I recognize that it's hard to define "energy-efficient" without knowing more about fan specifics. In a perfect world, fans would have a metric that quantified CFM/watt or something - a measure of how much air they move per unit of input power. I have not seen that for consumer-grade window fans, and so my next best idea was to find something that used a premium style of motor, which would have large bearing on fan efficiency. The best (and probably costliest) fan motors are (what's called) "ECMs" (electronically commutated motors), which rectify AC from the wall into DC before "inverting" it back into AC at whatever voltage and frequency is required by the fan. Without getting more technical, the presence of an ECM is probably a good indicator that the fan was built with low operating cost (i.e., high efficiency), rather than low purchase price, in mind.
We're in Chicago at a time of year where outdoor air is pretty cool. To run air-conditioning long-term when there's perfectly good and fresh outdoor air feels wasteful and silly to me. I'd hazard it does to some others of the Boglehead philosophy, too.
Here are the specifics of my situation that's led me here, in case any are curious.
- -Rented condo, 2 bed 2 bath, top floor of 4 in a large building
-Has been reaching mid/high 80s Fahrenheit during sunny days with mid 50s outdoor temp with windows open, curtains drawn and blinds closed.
-Exterior wall is large, south-facing, and has floor-to-ceiling glass. It's a greenhouse, and the daytime sun just roasts us.
-We have windows. However, they're all near the floor (where cool air sits) and hinged at the top (so as to shed rain), which means that they're serving, effectively, to dump out any cool air we have and trap hot air in the house. The windows hinge <30 degrees from vertical, and so there is not enough space for what gets described as a "box fan." We're probably limited, cross-sectionwise, to something around a foot tall.
-We do have a door to the outdoor balcony which makes a huge difference if opened. However, there is no screen door and so we can't leave it open without locking our cat somewhere. I am going to write our unit owners about having a screen door installed (Is it greedy or unreasonable to suggest that they should contribute to that effort? It seems like an obvious property improvement and, frankly, I cannot fathom how no previous tenants had the idea).
-Whole-house fan is probably not an option here.