Why wattage matters
Microwave instructions on food packages are written for a specific oven power, often 1,000 or 1,100 watts in the US. A weaker oven delivers less energy per minute, so it needs more time; a stronger one needs less. To a good approximation, the energy delivered is power × time, so:
New time = package time × package wattage ÷ your wattage
Worked example
A frozen meal says 3 minutes at 1,100 W and your microwave is 900 W.
- Package time: 3 × 60 = 180 seconds.
- New time: 180 × 1,100 ÷ 900 = 220 seconds.
- That is 3 minutes 40 seconds.
- Your oven is weaker than the package assumes, so there is no power level that helps: cook longer at full power.
If your oven were stronger, say 1,200 W for a 900 W instruction, you could instead keep the package time and set the power to 900 ÷ 1,200 = 75%.
Common conversions
| Package says | 700 W oven | 900 W oven | 1,200 W oven |
|---|---|---|---|
| 2 min at 1,000 W | 2 min 51 s | 2 min 13 s | 1 min 40 s |
| 3 min at 1,100 W | 4 min 43 s | 3 min 40 s | 2 min 45 s |
| 5 min at 1,000 W | 7 min 9 s | 5 min 33 s | 4 min 10 s |
Finding your microwave’s power
Check the label inside the door, on the back or in the manual. It lists the output power in watts; ignore the input power, which is higher. If you cannot find it, time how long it takes to bring 1 cup of cold water to a boil: under 2 minutes suggests 1,000 W or more, 2 to 3 minutes about 800 to 1,000 W, and over 3 minutes less than 700 W.
Tips for even heating
- Stir or rotate halfway through, since microwaves heat unevenly.
- Respect the standing time printed on the package; heat keeps spreading to the center.
- Cover loosely to keep moisture in and avoid splatters.
- Check the temperature: leftovers and ready meals should reach 165 °F throughout.
- The formula is less exact for very short times, popcorn and large dense foods, so check early and add time in 15 to 30-second steps.