Essentially there are two types of solar heating: passive and active. Any system that uses a fan is active. Passive systems rely on the fact that hot air rises, a phenomenon known as thermosiphoning. The beauty of a passive system is that it works very well, is very cheap to develop and produces near immediate returns on investment. If nothing else, a passive heater is a cool science project for school children. This is how I started mine. Here is a picture of the early prototype.
The simplest passive heaters are based on the greenhouse effect. Construct an insulated box, whose insides are lined with a metal and painted black and cover the top with glass or plexiglass. Drill two holes in the box and connect the top one to your room in some fashion. I used a window kit that came with a portable air conditioner. It is an extendable piece with a round hole. You slide it under a double-hung window and attach a duct to the opening.
Expose the contraption to sunlight and you get free hot air flowing. This works in any environment that has plenty of sun, even if the outside air temperature is very cold. People use such devices in Colorado and Montana, where it gets very cold but there is plenty of sun. In mild climates like California, you don't need to connect the intake and you can just let outside air flow into the box. In colder environments you need an additional opening to let room air into the box.
A moderate size box can heat a 500-600 sq foot room on a sunny day. I found that such a box can add enough heat to keep our 3,000 sq ft house warm during the day and at night in winter. In December and January, our house is in the sun from half an hour after sunrise (around 7 AM) till half an hour before sunset (around 4:30P). On a sunny day there is enough exposure to heat the inside to near 68F even if the outside temperature is in the high 40s to low 50s. Depending on the weather the overnight may drop as low as 55F but 60F is more common. With the box, we can keep the inside over 71F during the day and above 65F at night. That means no heater.
There are a few things to consider. The box only works when exposed to sunlight. As soon as the sun disappears the output drops to zero. That means you need to install some sort of one way valve to prevent cold air from coming in. Even a thick cloud can stop the box. If the inlet is outside air, that means cold air will flow in during that time. So you need a valve.A flap valve is ideal as it is a totally passive device. When the air heats it will push the valve open, and when the temperature drops gravity will close it up.
I built my box for less than $100 and used nothing but off the shelf parts. My biggest outlay was for a box. I got a large planter box at Ace hardware and painted it black with Rustoleum barbecue black paint. It is important to get paint that can withstand high temperature without outgassing. I lined the box with two layers of fiberglass insulation and mounted pieces of aluminum flashing on top. The flashing needs to be painted black too. I covered the top with a piece of leftover plexiglass that I picked up for a few $$ at Ace. Make sure the box is sealed properly so no hot air escapes.
I drilled a 4'' inlet on the side (bottom end) and another 4'' outlet at the "top." I fitted a grille in the bottom mainly to slow the air intake and a 4'' flexible elbow pipe in the top. These are standard pieces of aluminum that you find in any hardware store. The planter box leans against the windowsill putting it at a near ideal 50 degree angle and fitting the elbow nicely through the window piece I described earlier. When the sun hits the device, hot air at 110-120F starts flowing into the house. The temperature depends on the flow rate and that in turns depends on the diameter of the channel.
My box is a simple rectangle. It can be improved upon by using a more squarish box and directing the airflow inside to loop around a few times. A longer path means more heating. It is rather easy to add a computer fan with a thermostat at the outlet to create an active device. If you are really clever you can use a small solar panel to drive the fan. With a fan the airflow will be faster but the air will be cooler. 85F is more common. Be advised that fast flowing 85F air feels cold to the touch so don't blow it in your face. Even though it feels cold, it does heat the room.
For commercial devices look at SolarSheat and Cansolair. These devices cost almost $2K and require professional installation (meaning you need to drill a holes in the wall). Permanent installation is preferred though because the "interface" is the place of greatest loss, especially at night. Unless you want to remove the box daily an insulated interface is key. And if you live in California, a storm proof design is also needed.
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