Hold up ! Hee haw ! Whoa !
There is no such unit as "kilowatts per hour". I see it printed
right there on the sheet that somebody gave you, and it's as
wrong as it can be. The unit called the "watt" ... and all of its
multiples, like the milliwatt, microwatt, nanowatt, kilowatt, megawatt,
gigawatt, etc. ... are all rates of energy flow. The "per hour" is built
into them. 1 watt = 1 joule of energy per second. Glueing another
"per second" or "per hour" onto any quantity of watts makes it
meaningless, useless, misleading, and unhelpful. If you hadn't
attached a picture of that printed sheet, I would have thought that
you copied the problem wrong. But seeing that sheet, I'm worried
about what they're handing you in school. This one is misleading
and wrong. The more you try to understand it, the more damage
it'll do to your understanding of Physics.
The energy consumption of an appliance is measured in
kilowatt-hours, and is the product of the kilowatts the appliance
uses and the number of hours it uses energy.
The Siri family's clothes dryer uses energy at the rate of
2-1/2 kilowatts when it's running. If Mr. Soto sneaks into
the Siri family's house and runs their dryer for 3/4 hour,
it'll use
(2-1/2 kilowatts) x (3/4 hour)
= (2-1/2 x 3/4) (kilowatt-hour)
= 1.875 kilowatt-hours of energy .
