Are those aluminum framed panels? if not I can get you a much better deal on the wood framed panels, even if they are aluminum framed panels $1,500 seems rather high!
How to use SRCC solar collector ratings
It has been years sense I worked with solar hot water in MN, but I helped put a system on in St Cloud MN, near where I assume you are, it has about the same SF as 8 to 10 of those panels and was designed to heat 100% of the domestic hot water and keep a 1,800sf house at around 50 to 55F on it's own in the dead of winter, additional heat was with a masonry mass wood stove and of course insulation!
Now I know everyone on here who is from California says that 6 panels will not heat your shower water in the winter, but they also figure out excuses to bust out the winter coats when it gets down to 65F
These panels are 1980's tec, but what you can buy today for flat panel solar collectors are pretty much exactly the same! low iron dimpled glass, aluminum frames, tiny bit of insulation in the back... not much has changed.
Also, a BTU is the energy it takes to raise one pound (8 pounds to a gallon) of water one degree.