Mr Ho would like to replace all 60 units of air conditioners in his office building. He is considering between two brands of air conditioners. Information about the two brands of air conditioners is shown in the table below. Electricity Tariff from 1 July 2022 is shown in the table below. The usage of air conditioner units in Mr Ho's office building is shown in the table below.

1. Calculate the total usage of all units in one week.

Monday usage:
$50 \times 8 = 400\text{ h}$

Tuesday to Friday usage:
$4 \times 60 \times 6 = 1440\text{ h}$

Total usage per week:
$400 + 1440 = 1840\text{ h}$

2. Multiply weekly usage by the number of operating weeks in a year.

Usage for $51$ weeks:
$1840 \times 51 = 93840\text{ h}$

1. Calculate the purchase cost for 60 units of each brand.

Samsing purchase cost
$=60\times 1388=\char36{}83\,280$

Potasonic purchase cost
$=60\times 740=\char36{}44\,400$

2. Calculate the 4-year electricity cost for Samsing.

From part (a), total yearly usage of all air conditioner units is $14\,790$ h.

Samsing power consumption per unit $=3$ kW.

Energy used in 4 years
$=14\,790\times 3\times 4=177\,480\text{ kWh}$

Tariff with GST $=32.28$ cents per kWh $=\char36{}0.3228$ per kWh.

Electricity cost
$=177\,480\times 0.3228=\char36{}57\,290.544$

3. Calculate the 4-year electricity cost for Potasonic.

Potasonic power consumption per unit $=3.5$ kW.

Energy used in 4 years
$=14\,790\times 3.5\times 4=207\,060\text{ kWh}$

Electricity cost
$=207\,060\times 0.3228=\char36{}66\,838.968$

4. Calculate the chargeable servicing cost over 4 years for each brand, taking warranty into account.

Warranties cover servicing and maintenance.

For Samsing, warranty is for 2 years, so servicing is chargeable for years 3 and 4 only.

Samsing servicing cost
$=2\text{ years}\times 2\text{ servicings per year}\times \char36{}2100=\char36{}8400$

For Potasonic, warranty is for 3 years, so servicing is chargeable for year 4 only.

Potasonic servicing cost
$=1\text{ year}\times 4\text{ servicings per year}\times \char36{}2100=\char36{}8400$

5. Compare the total 4-year costs and make a justified decision.

Samsing total cost
$=83\,280+57\,290.544+8400=\char36{}148\,970.544$

Potasonic total cost
$=44\,400+66\,838.968+8400=\char36{}119\,638.968$

Since $119\,638.968<148\,970.544$, Potasonic has the lower cost after 4 years.

1. State one reasonable assumption used in the cost comparison.

Assume that the electricity tariff remains constant at $32.28$ cents per kWh throughout the 4 years.