Question

A man has a 40 watt and two 60 watt bulbs in a room. How much will it cost him to keep them lit for 8 hours per day in the month of June if the cost of a unit is #20.

Answer 1

Assuming than unit is 1kWh (kilowatt-hour).

Cost for light:

$Cost=Q\times UnitPrice$

where Q is quantity of energy spent.

$Q=P\times t$

where P is power of all bulbs (in kilowatts), t - time which bulbs were used (in hours).

$t=t_{1}\times N_{d}$

where t₁ - hours during 1 day which bulbs were used, N_d - number of days in June.

So we have

$Cost=P\times t_{1}\times N_{d} \times UnitPrice$

$Cost=(0.040+0.060+0.060)\times 8 \times 30 \times 20 =$ 768