Window air conditioning

I am stalling got curious about whether it is more cost effective to drive to Carleton and pay to park while sitting in the air conditioned library or to sit in my bedroom and run our thermostat-equipped window air conditioner.

Parking at Carleton is $10 for 4 hours, or $2.50 per hour.

I plugged a “Kill-A-Watt” power meter inline with my window air conditioner and ran it. The room was already cool, so this is the cost to maintain the coolness we had overnight.

When actively cooling (not just fan) the air conditioner uses 530 Watts of power. If it were to run solid for one hour at 530 W then it would use 0.53 kWh. At current rates we pay 9.764 ¢/kWh, giving a cost to operate of 5.1 ¢/hr. I recalculated using the current peak rate for people with smart meters (13.594 ¢/kWh) and they would pay 7.2 ¢/hr to operate the window air conditioner.

Of course these numbers assume constant operation of the air conditioner. Normally it cycles back to fan a lot. I ran mine for an hour with an inside temperature of 23.3 and an outside temperature of 28.5 (humidex 37). I read the actual kWh reading off the kill-a-watt. I actually used 0.28 kWh, or 2.7 ¢.

For kicks (and stalling) I decided to check the split ductless air conditioners a friend is thinking of installing. Of course, these units are designed for much higher capacity because they aren’t sized to cool a single room. Still if you can get away with single-room cooling and the ductless units are a luxury then it makes sense to see the difference in cost to operate.

I put in “ductless air conditioner watts” into google, picked the first link, and selected the biggest option of 24000 BTU/hr in the hopes this would approximate the system the friend is thinking about (2 cooling units run to a single compresser). (For comparison my window air conditioner is 6000 BTU/hr.) These units run at a spec of 2590 W while cooling. Using the same calculations this would run 25.2 ¢/hr at current rates or 35.2 ¢/hr at peak smart meter rates. Again, these are flat out cooling, not cycling on and off.

A full-house central air conditioner, lets say a 4 ton unit, runs at 48000 BTUs. I can’t find operating watts for these. Let’s assume the costs double from the 24000 BTU ductless. That gives 50 ¢ per operating hour (70 ¢ for the smart meter rate).

So there you have it. Running full tilt, a window air conditioner uses 5 ¢/hr, a 24000 BTU ductless uses 5 times that, and a whole house estimated at 10 times the window air conditioner. You cool more area with the more expensive methods, but I just need a cool corner to work in, and so it doesn’t make sense for me to pay $2.50 an hour to park at carleton when it’s costing me 5 ¢ an hour to stay home.

Of course, Carleton is already paying for the air conditioning so there may be an environmental benefit to going there, but my little air conditioner is already saving 90% of the power I’d use if we were on central air. I might choose to go to Carleton on this basis when the province’s power grid is really hurting.

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› technology     2008-07-08 12:49   ...1