I'm asking this here because this is primarily a huge office scenario and administrators will more likely have the answer I'm looking for.
Employees' desktop computers can be either left turned on for the whole night or switched off in the evening and turned back on in the morning. The latter will surely save energy. In the same time turning on and off is very harmful for the equipment - hardware often breaks specifically when turned on.
Both energy and hardware replacements cost money. With energy it's quite obvious - you pay every month according to what your power meter shows. With hardware replacements it's worse - you need qualified stuff to quickly diagnose the problems and once something breaks the affected employee will have to wait for some time while his computer is fixed/replaced and the data is recovered.
So the company has to choose between saving money on energy and saving money on computer maintaince and lost hours. Such decisions must be well though.
Is there any detailed study of how turning computers off each evening affects their lifetime and what losses are induced by it?
I work for a large state government where we recently began implementing power management. According to our calculations based on metering a sample population shutting off PCs during idle periods saves about $35/PC per year for a mainstream business desktop with LCD monitor. Your mileage will vary, so do some testing yourself.
Laptops are generally provide for less savings, CRT monitors and workstation class devices increase savings.
We looked into the issue of hardware failure at great length, and based on research, testing, and a production implementation that's about 6 months old, I can find no evidence supporting the assertion that turning a computer on and off "wears out" anything or causes other hardware related issues. We've observed no statistically significant increase in hardware failure. (If anything, it has gone down slightly due to refresh of older equipment.)
You will find other issues, such as:
These issues don't have magic bullet fixes. You need to communicate with end users, test your applications and test your older hardware.
First, if this is a "huge office scenario", you will hopefully have warranty contracts that covers most of the lifetime of the machines. In that case, if it breaks, it's the vendors job to repair it and you can just reap in the energy savings.
Beside that: While I would agree that there is a somewhat increased possibility that hardware will die during a power cycle, I consider this to be a problem of (really) old hardware and I can't see how one cycle every working day over a course of three to five years would cause a problem except on very crappy hardware that might die anyway whenever you look at it the wrong way.
One major issue remains, which decides the whole game in my opinion, and this is the harddisk. Desktop drives are not designed to run 24x7 and I personally experienced a significant amount of drive failures of non-raid-type drives used in 24x7 server scenarios.
So, in the end: Turn the machines off and save the energy. There is nothing else to gain.
Not switching PCs off may cause overheat which decrease hardware lifetime.
Sorry but I didn't get you where you say "hardware often breaks specifically when turned on." Does turning computers off and on damage them ?
Further, I would suggest having the machines to be put into suspend mode, if it is not possible to switch them off completely, so that at least some power is saved :)
How much does it cost in power to run the PCs 24/7? This is a question most companies can't answer off the bat. However, it should be considered that of 168 hours in a week, each PC will be used around 40 hours, or however long your working week is (at most, there's still lunch break and not everybody does all work on the PC all the time). This means you quadruple energy consumption by not switching them off.
My office desktop draws 250 W, so I use 1 kWh every four hours, or 42 kWh per week, if I left it running all the time. If you pay $0,15 per kWh, this means more than $300 a year, of which $225 are just waste. At that rate, the damage done by a power cycle every weekday would have to decrease the MTBF quite drastically to get economically meaningful.
I switch my PCs and laptops off whenever I know I won't use them for the next 30-60 minutes. I have used them for many years that way. In fact, I have never even heard of a PC breaking by switching it on and off.
In working as an energy consultant for businesses, I've seen this theme pop up over and over. Not just the PCs, your whole building is only used 25% of the week. The amount of energy that's used outside those hours, for all kinds of things, is astonishing, to say the least.
I wish I had detailed data, especially the type of numbers you want.
You might simply need to do A/B testing to see which is better.
However, I would say that the harm of turning stuff on-and-off is kind of hypothetical sounding to me:
First, what is the lifetime of a PC? Lets say it is 5 years. Turning it off and on again once a day really is not a lot. I remember, in my childhood, we turned off Apple's, IBM PC's and Atari class systems a dozen times a day, for years. Computers have only gotten more reliable since then.
Second, I doubt a higher failure rate can be can be uniquely associated to the daily-power down. Even if you leave your computer on all the time, you still have to reboot for system updates. In my experience, that is the source of most of my boot-time problems.
Third, the relative costs will continue to diverge over time. If you have to assume, the safe assumptions are probably: energy costs will continue to rise over time, hardware costs will decrease over time. So, even if this is break even now, this is a behavioral/cultural change, you need to start now so people will be doing it when the price benefits arrive, rather than suffering from a lag time.
The real costs to a power-down is that you have to close up all your windows and save your work. On my Macs, almost all my applications have an auto-save feature. For Windows, I usually use "hibertnate".