The majority of people eat out for many reasons. People may be extremely busy with commuting, work, sports, friends and families so eating out may save a few precious minutes for those that are not skilled at cooking.
We all know that restaurants do not have a good track record for being sustainable in the sense that they sell a product at 500% mark up, most do not recycle and compost as much waste as they should, and they use Styrofoam take-out containers which often little our streets. However, I would like to consider putting these deficiencies aside for a second to compare a restaurants ability to be sustainable compared to an individual at home.
In order for an individual to be able to cook at home this means that they need an assortment of tools. Every individual usually will not use up all of their produce or meat products immediately after purchase, so this means that the leftovers need to be cooled to delay spoilage Since it is not possible or practical for every one to own a clay pot fridge or a root cellar, almost all households are installed with a typical freon refrigerator. Refrigerators do not consume that much electricity compared other electronics such as your TV. However the problem with refrigerators, comes with the fact that they need to remain on 24 hours 7 days a week in order to preserve your food. This constant usage can cause the refrigerator to be the one of the main components of any electric bill.
Most foods are not as satisfying raw, so this means you will need some way to cook your food. Your options are limitless with bar-b-ques, grills, stove tops, microwave, toaster, and conventional ovens. Most homes are always fitted with one form of these, with the conventional stove and oven being the preferred. This means that in order for every individual to even have the means to cook their own food they will most likely need their own fridge and oven, not to mention all of the necessary cooking equipment such as knives, pots and frying pans.
Restaurants will already have all of these tools at their disposal, restaurants will already have the necessary fridges and cooking elements. If people would eat at restaurants all of the time, except for any non-perishable at home, this would eliminate the need for every household owning its own refrigerator and oven. Since you would have a significantly smaller demand for refrigerators and ovens, not as much energy is used smelting the metal, or materials used to manufacture them. However I have noticed with restaurants is that they need to keep the ovens, grills and such on for the duration of the whole dinner service in order to have it ready to make the meal when the chef requires it.
Not only that but the chefs that they employ are experts at preparing and cooking multiple dishes in short amounts of time. Most restaurants prep the necessary ingredients ahead of time for any dinner service, this combined with a chefs refined cooking skills allows them to prepare more dishes and quicker. By hiring experienced chefs, restaurants are able to make the process of cooking more efficient. Restaurants also have the means to make larger batches of food at one time. The more food you cook at once the more efficient it becomes since you do not have to repeat the same heating processes more than once. When you cook a soup in a larger pot you only have to get the pot up to temperature once, where as if you cook the same amout of soup in smaller batches you have to get the pot up to temperatur each time.
A large amount of food that people buy ends up in the trash before they even open it. Most people will do a large weeks worth or food grocery run, this becomes problematic as you now have a large fridge full of food. It is hard to judge exactly how much food you can eat in a week, as opposed to one day. It is hard to predict whether you might eat out, thus not eating the food that you had bought. The food may spoil before you even get a chance to eat it. Restaurants have the advantage of having chef's consistently checking product levels every day, its their job. This way restaurants are able to order only what they need, the only problem comes with not being able to predict the number of patrons for any dinner service. Restaurants have to basically guess how much food to get.
Since restaurants need to order larger quantities of the produce and meats, this means produce providers do not have to individually wrap every item. The food available at groceries stores is usually packaged in such a way that it meets the needs of an individual. Restaurants would be able to forgo this excessive packaging, less packaging also means that the produce provider as well as cities save money. Cities would save money since this means that there is less waste to process, and thus less labour and energy needed to recycle.
So could restaurants be more efficient? Possibly. Restaurants would need to become more affordable, and would have to change some of its operations, and practices but that is a discussion for another day.