I was doing a little more investigation about solar energy. I always get curious about it during the summer. I would love to mount up something to pre heat my DHW like I did with the coal furnace. Anyways, from what I've read, every square foot of direct sunshine has roughly 100 watts of energy or about 283.6 BTU per square foot at the Earth's surface. This is after it's been filtered by our atmosphere where some bounces back into space.
My 42000 BTU measurement earlier came from 160 square feet of solar collector and works out to be 262.5 BTUs per square foot at that heat output. It's under the 283.6 BTU sq/ft/hr, and it should be. Seems like it shouldn't be that efficient. That's like 92.5%. I wonder if my measurement was skewed somehow. I will do more measurements thru the summer and see what I get. I'll get a reading on a larger quantity of water over a longer time period. 2 gallons in 10 seconds is pretty tight lol. Off 1 second would skew results by 10% at that rate.
Thanks for everyone's interest.