An electric iron is rated 220V, 2 kW.
If the iron is used for 2h daily, find the cost of running it for one week if it costs ₹4.25 per kWh
Topic: Domestic electricity and safety
Answer
Given: electric iron rated 2 kW, used 2 h daily, for one week, at ₹4.25 per kWh
Energy used per day = power (in kW) × time (in hours) = 2 × 2 = 4 kWh
Energy used in one week = 4 × 7 = 28 kWh (28 units)
Cost = number of units × rate per unit = 28 × 4.25 = ₹119
Cost of running the iron for one week = ₹119
For a cost question, keep the power in KILOwatts and the time in HOURS, and the energy comes straight out in kilowatt-hours — which is what the electricity board calls a "unit" and what the rate is quoted per. Converting to joules here only creates work you then have to undo.
One week means 7 days, and it is easy to work out one day's cost and forget to multiply. The voltage rating of 220 V is not needed for this part at all — it is there for the other parts of the question. Not every number given has to be used.