Power off or sleep at night?

I have always simply left my computers on at night, not bothering to power them down since the power management on my machines usually kicked in and turned off the displays. Having spent so many years on the Windows platform with machines that took a tremendously long time to boot up I was simply used to leaving machines on all of the time. As a software developer—and one that for the last 10 years has created web server applications—I have adopted an always on, server kind of mentality.
 
There was that accessibility factor—I wanted my machines to be there when I need them, available the second I walk up while I have someone on the phone asking me a question that can only be answered online. Not "Hold on while I pull this up…" and then sitting there for a couple of minutes while the machine fired up. Having built and maintained online service businesses in the past I always needed instant access in the event there was a problem with our servers.
 
My switch to Mac changed that to a large degree. Macs go to sleep nearly instantly and wake up just as fast. Other than the network connection re-synching—which takes a few seconds—the machine is ready to go.
 
Energy costs and conservation are dominating the news lately so I also became curious about the impact of leaving my machine on all night long. Was it drawing that much energy? How much did that cost?
 
With the recent acquisition of an APC UPS I now had some additional data available to me via the LCD on the front of the unit. Among other things was how much power is actually being drawn through the unit, giving a pretty good picture of my Mac Pro’s overall energy footprint.
 
I grabbed our most recent power bill to look up my rates. The most expensive months for power in my area are June through September when people are running their air conditioners full time. The rate is $0.0747 per kWh, compared with a $0.0590 per kWh rate for the remainder of the year.

Some Interesting Data
With all of the information available on the LCD display of the APC unit I tried viewing the power draw from the Mac Pro, both at near idle and under reasonable load. Keeping in mind that my Mac Pro is the latest generation version with dual quad-core 2.8GHz processors, 3 internal hard drives and two 20" LCDs attached, here’s what I found:
 
  Watts Used Battery Load Time on Battery
On and Idle 207 23% 22 minutes
Medium CPU Usage, HD activity 280 32% 16 minutes
Idle, Monitors Off 150 17% 31 minutes
Sleep Mode 0 0% 785 minutes
 
In sleep mode the Mac Pro draws so little power it didn’t even register on the APC.
 
Since normally I was leaving my machine in Idle mode with the monitors auto-sleeping I was burning 150W for the 6-7 hours I slept at night. Yes, I know, I’m on the computer a bit too much. Realistically I am completely away from my machine and not requiring it to be active for about 12 hours a day. So what does that translate into?
 
12 X .150kW = 1.8kWh * $0.0747 = $0.13 per day or $4.09 per month during the summer.
 
For me, from a purely economic standpoint, the $4 per month in savings is not all that much. A value meal at Taco Bell? A gallon of gasoline (soon to be half a gallon)? What does bother me is the environmental impact of having the machine on and at full power all the time. It seems as wasteful as keeping a couple of 75W light bulbs burning on throughout the night in case I might need some light.
 
Though OS X has some great power scheduling options I’ve found that my erratic work hours doesn’t lend itself to a schedule so now I’m just getting into the habit of letting my Macs sleep when I’m not in front of them.