Solar power is becoming more popular, and it seems a lot of people who have solar panels have a DC to AC inverter. It seems ironic to convert DC electric to AC, and then the the AC gets converted back to DC to run a computer. I believe all digital hardware runs on DC power. Do LED light bulbs, LED TVs, LCD TVs, have to convert AC to DC? How about motors. What are the trade offs between AC and DC motors?
TVs, PCs and LED bulbs all run off varying DC voltage, although you'll find interestingly enough that the most efficient DC->DC converters are actually DC->AC->DC converters. (See resonant converter)
I'll expand a bit more on the motor front. AC induction motors (that run directly off AC mains) have been the standard industrial power source for many many years. They are cheap and very reliable. A large portion of the electricity generated (see below) is used to drive a AC induction motor in one form or another (as a water pump, air pump, conveyor belt and so on). Newer more efficient AC synchronous motors (being used in some electric cars, washing machines and more), have variable-frequency inverters inside them anyway, and so can be fed with AC (which is rectified) or DC with minimal loss in efficiency. DC motors are usually much smaller than both these types by comparison, and even then, they are often used with pulse-width modulated (PWM) drives which would rectify any AC before adjusting the voltage.
Induction motor usage stats: 30% of total energy (according to http://www.mpoweruk.com/motorsac.htm)
80% of total mechanical energy (according to http://electrical4u.com/construction-of-three-phase-induction-motor). No actual studies provided by these sites though.
I found some similar questions
I'll bet real money that the AC vs DC war question has been asked here before.
AC is hugely advantageous for long distance transport of electricity (i.e. the 'grid'), because it can be stepped up to much higher voltages with a relatively simple & reliable transformer. With higher voltage, to deliver any given amount of power you need less current - less current means thinner wires - and over tens, hundreds or even thousands of km/mi, less cooper means far far $cheaper, because power loss in wires is proportional to the square of the current.
But for short distances & several reasons, DC distribution is indeed seeing a come-back, in server rack power distribution, in small-scale renewables systems, etc.