Ontario, Canada
30' round, I use 2 at 2x20 feet, only 1/3 of what was recommended and on average we're 5-10 degrees warmer than all the others around town without any heat.
Last year, we were swimming a month before all others, a month longer and in mid summer I had to bypass because the pool got to be 95 degrees on full sun days. This year, I was doing a lot of landscaping and some plumbing work and didn't get them hooked in til a couple of weeks ago, since then, we're about 7 degrees warmer than the neighbour's.