All of the above.
Explanation
The kilowatt-hour (kWh) is the commercial unit of electrical energy, commonly referred to as one "unit" of electricity. It quantifies the total energy consumed by an electrical appliance over a period of time based on the formula:
Energy ($E$) = Power ($P$) $\times$ Time ($t$)
By definition, 1 kWh represents the energy consumed by a device with a power rating of 1 kilowatt (1000 Watts) operating for 1 hour.
Option Analysis
- A 1000 W appliance is used for 1 hour. is Correct: An appliance with a power rating of 1000 W used for 1 hour consumes:
$1000 \text{ W} \times 1 \text{ h} = 1000 \text{ Wh} = 1 \text{ kWh}$. - A 100 W appliance is used for 10 hours. is Correct: An appliance with a power rating of 100 W used for 10 hours consumes:
$100 \text{ W} \times 10 \text{ h} = 1000 \text{ Wh} = 1 \text{ kWh}$. - A 1 W appliance is used for 1000 hours. is Correct: An appliance with a power rating of 1 W used for 1000 hours consumes:
$1 \text{ W} \times 1000 \text{ h} = 1000 \text{ Wh} = 1 \text{ kWh}$.
Since all three scenarios result in the consumption of exactly 1000 Watt-hours of energy, they all represent 1 kWh.
Key Takeaway: One kilowatt-hour (kWh) is equivalent to 1000 Watt-hours. In SI units, 1 kWh equals $3.6 \times 10^6$ Joules.