System Savings Data

Measuring payoff for installation of the system simply involves collecting annual data on propane use. Consider that there is always a base use of propane used for the drying of clothes, and we do operate a wood stove during cold periods. This year has provided a lot of sunny days, which has helped in propane savings.

Domestic hot water use has remained constant, as our home has three full-time residents. One factor in savings on DHW use is that clothes washing and dishwasher running is generally done in the early afternoon, when the DHW tank is fully heated and has time to recover with solar collection. Showering is generally done during the early morning, but the well insulated tank is still hot from the day before.

Propane usage for the year (Jan 1 - Dec 31) is somewhat dependent on when the tank is filled. I have tried to be consistent on fill times not only to avoid skewing the data, but to take advantage of filling at lower pricer periods. I just did my annual October "top off" of the tank to take advantage of the lower pre-winter prices.

All that being said, here are the figures:

2007 propane usage - 1390 gallons, average price was $1.81, total cost $2728

2008 propane usage - 1180 gallons, average price was $2.11, total cost $2763

2009 propane usage - 892 gallons, average price was $1.69, total cost $1578

The system as installed in the summer of 2008. So, thoerectically savings for that year should have only been half of that this year. The numbers bear that out.

Gallons usage is the main factor in calculating gain. Cost ($) savings is kind of a mixed blessing. Lower propane cost does save me real dollars, but lengthens how long it takes to achieve a payback.