# How do I adjust cooking time for an under-powered microwave?

Example:

I have an 800-Watt microwave. Cooking instructions for a certain product say to cook on HIGH for 4 minutes in a 1,100-Watt microwave.

How do I properly adjust the cooking time so that my microwave will properly cook the product? Is it simple math (such as `4*{1100/800}=5.5`, or 5 minutes, 30 seconds), or are there more factors involved?

For the purposes of my application, presume altitude is at or near sea level.

• To those suggesting to just get an 1100W microwave: There is no reason to, unless the poster either needs to work as fast as possible in a commercial kitchen, or wants to replicate involved recipes exactly without bringing in an off-spec microwave as an unwanted error source. Nov 13, 2015 at 20:23

1100 Watt means 1100 Joules per second (energy over time). 1100 Watt over a period of 240 seconds therefore is 264000 Joules. To deliver 264000 Joules of energy with only 800 Watt takes 330 seconds (5.5 minutes), as you expected.

As KatieK noticed, there are some additional concerns. A good recipe will tell you to let the cooked product stand for a while. This allows heat diffusion, so all those Joules of heat will be distributed well. But if you cook at lower power, then the heat will already be diffused more. I.e. at lower power, you don't need to rest the product as long.

Another difference might be that you're not just heating the product, but you're relying on a secondary effect such as killing germs. For that, you'll need to have the entire product above a critical temperature for a certain time. This in general means that you don't need to adjust the times as much as you'd expect by the simple formula.

On the other hand, a warm product will lose heat to the environment over time. I.e. not all of those 264000 Joules will stay in the product you're heating. And with more time spent at medium temperatures, there's more heat loss in a low-powered microwave.

• Each microwave oven has a different magnetron (the thing that generates microwaves), the actual heating efficiency of that plus the cookign chmaber desing is what counts, so it's never an exact formula, and the difference can be quite large
– TFD
Nov 13, 2015 at 21:15
• Your microwave is likely smaller than one of the 1100 watt jobs. Calculating microwave watts per cubic meter can be helpful. Sometimes the less powerful units come out with the same energy density as the larger models, sometimes not. You want Microwave energy, not Amps X volts from the wall. The cavity magnetrons in various units run at different efficiencies, and it's the output from that into the oven cavity that you're interested in. Feb 12, 2016 at 16:34
• So to hopefully shed slightly more light on this.... Usually 1100W microwaves are 1.1 square foot (although no recipe I have ever read makes this clear). I have a .7 square foot microwave with 700W, do i not need to adjust cooking time at all ? It would seem that adding 35% to the cooking time is too much. Jan 14, 2018 at 22:44
• @xyious: The volume hardly matters - the microwaves bounce around until they hit the food. Jan 15, 2018 at 8:19

There is no simple math equation that will always get you the proper cooking time. Each 800W microwave may differ, even within the same models, due to hotspots.

You can estimate that a 800 W microwave will be 72% as effective as a 1000W model, and that you want to increase cooking time to about 5 minutes. But since you can't fix over-cooked food, check your food early, and check often.

• Good answer. Thankfully microwaves don't work like conventional ovens, where more power = burned exterior, underdone interior. Aug 25, 2011 at 4:18
• The 1100W microwaves also have hotspots, and hopefully the 4 minute cooktime reflects that. In fact, at only 800W the effect of hotspots is less: the extra minute will allow more time for heat diffusion. Aug 25, 2011 at 13:12