first thing: around the perimeter of the porch, I have 30 "bays" that are about 16 inches wide each. Essentially that means if I want to light each bay, I need 30 lighting fixtures.
For the low-voltage incandescent lights, the best thing that I've found is a 10-watt flood for $7.00. It's actually a metal fixture with a glass lens and metallic reflector. 30 of these would run me about $210, and I'd need a power supply capable of 300 watts continuous power. I'd probably want to dim them as well, but that's another category entirely, since I'll probably want to dim whatever lighting I put in.
LEDs are nice, but LED fixtures are freakin' expensive. There are some compromises, like regular fixtures with LED replacement bulbs, but it would still double the cost at a minimum.
And then my friends at Marlin P Jones chimed in with a sale on 5000 mcd white LEDs. bulk package of 500 of them for $105.00. 20 degree illumination angle (works out to about 0.5 lumen per LED). Three of these in series settle very nicely into the 12-volt range (with a couple of resistors for balancing and current limiting), and six of them together provide some pretty decent accent lighting.
For now, let's call a set of three one unit. One unit consumes 20ma at 12v, or 240 mw (just under a quarter of a watt.). If I put three units in each bay, that's 90 units, the total power consumption is still under 22 watts. Four units in each bay would bump that up to 28.8 watts, and that's still less than ten percent of the power consumption of the incandescent bulbs. Plus the power supply to run those beasties would be a lot cheaper.
Downside is soldering that many LEDs. Designing and etching circuit boards is a bit tedious too, but it's not like I haven't done it before. Sounds like it might be a fine winter project. :)
Incidentally, a more pure measure of lighting efficiency would be lumens per watt.
A 10-watt halogen bulb is about 150 lumens, or about 15 lumens/watt.
A 240 mw LED unit (above) is about 1.5 lumens, or about 6.25 lumens/watt.
The big difference is that the LEDs are highly directional, with a 20-degree beam spread. The 10-watt halogens are floods, and light up everything.
There are some seriously high-efficiency LEDs out there, like the Cree XR-E with a typical efficiency of 69 lumens per watt (up to 80 peak).