For solar heating, you have to have available solar energy, such as a clear, unobstructed south-by-southeast opening, not blocked by trees or other buildings. In most North American communities, 8-12 square feet of solar hot air collection area all year long will sufficiently warm a composter in a heated home. Contact your state solar energy association for collector designs.

There are many designs for heating small outhouses with composters in them. Some designs direct solar heat onto the composting mass through a south-facing window, some onto the leachate, and some onto the composter.

Note: Direct solar heat can dry out the surface of the mass, creating a crust that actually insulates the center, so you might actually need more heat. Turning the material and adding water helps.