Does it take more energy to turn on a light than to leave it on?
No. There's no power surge when you turn on a light. Turning the light off ALWAYS saves electricity, even if it's for just a second.
Does it take more energy to turn on a computer than to leave it on?
No. There's no meaningful power surge when you turn on a computer. Turning the computer off ALWAYS saves electricity. Of course, you can also use the power saver feature.
Is there ANY consumer device that uses more energy when you turn it on than when it's already on?
No, not in practical terms.
I don't believe you. Everyone says there's a surge when you turn on computers and stuff.
There's a surge but it's so tiny you can't easily measure it. That's because it happens for only a fraction of a second, and the surge itself is modest. It's certainly not costing you any extra money, not even a penny. So there's no surge in practical terms. As far as you're concerned there's no surge at all.
Think of it this way: If a device used twice as much power as normal for one full second when you turned it on, that would mean that it cost you one whole extra second of electricity. Big deal. That's a fraction of a fraction of a fraction of a penny. And in fact, the surge doesn't really last for a whole second, it lasts for only a fraction of a second, and the surge isn't close to twice as much power as normal, it's much less. Bottom line: Surge is so incredibly insignificant it's really like there was no surge at all, for all intents and purposes. There is never a penalty for turning on a household device.
Does it take more energy to cool a house in which the AC has been off all day, than to keep the AC running at, say, 85 degrees during the day?
No. Cooling a hot house down at the end of the day always takes less energy than leaving the AC running all day, even if it's running on a high setting.
Does a 240V device use more electricity than the same device designed to run off 120V?
No. The electric company charges you for watt-hours, not volts, and the wattage is the same. To figure volts you use the fomula V x Amps = Watts. A device that uses twice as many volts will use half as many amps, so the wattage will be the same -- and so will the cost.