Wednesday, March 4, 2009

A small energy savings

Last weekend, our garage door stopped working. It is one of the older electric garage door openers and had been having little difficulties for a while, but this time it went kaput. The problem is that we park our bikes inside the garage (no, the car doesn't fit) and every morning it's opened twice, once when my daughter leaves for school and once when I leave for work. Likewise twice in the evenings. I tried fixing it for a while but I'm no garage door mechanic so I gave up, having better things to do. I installed a manual slide lock so it could be secured from the inside and it's good to go. A small inconvenience to open it manually. 

Then I thought about it. I realized this was an opportunity rather than an inconvenience. I can save energy costs! I was inspired some time ago by the Fort Collins net-zero electric energy home, and maybe this is one of many steps I could take to reduce my energy consumption at home.

I did the calculations (see comments for my actual calculations) and found that I will be saving $68.58 per year on energy costs by having a manual garage door opener in place of an electric one.

I was speaking with a coworker about this very calculation, and she pointed out that the marginal utility of having an electric garage door opener versus a manually-opened garage door is highly in favor of the electric opener, given the cost savings. For some people, spending $5.71 per month to have an electric garage door opener is worth the cost. For myself, it's not; we only park the bikes in there, not the car, and don't use the remotes -- just open it from the inside anyway. 

For now, I'm happy with the arrangement. The cost savings is definitely worth the marginal utility loss. 

1 comment:

  1. How I arrived at garage door opener energy cost:

    First, I used the table from http://www.absak.com/library/power-consumption-table# to find the energy output of garage door opener (350 W) and associated two lights (120 W).

    Second, I used the load calculator form from http://www.absak.com/pdf/docs/loadeval.pdf to calculate the load. A typical garage door opener run at .133 hours/day (8 minutes, 2 minutes per use at 4 uses per day) plus the two lights on mine (which stay on for about 5 minutes after door is used) at .333 hours/day each. There's also the Load Correction Factor: "Load Correct Factor compensates for losses in the system. Batteries and other power system components are not 100% efficient. We have found that increasing load value by 30% adequately factors in these losses." So the corrected Watt-hours/day is 124. This translates to 45.26 kWh/year.

    Next is standby power, also known as phantom loads. According to http://standby.lbl.gov/summary-table.html, a garage door opener's average standby power is 4.48 W, or 40.32 kWh/year. Amazingly, this almost doubles the load!

    Adding the two loads gets me 85.58 kWh/year total. At my current utility rate (0.06678 per kWh per month, or 0.80136 per kWh per year), this results in a cost SAVINGS of $68.58 per year. Just for the garage door opener!

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