There4 is no real way to tell without testing what fan size will be required to get the desired slight vacuum under the slab needed. It depends on soil, leaks between the basement and under-slab area, etc. You only need a few inches of water of vacuum under the slab, but the leak rate sets the flow rate and that determines the fan size.

I thought this reminded me of something... http://ths.gardenweb.com/discussions/3277811/radon-mitigation-detector?n=2 Yes, the radon fan requires electricity. Unless you have one that runs on hamster power and some very active hamsters. You really should move the fan to the attic, and install a manometer in a visible area before the fan so you know it is working. This is normally a PVC pipe, and not all that difficult to manipulate. If you know how to move electrical, it isnt all that hard to put an outlet box up there for the fan either, otherwise you could call someone to come do it. It will be worth knowing how to deal with the radon fan anyhow, as they can fail like any motor driven device and usually all it requires loosening a couple fernco boots and hooking up the new fan.

R408.3 Unvented crawl space. Ventilation openings in under-floor spaces specified in Sections R408.1 and R408.2 shall not be required where: 1. Exposed earth is covered with a continuous Class I vapor retarder. Joints of the vapor retarder shall overlap by 6 inches (152 mm) and shall be sealed or taped. The edges of the vapor retarder shall extend at least 6 inches (152 mm) up the stem wall and shall be attached and sealed to the stem wall or insulation; and 2. One of the following is provided for the under-floor space: 2.1. Continuously operated mechanical exhaust ventilation at a rate equal to 1 cubic foot per minute (0.47 L/s) for each 50 square feet (4.7m2) of crawlspace floor area, including an air pathway to the common area (such as a duct or transfer grille), and perimeter walls insulated in accordance with Section N1103.2.1 of this code; 2.2. Conditioned air supply sized to deliver at a rate equal to 1 cubic foot per minute (0.47 L/s) for each 50 square feet (4.7 m2) of under-floor area, including a return air pathway to the common area (such as a duct or transfer grille), and perimeter walls insulated in accordance with Section N1102.2 of this code; 2.3. Plenum in existing structures complying with Section M1601.5, if under-floor space is used as a plenum. http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_4_sec008.htm

So I have two radon sources: my crawlspace dirt and my water cistern off-gassing radon (confirmed to be 20,000 pCi/L by water labs). I am suspicious about that high of a reading, sounds like an error. If it is true you are on top of a uranium mine and your radiation exposure is similar to living at Chernobyl.... If these levels are real your area would need to be condemned. What are your air levels reading? You would be better off spray foaming your floor joists, installing heat tape on your plumbing so it doesn't freeze (not touching the foam or anything else), and ventilating the crawlspace with outdoor air. You want to make your house tight so radon doesn't get in, not make openings to try to blow conditioned air out (which is very inefficient also). You should also not have ductwork running through this crawlspace. You will be better off using a granular activated charcoal system for capturing the radon in the water, and the filter material may need to be registered as radioactive waste. I guess doing the external cistern could work. It would have to be very large and would have to sit long enough to off-gas which is not practical for most homeowners but if you have the space to do it (and it sounds like you do) then it should work.