I read on some hardware vendor’s site they had an RTC circuit that could be programmed to set a pin “low” (I think) at a scheduled time–such that the pin could be wired to the RPi to wake it up at a specific time. I’m wondering if the OSPi RTC has the same capability… so I could program the RPi to wake up, say every hour, do some processing (at boot), then set the next wake-up time and shut itself down. This would help with saving power, especially in the Pool Water Monitor I’m building.
I wonder if it would just be simpler to build your water monitoring system around an Arduino. They have lower power consumption than an RPi, and with a cheap RTC shield and a WiFi shield, you would be able to log and retrieve data via the web.
If you are using the OSPi for irrigation scheduling it would need to be running all the time in order for the scheduling program to work unless you write your own program to activate the valves when the OSPi wakes up.
As far as power usage, I have seen an estimate on the Raspberry Pi web site of a yearly cost of something like $3.50 for running the Pi 24/7.
I agree with Dan, the effort needed to accomplish what you are asking might not be worth the savings you would yield. Don’t forget the power supply will inherently draw some power just being plugged in. Here is some data (http://www.raspberrypi.org/phpBB3/viewtopic.php?f=63&t=6050&p=291334&hilit=watts+power#p291334) on the power draw of the Raspberry Pi with and without a wifi dongle. The power draw at idle was half a watt which is what a power supply will draw with the Pi being off. I really don’t think you would see ANY real world savings doing what you are discussing.
With all of that said, you could argue having it turned off most of the day might reduce heat damage if the controller is outside. However, the Pi can handle a fairly high temperature from my understanding without failing.