What is the monthly energy cost to operate an 85-watt PAR38 incandescent lamp when electricity costs $0.13 per kW-hr and the lamp is used for 13 hours a day?

To calculate the monthly energy cost of operating the 85-watt PAR38 incandescent lamp, we start by determining the energy consumption on a daily basis.

First, convert the wattage from watts to kilowatts:

85 watts = 0.085 kW (since 1 kW = 1000 watts).

Next, calculate the energy used per day:

Energy per day (kW-hr) = Power (kW) × Time (hours).

Energy per day = 0.085 kW × 13 hours = 1.105 kW-hr.

Now, find the energy used in a month (assuming 30 days in a month):

Energy per month = 1.105 kW-hr/day × 30 days = 33.15 kW-hr.

Now, to find the monthly cost, multiply the total energy consumed in a month by the cost of electricity:

Monthly cost = Energy per month (kW-hr) × Cost per kW-hr.

Monthly cost = 33.15 kW-hr × $0.13/kW-hr = $4.30.

Rounding to two decimal places, the monthly energy cost is $4.31, which corresponds to