Precisely calculating your heating and cooling costs is quite involved. To figure your cooling cost, you have to calculate the square footage of each room and its windows and then consider all heat sources, including solar heat, body heat, equipment heat and heat from lights. You must also consider peak hours and off-peak hours relative to each heat source. Taking all these factors into account, you can calculate your total heat load and your cooling cost. Similar factors apply when calculating heating cost. Fortunately, you can use simple rules to quickly compute a conservative rough estimate of your heating and cooling costs.

Determine your cost per kilowatt hour, or KWh. Refer to your most recent electric bill. For example, assume your cost per KWh is $0.75.

Find the rated power consumption of your heating, ventilation and air conditioning or HVAC unit. Refer to your owner's manual or find your model number and go to the manufacturer's website. As an example, let's assume it's 2,500 watts or 2.5 KW.

Determine the number of hours you use the unit for the period in question. For example, if you are interested in your monthly cost and you run the unit 6 hours a day, 7 days a week: Total hours usage = 6 hours/day x 7 days/week x 4 weeks/month = 168 hours/month.

Calculate the Kilowatt-hour usage, or kWh, using the formula: KW x total usage hours. Continuing with our example: Total Kwh = 2.5 kW x 168 hours = 420 KWh/month.

Calculate total cost using the formula: Total KWh x cost per kWh. Continuing with our example: Total cost = 420 kwh/month x $0.75/KWh = $315/month.