Immersion Chiller Qs

I've read through numerous threads on the pros and cons of 3/8 vs 1/2 inch immersion chillers. I'm building a keggle and will eventually be doing 10 gallon batches (at least some of the time), so will be building a 50 ft chiller (might make it shorter and use the extra tubing for a prechiller). Tap water here is currently 65F.

My understanding is that 1/2 inch will cool somewhat faster, but use more water. My main question (and I understand that there are lots of variables), is how much more water, in relative terms, will the 1/2 inch use compared to the 3/8? 25% more? 50% more? 100% more?

Thoughts?

I can't really answer your question, but I can give you a few options on saving your water...

I usually go through about 5 gallons of water while cooling my wort. Sometimes more or less. I've got very good tap water, so instead of running the exit hose into my sink drain, I run it into buckets. Over the summer I use it to water my plants. In the winter, I usually boil half in 4 gallon pot and stick it in the snow. From there (after it's cooled) I bottle it for drinking water. The other 2-3 gallons gets used for cleaning my brew kettle, mash tun, etc.

The bigger tubing will have a greater surface area and will be more efficient at heat transfer. It's not just about the amount of water that you can flow in the tube. You can always run the water slower in the bigger chiller.
When the wort is near boiling, the exit water will be pretty hot with either size, but as the wort cools down, the bigger copper will be more effective.

Water use is directly related to how fast you run it, not chiller size. Certainly you could flow more water through a larger tube but you have that control by turning the valve on the hose bib. Run it slower and use less water (and chill slower of course).