A while back i was adding instruments to a rack already loaded with sensitive equipment and started to worry about blowing a fuse in the middle of a delicate experiment. A colleague lent me a power monitor made by the Watts Up? Corporation. You plug it into an outlet, then plug your devices into it and it reads out statistics on your power consumption in Volts, Amps, Watts or Dollars. It does peak detection and a number of other sophisticated things, but i was struck by how little i actually know about grid power usage. I consider myself fairly well informed (i work with electronics for a living), but i did not know, for example, that my desktop draws 0.9 A at rest and 1.3 A when working hard. At 120 V, assuming a rate of $0.15/kWh, it costs $0.39/day to leave my computer running ($0.56 if i leave it running an experiment). Good to know.
I suspect i am in the American majority in having a vague sense that i should reduce my power consumption without really knowing how to effectively do that. I'm sure that some of the 'green consumer' advice we get is good and some of it is hogwash, but i've never taken a quiet stroll through the data to decide which is which.
I'll start at the small end. How important is it to unplug chargers when not in use? Any power 'wasted' by a device turns to heat. In the absence of cooling fins, all heat is radiated away. So if we know the surface area of a device and its temperature, we know how much heat is being wasted. In the extreme limit, a charger that was a 4 inch cube heated to the boiling point would be wasting 42 W. But that's unrealistic. My cell phone charger is about 2"x1"x1" and isn't warm to the touch. Even assuming it is 5 ˚C above room temperature, it wastes 0.18 W (costing me $0.24/year). Not worth the hassle of forgetting to plug it in even once a year.
Looking up, i see that each of my labs is lit by 16 32W fluorescent bulbs, or 512 W. This costs $0.61 per room each day if i work 8 hours (ha), and $1.22 per room if i forget to turn them off one night. That's starting to look like real money. Whatever the advertisers say, fluorescent bulbs don't last any longer than incandescents, but each of those 32 W fluorescents probably replaces a 100 W incandescent. Over the 1000 hr life of each bulb, that extra 68 W costs $10.20. So even though they cost a few bucks extra and are heavily over-advertised, i should probably replace my incandescents at home with compact fluorescents. (grumblegrumble)
A quick search of Energy Star ratings tells me that modern home refrigerators use 280-580 kWh/yr at a cost of $42-$87/yr. (1 kWh/yr = 0.114 W) This number does seem to be very time dependent. Refrigerators from the '90s routinely ran over 1000 kWh/yr or $150/yr. Since a new refrigerator can cost $500-$2000 and lasts 10-15 years, i'd say from a cost perspective its probably not worth replacing a nice refrigerator that isn't nearing the end of its life anyway. (Unless the price of electricity goes up.) On the other hand, my parents have an enormous deep freezer that has been in our basement for my entire life. When we become real people and have our own house, my wife and i definitely want one; but that might be a case where buying new actually is cheaper.
Frankly, i suspect all of this logic is wiped out by the cost of heating and cooling a home. The insulative power of materials is a tricky thing to measure, but the standard unit seems to be R-value measured in (m^2*K)/(W*in). This has the advantage that you add the R-values of multiple insulation layers, but its a weird unit to think about. Most decent insulation has an R-value of about 1, so 5 inches of decent insulation lets through about 0.2 W/(m^2*K), which is a nice physics unit that i'm used to. If we imagine a standard 2-story, square, 200 m^2 house with no windows, a flat roof and 2.5 meter ceilings, it would have 200 m^2 of wall and another 100 m^2 of roof. If you try to pull a 10 degC difference between inside and outside, you need to move 300 W of heat. Total cost: $394/year.
Huh. Actually, that's not bad. However...a single square meter of single-pane glass will cost you 70 W = $92/year under those conditions. Glass is effectively a conductor, but it does block convection. Even double-pane glass with radiation coatings only get this down to ~10 W/m^2 = $13/yr/m^2. If you've used fiberglass panels (R = 0.44) instead of, say, polyurethane foam (R ~ 1), you spend an extra $500/year. On the other hand, installing blown foam costs about $15/m^2, or $4500 for our fictitious house. You'd recover your investment in nine years in modest climes. Definitely worth it in extreme climates, but very geography-dependent. Same goes for double-pane windows. They range $200-$500/m^2 and may save around $80/m^2/year.
Of course, this assumes you're running an electric heater. Air conditioning is even trickier. First, cooling power is often rated in 'tons', which apparently means the same cooling power as 1 ton of ice melting over 1 day. This works out to 3517 W or $4620/yr if it ran continuously. Next is efficiency ratings. There seem to be at least four competing systems, most of which mix imperial and metric units. I like the Coefficient of Performance (COP), which is (Watts removed from house) / (Watts of power used). It seems that COP values of 2-5 are common for cooling systems. Of course, if you switch the input and output pipes on your AC, you could theoretically get that same boost to efficiency by 'cooling' the outdoors to warm your house. This arrangement is sold under the name 'heat pump', which i guess makes sense.
This is rapidly becoming too situation-dependent to carry further. The pattern here seems to be that most of the eco-friendly advice makes good economic sense most of the time, but a lot of it is in the margins and many power usage decisions are interlinked. You really can save money every year by installing better insulation, getting a newer fridge or switching to CPFs. But at current electricity prices, you won't recoup your investment for several years. As for the Watt's Up meters, they range $50-200. Its possible that having better information could help a homeowner save $200 in the course of a decade by making better choices. Probably not worth buying for an apartment-dweller.
