It may be expensive, but so far all of my buildings have been MOSTLY self-sufficient using solar energy. My cobblestone processing plant has multiple overclocked macerators and electric furnaces, but I put an array of 25 solar panels on the roof and that has still been more than enough to maintain its power. Even with all 5 production lines running constantly, the facility can self sustain for ABOUT 10 Minecraft days, assuming you start with a full MFSU. Note this is very rough, I didn't use a timer, I just counted Minecraft days. Granted, this takes TONS of copper wiring to make, but it's definitely been more than I needed for that plant.
I'm working on an automated cactus factory right now, one that produces dye and buildcraft waterproofing, and it will use much less power even when both production lines are active. I expect to use even less solar power for it.
Of course, my next project after this is going to be a huge recycling plant to turn all the excess cobble and cactus into scrap, and a huge array of mass fabs. That will probably require more than just solar power could provide. For that, I may either set up an ice-cooled CASUC reactor, or go the super manly method and build a giant lava tank warehouse and start draining volcanoes to power a huge array of geothermals. There's something magical about the loud hum of 50 geothermal plants taming mother nature's lava fury into raw power!
And of course, for the awesome factor, I may build an underground nuclear plant for backup power, and hook it up to all the completed facilities for emergency power at the flick of a switch. I certainly have enough Uranium saved up for it now, over a whole stack! But since I also have about 3,000 buckets' worth of lava also stored in my home base's basement, that's something for waay down the line.
[It isn't harder than nuclear in my opinion and lasts forever]
-> directly sends compressed plantclump to extractor 
-> directly sends extracted fuel to generator 
.