I have found that the biggest challenge in window farming is lifting the water to a point high enough to irrigate the entire garden. I tried many types of pumps and now know that the simplest solutions are the best – and sometimes the oldest solutions are the best too. So I have turned to Archimedes screw pumps. Recent work with half-inch flexible tubing (from AutomationDirect) wrapped in a spiral around a 1 inch PVC pipe as a core (from Home Depot) and rotated by a small DC gear motor (6 volts and 250 milliamps-from Jameco) can easily lift water over 6 feet. I have been able to drive the motor very nicely with a 10 watt solar panel that puts out 21 volts in full sun. The motor will turn even though the panel is not pointed directly at the sun or there are clouds.