2.5 Power vs. Energy in Everyday Life

Spot the Power Label!

Take a look around you right now. Chances are, you’re using a laptop, phone, or tablet—and it’s plugged into a charger. Flip that charger over (or look at the label), and you’ll likely see something like this:

Credit: Jennifer Clemons

Quick Check: Is Watt a unit of energy or power?

Remember from our earlier lesson:

• Power = how fast energy is used → measured in Watts (W)
• Energy = total amount used → measured in Watt-hours (Wh) or kilowatt-hours (kWh)

So Watt (W) is a unit of power—not energy!

Let’s Calculate: How Much Energy Does Your Laptop Use?

Suppose your laptop charger is rated at 65.0 W, and you use it for 10 hours each day.

$\text{Energy = Power × Time = 65.0 W × 10 hours = 650 Watt-hours }\left(\mathrm{Wh}\right)$

But utility bills don’t use Watt-hours—they use kilowatt-hours (kWh).

Since kilo = 1,000, we convert:

$\text{650 Wh × }\left(\frac{1 \mathrm{kWh}}{1,000 \mathrm{Wh}}\right)\text{ = 0.650 kWh}$

So your laptop uses 0.65 kWh per day.

What Does That Cost?

If we estimate electricity to cost about $0.15 per kWh (check your bill for your exact rate!).

$\text{0.65 kWh × \$0.15}/\text{kWh = \$0.0975 }\approx \text{ \$0.10 per day}$

That’s just 10 cents a day—or about $3 per month to run your laptop!

Depending on how much your electricity costs, you can now determine how much that one day of laptop use costs you each day (or month or year).