Standby or hibernate
Does either save energy for a PC?
If I'm trying to maximize my energy savings, is it better to put my computer in standby or hibernate it overnight? Does hibernating my PC increase wear and tear?
When you hibernate your computer, Windows has to write the contents of memory out to the hard drive so that the system can be restored to its previous state by reading the hibernate file back into memory when the PC is turned back on. This uses a small amount of extra electricity up front and when the computer is turned back on, but you're using no electricity at all while the computer is turned off.
Standby, on the other hand, uses a trickle of electricity to preserve the contents of memory and the current processor state, and this allows the PC to wake up faster from standby than from hibernate. There's less up front energy used to put the computer to sleep, and less energy needed to wake it up afterwards, but in standby your PC is using energy continuously, albeit a tiny amount.
I measured my Lenovo Thinkpad on my Kill A Watt meter to see how much electricity it uses when in standby, and when hibernated. It turns out it's hard to tell the difference in power draw between standby or hibernate because both draw only one watt, which is probably just what the laptop needs to keep its battery charger circuitry engaged (the battery was at 98% charge when I did both).
When you put a laptop in standby for an extended period without leaving it plugged in, you'll find that the battery charge drops quite a bit. My laptop loses about 20% of its charge when left in standby over the weekend. When I hibernate the laptop for the weekend, it loses about 5%. So my general approach on when to use standby or hibernate is to do standby when I'll be back on the PC within 12 hours, and use hibernate for longer periods.
Going for a long walk?
I doubt the wear and tear on the hard drive from the hibernation is worth worrying about. It does involve some extra writing to and later reading from the hard drive, but unless you are hibernating your computer several times a day it won't make much difference.
You'll find much greater energy savings if you look at minimizing your overall computer device usage when you aren't using your PC, than in worrying about whether standby or hibernate is better. There is a lot more savings to be had by keeping on only those devices you need to use the computer at the moment. For example, there's no point leaving your printer on 24x7 if you only print a few pages a week; there's no point having external speakers turned on (or even plugged in) when you're just doing word processing or web browsing, assuming you have built-in speakers, which most PCs have. And you'll save a lot more by turning off all the other computing devices - printer, speakers, cable or DSL modem, wireless router - whenever the computer isn't in use, than you'll save by using hibernate instead of standby. Each of these devices can use 10-50 watts when left on, and you don't need them on when you're not using them.
The best way to control your PC peripherals is to put them on a power bar, and turn the power bar off when the computer is not in use. This not only saves more energy than just putting the computer into standby or hibernate, but having the peripherals and the computer on a power bar, assuming it provides surge protection, will reduce the risk of your equipment being damaged by voltage spikes.
Saving on computer energy usage is just part of the much larger problem of cutting overall energy use. See my page How to save electricity for a comprehensive methodology for getting your energy consumption down drastically. I applied this methodology in my own home and my family of four has been able to cut our use to about a quarter of what the average family in our area uses.