Final answer:
To calculate the monthly energy cost to operate an 85-watt incandescent lamp on for 13 hours a day, multiply the wattage by the hours, convert to kWh, then multiply by the days and the cost per kWh. The cost is $4.31 per month.
Step-by-step explanation:
The question is asking us to calculate the monthly energy cost to operate a 85-watt PAR38 incandescent lamp that remains on for 13 hours a day, at an electricity rate of $0.13 per kW-hr, and assuming there are 30 days in a billing month.
First, to find the daily energy consumption of the lamp in kilowatts, we multiply the power (85 watts) by the number of hours it is on (13 hours).
85 watts × 13 hours/day = 1105 watt-hours/day
Since electricity is billed per kilowatt-hour, we convert watt-hours to kilowatt-hours (kWh):
1105 watt-hours/day ÷ 1000 = 1.105 kWh/day
Next, to find the monthly energy consumption, we multiply the daily kWh by the number of days in the month:
1.105 kWh/day × 30 days/month = 33.15 kWh/month
Finally, we calculate the monthly cost by multiplying the monthly kWh by the cost per kWh:
33.15 kWh/month × $0.13/kWh = $4.31/month
Therefore, the monthly energy cost to operate the 85-watt incandescent lamp is $4.31.
Comparatively, a Compact Fluorescent Light (CFL) or Light Emitting Diode (LED) bulb would be much more cost-effective as they consume significantly less power for the same amount of light output.