We are considering investing in newer, more energy efficient systems for our home, including heating and air conditioning, photovoltaic systems, and solar hot water.

Understanding one's PG&E bill is a prerequisite for such any such decision, and my goal recently was to determine if I could use my energy bills to determine what percentage of our energy use was going towards heating and cooling the house. It turns out that for our particular heating, ventilating, and air conditioning (HVAC) system at least, I could.

I started by logging onto our PG&E account (we have paperless bills) and utilizing their online tools to create and print two simple bar graphs showing our gas and electricity usage for 2008.

Our gas use went up dramatically during the four coldest months, and otherwise remained pretty constant.

Electricity use was highest during the summer months and formed a neat bell curve in the winter. Electricity use was lowest in the spring and fall.

I then made the assumption that primary reason for the seasonal fluctuation of our energy use is from heating the house in the winter (gas) and using the air conditioner to cool the house in the summer (electricity). The bell curve on the electric bill in the winter was likely due to turning on the lights earlier in the evening during the shorter days. I would not be able to account for any seasonal fluctuations in gas use due to hot water heating or differing cooking patterns, but felt that was negligible as compared to the HVAC.

Now for some junior high school level math:

To determine gas use for heating, I found the total gas use for May through October--six months where I knew we were not using the heater--and subtracted this number from the total gas use for the six remaining months. This gave me a pretty good idea of the total amount of natural gas we were using for heating the house.

To determine electricity used for cooling the house, I found the total kilowatt hours used for the spring/fall months (April, May, October, November), and then subtracted this from total electricity used during the four months when were used the air conditioning (June, July, August, September). This gave me a rough estimate of the amount of electricity being used to cool our house during the summer.

The results, again based on the assumption that the major source of seasonal energy use fluctuation for our home is the HVAC, are as follows:

In 2008,
--25% of our total annual electricity use went towards cooling the house in the summer.
--66% of our total annual gas use went towards heating the house in the winter.
--42% of our annual PG&E bills went towards indoor temperature management. This is pretty much on target with the U.S average, but slightly higher than the rest of California Reason: I would guess that the majority of California's population lives in milder climates than Chico, coastal regions--think San Francisco Bay, Los Angeles, and San Diego--which require less heating and air conditioning.
--We used about 30% less electricity and 31% less gas than the average PG&E customer.