therefore, 20Wh = 0.02kWh.

On standby overnight, the phone lost 2pc of the full charge, that is, 2pc of 0.02kWh = 0.0004kWh. Obviously, this is not zero but it is a miniscule amount. Is it worth worrying about?

Let’s compare it to leaving a light bulb on. A modern LED light is typically rated at about 5W, so, if it is left on for 8 hours, it will consume 40Wh, or 0.04kWh. This figure is 100 times the energy consumed by the phone on standby!

What about the cost? Electricity in Kenya costs just under Sh30 per kWh. Thus, leaving my phone on the charger overnight costs me about Sh30 x 0.0004kWh = Sh0.012, or 1.20 cents! If I do that every day, the monthly cost will be 36 cents! I will not notice it in my electricity bill.

Clearly, there are better and more effective ways of saving electricity, like ensuring that lights are switched off when not in use and boiling only the water that is needed (instead of filling the kettle every time).