In writing yesterday about evaluating the cost effectiveness of our fancy whole-house fan I mentioned that I only had a way of measuring how much air-conditioner run time was saved by using the fan, not the actual dollars and cents difference. If I knew how much power the air-conditioner used I could make a rough guess on reduction in the energy bill due to the fan use.
Today, I found a way to measure the power and can now fill-in that gap. The number: ~4kW. (You have no idea how long I've wanted to know this number. This made my day.)
This is an astoundingly high number but I don't know how it compares to other air conditioners. Obviously power consumption depends on size; the window units are rated between 1 and 1.5kW. Our air conditioner is old and I bet its efficiency is not that great but I have so little documentation on it that I can't compare it to other more modern units of equivalent size. Maybe that will be my next bit of research.
Now for some math: Even with a power bill, it can be hard to figure out the $/kWh we all pay. Let's assume somewhere around $0.1/kWh, a ballpark number that makes the math easy. Using $0.1/KWh and looking back over this summer's usage of ~8/day, the energy cost just for cooling is $3.20/day; 30 days in a month brings the total to $96/month.
As far as the cost effectiveness of the fan goes, well, there's still a few more complications. The easiest one to address is the energy consumption of the fan itself. Looking this up in the manual shows the fan is rated at 292W on high (which we most often use) . This is a 1:13 energy consumption ratio between the fan and the air-conditioner. For ballpark analysis purposes, we could probably just consider the energy consumption of the fan as negligible. Its a bit of a stretch but my work so far has been just as imprecise and I can live with this.
The second issue is very much related to what I discussed yesterday: how many hours over the course of the summer am I using the fan when I would normally be using the air-conditioner? The results of the experiment I wrote about yesterday show a 25% reduction (using awful experimental techniques) in air-conditioner usage when I use the fan. Many days of the summer, though, I can't use the fan at all as the overnight low is still above the indoor temperature. If I get really committed I might go back through my temperature data and try to make some kind of estimate; that's more work that I want to mess with right now. For tonight, I'm once again going to table this.
Can you share how you measured the consumption of your AC?
ReplyDeleteIn high-school physics, we had an assignment to turn off and unplug EVERYTHING in the house, verify that the power meter wasn't spinning, then turn on one thing (light bulb? fridge?) and measure the metered use over a relatively short period of time. The technique obviously has a high "noise floor," but with a device as large as your central air, you might still get a good S/N.
You’re so right about not letting the little things get to us. 7 days without an AC is quite a bit of a wait, though, so I totally commend you for your patience there, especially if it happened during the warmer seasons. The last time I had my AC repaired, I waited three days for the repairmen to show up, and they took all day to fix it, but it’s since been running without a hitch, so the wait was definitely worth it.
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