One thing with Microwaves is the magnetron degrades of time from use. So your 1000 watt micro slowly becomes a 900 watt. and so on. Si the point things start to wear out its time to ask yourself is this costing me more to fix then getting a new one.
Yeah if you buy $500 microwaves but a $100 microwave at my place gets recycled soon as it dies. FYI my current one is 9 years old. But I might use it once a week.
Also some simplified math to calculate your electricity cost.
The cost of using 1 watt 24/7 for a year will cost you 60 cents. So a 100 watt light bulb on 24/365 costs $60
I did the math with the light bulbs at the condo complex where I used to live. Convinced the council to spend $2500 on compact florescent bulbs and saved us almost $10,000 a year. And that was 2002.
If you (most likely) use it less then 24 hrs a day then divide by 24, times by # hours. This is also how I calculate what my computer is using to decide if bitcoin mining will pay for itself.