I have SIP plans for a 3,800 sq ft home in Ruidoso NM with radiant floor hot water heating.  I have access to Nat gas at the property line.  Its south facing with a large passive solar concrete slab exposed to the sun in about 40% of the house.  How would one figure what would be the best investment; solar hot water heating for DHW and radiant or in PV cells  for the house?

Austennn,

You might want to post this under the Solar and Wind Forum.  Does your state have a net metering law to buy back the electricity from your PV panels?

solar hot water has about a 2.5 to 5 year payback, solar PV has between a 10 and 25 year payback.

Hands down, I'd recommend you go with solar thermal to heat your hot water.  PV is not nearly as efficient and much costlier.

Hope this helps.

Ed