Thank you, Brock, Engineer, and Joe for your replies. The earth-sheltered house will be about 2000 sq. ft. and cut into the side of a hill, so not actually below ground at all. But the several feet of soil on the 3 sides and the roof maintain the interior temperature at the same temperature as the earth, an even 50-52 degrees 24/7 even when it's -30 outside (doncha love those Canadian winters!).
Add to this the passive solar heating (the sun's heat soaking into the concrete floors and walls through the one exposed side on the south) and your house only needs heating (geothermal or anything else) on the coldest of nights and long cloudy-day periods. So I think you three are right about this kind of low-load system not being enough to justify the geo system. Thanks again!
...Terry