Egg white powder is a shelf-stable alternative to fresh egg whites. It is also pasteurized, which means that using it significantly reduces the risk of foodborne illness. Egg white powder is a unique product in many ways; however, there are a few effective emergency substitutes if you find yourself out of it or unable to use it for some reason.
Your best bet: Fresh egg whites
Once egg white powder has been reconstituted with the addition of water, there is little difference between it and fresh egg whites. Fresh egg whites taken from whole eggs are the closest match to how rehydrated egg white powder looks, tastes and works in a recipe. Of course, a key difference is that egg white powder has been pasteurized; however, this primarily affects food safety and not texture or appearance. They will both function exactly the same when cooked or whipped into a foam.
Note that fresh egg whites do have a drawback in that they are more labor intensive to use. You may have to crack and separate multiple eggs in order to get enough of them for a recipe. In addition, they do not have a long shelf life.
Use 3 egg whites to replace two tablespoons of egg white powder.
A decent second choice: Meringue powder
Meringue powder consists mostly of egg whites, which makes it an ideal alternative if you are replacing egg white powder in a meringue or similar application. It is not normally used as a substitute in any recipe where the egg white powder would not be beaten to stiff peaks. Meringue powder is commonly used for making royal icing as well as meringue for macaroons and lemon meringue pies. You can also use it to make angel food cakes.
Aside from egg white powder, meringue powder may include cornstarch to prevent clumping along with citric acid and food gums to increase its stability. Those other ingredients can affect how well it works when used as a substitute for pure egg white powder, which consists entirely of dried egg whites.
Use meringue powder as a 1:1 substitute for egg white powder.
In a pinch: Aquafaba
Aquafaba is an exotic term for the liquid in a can of chickpeas. This liquid can be drained and whipped into a foam that is very much like egg white foam. Like fresh egg whites, it differs from egg white powder in that it is a liquid and not a powder; however, you can use it in many of the same ways. Use it to make a vegan meringue by whipping it with cream of tartar. Unlike other vegan egg white powder substitutes, it can stand up to heat. As an egg substitute that is completely free of animal products, aquafaba is suitable for vegans as either a replacement for egg white powder or whole eggs.
Use 1/3 cup aquafaba to replace 2 tablespoons egg white powder.
Flax seeds are rich in mucilage, which is a soluble fiber. The mucilage content makes flax seeds an effective egg white substitute in applications where it is whipped into a foam or used as a binder. In order to release the mucilage, soak flax seeds in water. Within a few minutes, the water and seeds will take on a texture similar that of reconstituted egg white powder. Note: Flax seed foam does not hold up to heat, so it cannot be used in macaroons or similar applications where it is heated.