--The system that we are looking at costs about $3,900, installed. We would use it in conjunction with our present hot water heater. I have been told that there is a Federal Tax Credit available that will account for 30% of this, which could potentially knock $1170 off the price, bringing it down to $2,730.

--Various websites estimate a 50% - 80% savings on hot water expenses by installing a solar hot water heater. It is unclear whether they are talking about solar hot water heaters used in conjunction with gas storage, electric storage, or on-demand gas or on-demand electric hot water heating units.

--We presently have a 50 gallon gas hot water tank. It is about 5 years old. Literature on the heater says that it uses 258 therms/year (based, I presume, on the hot water needs of the average household). This comes to 21.5 therms/month.

--A close look at ur PG&E bills shows that, when you take winter heating out of the equation (gas), we use a total of 8-12 therms/month for heating water and cooking on our gas range.

--According the U.S. Department of Energy, hot water heating accounts for 15.5% of residential energy use, while cooking accounts for 4.5%, though this does not delineate between gas and electric. Most of our cooking is done via our gas range, though some uses the electric microwave and the electric oven.

Here's where things get sketchy. Using the Department of Energy statistics, I might ballpark that we use in 9 therms/month for hot water heating, which would mean that we are more than twice as efficient with our hot water heater as the average American household.

That seems a bit surprising, but plausible. Much of our laundry is done on cold cycles, our showers are short, baths are few, and the temperature of the hot water heater is set to "warm" rather than on the highest setting.

And then there are some X-factors to consider:

--Readers of this blog know that our family is growing, which will result in increased hot water use.

--The cost of natural gas will continue to rise, but it is hard to predict how quickly.

--On the flip side, our 50-year-old pipes were recently updated from metal to PEX (polyethylene, i.e. plastic), which should improve our ability to use hot water more efficiently in the upcoming years. Previously, the cool metal pipes absorbed much of the heat from the water prior to it actually coming out tap. This resulted in as much as a minute or more of waiting around for the water to warm up.

So let's say that over the next few years we use 10 therms/month for hot water heating, at a rate of $20/month.

Being conservative, if the solar hot water heater cuts this by 50%, then we would save $120/year on hot water heating. With these figures, the $2730 system (including tax credit) would pay for itself in about 23 years. If our heating costs are cut by 80%, then this number is reduced to just over 12 years. For comparison, most websites cite 4 - 8 years as the time it would take for a solar hot water heater to pay for itself.

So, will we be moving forward with installing a solar hot water heater?

The answer is not at this time.

In short, there are just too many unknowns, and at almost three grand, this is not "low-hanging fruit." First of all, I think it will be helpful to gain more data in the form of another year's worth of PG&E bills to see what effect having a baby and having new pipes has. Perhaps we can even do a test to determine more scientifically how much gas we use on an average day to heat water.

But the biggest unknown is the system itself. How, exactly, does the system work? How much will we need to alter our water-use style in order to gain the most from it? Just because most people see 50% - 80% reductions in the cost of heating water doesn't necessarily mean that we will--Most people, it seems, use more than twice as much gas as we do for heating water. And finally, have we truly exhausted the simple, more budget-minded strategies of using hot water efficiently?