In the U.S., wood and natural gas energy is so inexpensive that it is hard to justify using solar or other forms, such as electricity, based on energy price considerations alone. In some cases, solar energy systems have a government subsidy, such as a tax credit or even an outright grant, which makes solar feasible.
Solar drying time depends on the amount of energy collected. During six months of the year, the roof collector at 45 depress will collect, on the average about 1000 BTUs per day per square foot using a cover that is 85% efficient (glass is better, but it is also breakable band heavy). That means that each square foot will evaporate 1 pound of water per day, on the average...at 100% efficiency (no building heat losses and no vent losses).
If we consider a 1500 BF solar kiln with 150 square feet of roof area, and we have oak lumber going into the kiln green and this means about 2200 pounds of water to be evaporated per MBF, and also assume that the kiln is overall about 60% efficient (probably a bit high), then the kiln has 3300 pounds of water to evaporate, which will require 3.3 million BTUs of energy. With the useful energy in the kiln at about 600 BTUs per square foot, that means will will have about 90,000 useful BTUs per day collected, on the average. So, we are looking at 37 days to dry this lumber. Now, as we move past Sepatember, the solar energy input drops (lower sun angle and fewer hours of daylight), so drying time is extended. By November, drying is really slow and stays slow until March, at which point is begins to increase again.
Can you run a business that only produces dry wood during four or five summer months? Oftentimes, the answer is NO, so then a DH unit looks very attractive. With DH, to save the expensive electrical energy, the kiln is usually totally enclosed and well insulated. The capital cost is a bit higher per year than a solar kiln, but the cost is spread out over many 1000s of BF, so the cost per BF is lower. Note that with this design, it becomes reasonable to consider a separate solar collector that would provide hot water heating. But, as mentioned, in the U.S., hot water or hot air heated with natural gas or wood is a lot cheaper than solar, electric and most other fuels.
Here is a comparison of providing 3.3 million BTUS (April 2014 prices)
Electric heat $145
Electric DH $72
Fuel oil $93
LP $76
Natural gas $28
Wood variable $39
We must also factor in the burner needed. With electric, none or small; DH uses a compressor; the others use a furnace of some sort, with natural gas being the cheapest. Wood looks cheap, but there can be fuel preparation and storage costs.