Apparently a Microwave typically uses about 1.5W on standby, so that's: 24 * 3600 * 1.5 = 129600 Joules per day
So, if it draws about a kW in use, that means that if you use it for less than 130 seconds per day, or just under 2 minutes, you'll actually use more power on standby than on cooking / heating.
I think I use my microwave more than 2 mins each day, but there are days I don't use it at all, so I'm not sure how it would even out.
1.5W is a crazy amount of standby power. Not that I don't believe older models may use that much, but there are now regulations and standards for that kind of thing which push that to a much lower power.
