Explain why kilowatt hour is a unit of energy not power.

$$
\text{Total Power} = 4\, \text{bulbs} \times 15\, \text{W/bulb} = 60\, \text{W}
$$

$$
\text{Total Power} = 60\, \text{W} \times \frac{1\, \text{kW}}{1000\, \text{W}} = 0.060\, \text{kW}
$$

$$
\text{Energy} = \text{Power} \times \text{Time}
$$
$$
\text{Energy} = 0.060\, \text{kW} \times 5\, \text{hours} = 0.30\, \text{kWh}
$$

$$
\text{Cost} = \text{Energy} \times \text{Tariff}
$$
$$
\text{Cost} = 0.30\, \text{kWh} \times \text{R1.50/kWh} = \text{R}0.45
$$
The cost is $\boxed{\text{R0.45}}$.

$$
\text{Power} = 100\, \text{W} \times \frac{1\, \text{kW}}{1000\, \text{W}} = 0.100\, \text{kW}
$$

$$
\text{Time} = 3\, \text{hours} + 30\, \text{min} \times \frac{1\, \text{hour}}{60\, \text{min}}
$$
$$
\text{Time} = 3\, \text{hours} + 0.5\, \text{hours} = 3.5\, \text{hours}
$$

$$
\text{Energy} = \text{Power} \times \text{Time}
$$
$$
\text{Energy} = 0.100\, \text{kW} \times 3.5\, \text{hours} = 0.35\, \text{kWh}
$$

$$
\text{Cost} = \text{Energy} \times \text{Tariff}
$$
$$
\text{Cost} = 0.35\, \text{kWh} \times \text{R1.50/kWh} = \text{R}0.525
$$
The cost is $\boxed{\text{R0.53}}$.