From a Google Search.....
I read somewhere that the iMacs use around 2 watts when sleeping. Thats .024 KWHrs when sleeping for 12 hours. Assuming it uses the full 300watts when starting up for 20 seconds that would be 0.00167 KWHrs. Sleeping uses more power.
You have a computer like in iMac G5 20", which uses about 105 watts, and you're smart enough to turn it off when you're not using it. You use it for two hours a day, five days a week. That's ten hours a week, or 520 hours a year. So your 105 watts times 520 hours = 54,600 watt-hours. Divide by 1000 and you have 55 kilowatt-hours (kWh). If you're paying 10¢ per kilowatt-hour, then you're paying $5.50 a year to run your computer.
That's quite a range, $5.50 to $405 a year. It really depends on how much you use your computer, what kind of computer it is, and how much you pay for electricity. I used to have only one example somewhere in the middle but then I'd see people on blogs and messageboards misquoting it by writing, "Mr. Electricity says a computer costs about $150/yr. to run" No, that is not what I said. I said that with the particular set of assumptions I used, the computer system cost that much to run. Your situation is almost certainly different, and you need to consider all the variables, most especially how much you use the computer, what kind it is, and how much you pay for electricity.
It's a myth that it takes more energy to start a computer than to keep it running. It doesn't.
You'll always save energy by turning your computer off when you're not using it. If that's inconvenient you can always use the sleep or standby mode.
And no, you won't wear your computer out any faster by cycling it once a day, or even a few times a day. Modern computers aren't that fragile. I did hardware troubleshooting at Apple, I know what I'm talking about.