I try to be a good energy steward: I don’t leave unnecessary lights on, I recycle and participate in community composting, and I never (well, hardly ever) take a hot bath any more (we have a drought in California).
But I do preheat my oven when I bake cakes, cookies, pastries, soufflés, breads, and the like.
For most baked goods, the wrong temperature can make the difference between success and failure, or at least between fabulous and not very good results. Time and temperature affect the texture and flavor of baked goods, and starting in cold oven changes both of those variables. Unless a recipe calls for a cold start, you are likely to get unfortunate or at least less than optimal results when you skip the preheating step. Who needs the aggravation?
The fine crumb or flakey, layered texture of cookies, cakes and pie crusts comes the expansion of trapped air and/or moisture in these batters and doughs (along with chemical leaveners such as baking soda or powder or yeast). And that expansion is produced by heat. Many batters and doughs require a push—a good hit of heat at the beginning—for optimal rise, texture, and browning (and remember browning is flavor).
A slow start in a cold oven is apt to produce cookies that are hard and dry instead of tender and crunchy, or that spread all over the pan before they firm up, or that don’t brown at all; pie pastry that is pale and crunchy rather than flaky; biscuits that don't rise and/or aren’t flaky; cakes, intended to be airy and light, that come out with a tough layer at the bottom or a coarse rather than fine crumb. Meanwhile, cooking times will be lengthened by about the time it takes to preheat the oven in the first place. Of course, results vary by the type of batter and there may be notable exceptions.
Here, I know some of you are thinking about the famous cold oven pound cake. Why does that work? My guess is this: Pound cake batter is rich and dense and normally bakes—in a preheated oven—for at least an hour (often more) at a low-ish temperature of 325. This means that the batter heats up slowly anyway and takes a long time to bake. Starting the pound cake in a cold oven doesn’t change the outcome much because it slows down the baking time by only a small percentage—only 10 or 15 minutes— of the total 1 1/4 hour baking time. No harm, no foul!
Compare the poundcake scenario to cookie-baking (for one example): Many cookies take only 10 to 15 minutes in a preheated 350 to 375° F oven. Starting in a cold oven means the dough heats slowly for 15 minutes before the oven reaches the intended temperature. By the time the oven is up to temperature, those little lumps of dough may have spread all over the pan or dried out or failed to rise, or who-knows-what, because the baking conditions have been changed drastically rather than slightly (as for the pound cake). If you are still having a hard time imagining the effect of starting cookies in a cold oven, imagine starting pancakes in a cold pan?
More questions about preheating? Here are my answers to three other FAQs:
Unless you are baking bread or pizza, you only need to preheat until the oven reaches the set temperature. This takes 15 to 20 minutes, depending on your oven and the intended temperature (read more about the particularities of ovens on this Hotline thread. If you want 345° F degrees, you’ll obviously wait a little longer than if you only want 300° F.
If you have an oven with digital controls, simply look at the little screen that indicates oven temperature. You can slip in that first batch of cookies as soon as the temperature on the screen matches the temperature that you set—in some ovens, you’ll hear a beep or see a light go on.
If you have an old gas oven (like mine) without an electric screen, the gas will shut off when the oven is preheated. You’ll either hear it shut off or, if you weren’t paying attention, you’ll notice the silence. If the room is noisy, bend down and listen at the (closed) door!
Note that ovens cycle on and off to maintain an average temperature, so there may be as much as a 25-degree swing above and below the set temperature. If you wait for a while after the gas shuts off, it will eventually start up again as the temp in the oven drops. I normally load the oven when the gas is silent.
Bread and pizza are exceptions. Recipes for artisan loaves and really great pizzas often call for preheating the oven for up to an hour ahead of time, with a pizza stone or a Dutch oven inside. This is meant to get the stone or Dutch oven as hot as possible, in order to simulate the quality of results produced in professional bread ovens and to mitigate the amount of heat loss that occurs when the door to a small oven is opened and the product is loaded in.
If you preheat the oven for longer than necessary, you will have wasted energy, but your baking results will not be harmed because the oven continues to cycle on and off to maintain the set temperature.