In theory, the best time to aerate would be on calm windless nights or very cloudy days. Plants give off O2 during photosynthesis when you have sunlight and wind is a natural aerator via wave action. That being said, I've heard many times (by people much smarter than myself) any aeration is better than no aeration. If you can afford a system with a battery and controller great, but a direct drive system correctly sized for your pond is a fine option, imo.