**The main difference between a 900-watt microwave and a 1000-watt microwave is how long it takes to cook food items.** According to Microwave Cooking For One, it normally takes a 900-watt microwave longer to heat up something than a 1000-watt microwave.

Microwave Cooking For One contains a large chart that uses a basic mathematical formula to lay out the differences between cooking times, rounded to every 5 seconds used by a 1000-watt microwave. For example, something that can be heated in a 1000-watt microwave in 10 minutes would take a 900-watt microwave approximately 11 minutes and 7 seconds to heat.