How we tested
Before we started to test solar cookers, we were skeptical—could anything simply left in a pot or box to cook in the sun actually taste good? But we were proved wrong—at least partially. We also didn't anticipate just how much fun solar cookers would be. Whenever we had a sunny day, we wanted to try cooking something new. But these results still beg the question: Are solar cookers more than just a toy for a food hobbyist?
Cooking in the sun has a long history. The contemporary impetus for using solar cookers is largely economic and environmental. Solar cookers don't require any fuel, they don't create smoke pollution, and they use minimal water; all factors that make them attractive for use in developing regions around the world. Over 100,000 are reportedly used in India and China. Most solar cookers are produced by nonprofit organizations; profits from cookers sold in the U.S. or Europe subsidize cookers shipped elsewhere. In the U.S., sales are geared to several audiences: those who advocate environmentally sound practices, people in sunny climates who want a fuel-efficient way to cook that doesn't heat up the kitchen, outdoor enthusiasts who like their portability, and food hobbyists who enjoy experimental cooking.
There are two main types of solar cookers: concentrating or parabolic cookers and hot boxes or ovens. We tested one of each, as well as a hybrid form. The Hot Pot Solar Cooker ($100) is a large, insulated pot that sits on and is surrounded by reflector shields (metallic panels that concentrate the sun's rays at the pot). Its temperature stayed below 200 degrees. Our hot box model, the SOS Sport Solar Oven ($140), is a box with a specially molded insulating lid and insulated sides—its internal temperature reached 250 degrees in our tests. The Sun Oven ($200) is a combination of both types: The box has insulated sides and a glass top, and it’s ringed by reflectors. It reached an internal temperature of 350 degrees.
To cook effectively, it is necessary to understand some of the science that makes solar cookers work. It’s not the sun's heat that cooks the food, but rather the sun's ultraviolet rays. The sun must be high in the sky in order for the ultraviolet radiation to penetrate the atmosphere. For example, from November through March in the Northern Hemisphere, when the sun is low on the horizon, its light passes through more atmosphere to reach the earth. This screens out most of the UV rays—that's why it’s difficult to get a tan in the winter. When the sun is overhead, light rays pass through less atmosphere, so less UV radiation is screened out. A solar cooker works like a one-way lobster trap. It lets UV light rays in and then converts them to longer infrared light rays that can't escape. Infrared radiation has the right energy to make the water, fat, and protein molecules in food vibrate vigorously and heat up.
Practically, this means that successful cooking requires a clear sky, with the sun at least 45 degrees or more above the horizon for a significant amount of time (depending on what you're cooking, from two to eight hours). There are two easy ways to test for suitable conditions: 1) if your shadow is shorter than your height or 2) if the UV Index in your area is 7 or higher.
To determine local UV Index:
The EPA has a site that will calculate the local UV Index for that day based on zip codes: