I think it obviously depends on what you're cooking. For anything that's going to be in the oven a long time, it probably is a waste to preheat. But there are a few things that come to mind that definitely need the oven to be good and hot when the food goes in. Pizza is a good example; it bakes much better on a good hot stone in a nice warm oven. Same with bread, to get that good initial rise. For a roast or casserole? Probably not.

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I determine whether, or how much, I preheat depending on the food. I never wait for my oven to get to 400 before sliding my pizza in, but I always do it for baked goods. My old oven takes eons to heat up, and it must be cranked sometimes 20 degrees higher on the dial, so if I didn't preheat, I'd never get the correct temperature during any part of the process.

As for waste -- since I keep my apartment cold most of the time with my horrid baseboard heaters on as low as possible, preheating is a nice, tropic vacation.

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I always pre-heat for creme brulee. I almost never pre-heat for general food. Most bread can be cold-started. Pizza should never go in cold.

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My rule of thumb is if it's going to take 20 minutes or more to cook, I don't generally preheat. But, I think it's a good idea to have a thermometer in the oven so you know when you're at the proper temperature. The old rule at my house was it took 10 minutes to preheat, but I found my oven was generally at temp in 4 or 5.

If you're basking "flaky" type pastries, pre-heating, I've learned the hard way, is essential. Nothing like ruining a beautiful beef wellington by not having the oven hot enough and the pastry didn't rise... :(

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As has been stated, it depends. For instance, I follow Alton Brown's roasting method for a turkey, which calls for a HOT oven to start. If you put the turkey in a cold one, you'd defeat the entire purpose of the initial high heat. Same with his beef roasts, which he finishes on high heat (after removing the roast and increasing the heat of the oven) - the gradual 300-degree rise in temp would contribute to drying out the meat before creating a crust.

Some baked goods need even heat for even cooking, and preheating is more a method of getting this. Preheating also shortens a lot of shorter cooking times. ie.- if it's 10 minutes at 425, and it takes your oven 10 minutes to reach 425, you can't expect the dish to be ready in 10 minutes if you start cold.

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I feel its necessary to preheat oven before.. as you said sometimes it may not. It depends upon the recipe. Yes we need to think about the fuel costs as it became the prime concern.

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It would be entertaining to test the pizza theory.

My understanding is, the preheating (presuming the temperature is correct) aids in the estimation of cooking times in recipes. Still, most recipes give you a 'window'.

As noted above some dishes are more sensitive than others.

Monika noted the use of the heat. We, in Florida, wait until winter to bake. Usually the windows are open and the heat can escape from the house :)

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I think the "preheated oven" is more about standardizing the cooking time, rather than changing the outcome of what you make. Some ovens take longer to get up to temperature than others. If your oven takes an extra long time to get up to temperature, then your baking time might need to increase if you put it in cold. Most people don't want to see a recipe that says "Remove from oven when done." :P

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It is for the standardization of the cooking times for the food set by the food producer. Because of the variations in ovens used around the country, pre-heating of an oven allows for a standardization of the time it takes for a food to be heated through in an oven.

These times are especially important when microwave heating. Because the wattage on microwaves can vary from 800W to 1200W, cooking times in the microwave is usually standardized at 1000W ovens.

Believe me, I had to run studies on different heating times of mac n cheeses while developing at one of my past companies. Literally hundreds of cycles were put on 5 different types of microwaves in order to figure out the right instructions on how to heat the product through.

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My oven actually has a timer on it that tells me when it's hit the right temperature, usually about 5 minutes! So that's how far in advance I turn it on:)

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Oddly, we'll be testing this in a couple of weeks over at Kamikaze Cookery - http://www.kamikazecookery.com

We'll be testing a roast duck breast and a frozen pizza - results should be interesting!

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I have to admit, I don't pay a ton of attention to preheating. My oven has a sensor or something and it beeps when it is hot enough - after just a few minutes. I think the only time I really preheat is with bread making or pizza - things where I'm going to be counting on a real oven spring.

Just my two-cents for what it's worth. I definitely think about our gas bill every time the oven goes on too.

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My Kenmore electric wall oven (5 yrs old) uses the broiler to heat the oven until it reaches the set temp. So, if I put anything in before it reaches the set temp, it gets broiled. Also, if I decide to increase the temp of the oven while something is in it, it will get broiled until the new higher temp is reached. It's a pain and I've nearly ruined many a dish because of it.

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