I haven't played with Peltier devices in a long long time but as I recall they weren't the most efficient devices in the world. I believe they're used in some solid state coolers now and in some electronics for very local cooling. Enough of that.

I have two solar things. One a 4x20' mat and the other the pipes I mentioned. Flow is split three ways. To returns, mat, and field. The mat and field have their outputs going through separate hoses I drop in the deep end so it makes it easy to measure GPM using a timer and calibrated bucket. Measure the water temp at the skimmer inlet and the temp of the water out of the hose and that's all you need. Delta F and GPM. Convert GPM to pounds per hour and multiply by delta F = BTU per hour. I'm not where my records and notes are but rethinking this the 40 KBTU was for BOTH heaters. Something like 10 GPM through each and a 4 deg rise. Haven't looked at this in more than a few years so my memory is a little cloudy....like pool water sometimes.
Al