7.1 MillionBtu to heat a 1718 sq. ft. house through 1084 heating degree F days is 3.8 Btu/ft&2/HDD(degF), which is good but not great, and if the hot water is included, the figure becomes 5.7 Btu/ft^2/HDD(degF). I assume the 7.1 MillionBtu figure does not include the 4.1 MillionBtu loss in the heating system, which presumably is also lost in the house, and presents a problem during the cooling season. This is in a desert climate that should benefit from high mass construction.

Supplying 6.3 MillionBtu from electricity for the heat and hot water is not bad, but since they were trying to meet net-zero source energy, they could have done better with a natural gas heater, assuming natural gas was available in the subdivision. That would have reduced the source energy to about 1.9 MillionBtu using the national average factors. Alternatively, they could have used an air source heat pump, as Dana pointed out, and probably done almost as well.

No slab insulation, but it's Tucson, with bone-dry soil (which is fairly insulative) and an optimal subsoil temp. A modest amount of slab-edge insulation would be worth it when net-zero is the design goal. I saw no mention of it, so I'm assuming it's not there. Look for yourself- it's a fairly readable document (and thanks for digging it up, toddm.)

http://www.toolbase.org/PDF/CaseStudies/TucsonZEH1Report.pdf