Answer by Robert Frost:
The International Space Station (ISS) obtains ALL of its power from the Sun. The ISS, like Earth, is 149 million kilometers (93 million miles) from the Sun. At that distance the power received from the Sun is about 1.367 kilowatts per square meter. That power can be gathered by arrays of solar cells.
A solar cell utilizes the photoelectric effect. Photons from the sun strike a semiconductor surface, knocking loose electrons. Electrical conductors route those electrons along a path, creating an electrical current.
The US Segment provides eight large solar array wings (SAW) to absorb that solar energy. Each wing has two solar array blankets, each containing 16,400 photovoltaic cells. Each of these blankets provides about 105 square meters of solar cells, for a vehicle total of 1680 square meters. All together, at beginning of life, that totaled to 124 kilowatts of solar power (although 80 kw is typical). But that number varies depending on the angle of incidence to the Sun.
Here's a snapshot of the power gathering right now:
From those numbers it looks like the arrays are supplying about 52 kilowatts.
Each of those solar array wings are referred to as power channels. If you watch NASA TV, you will hear them referring to those channels by alphanumeric names, as depicted in this diagram:
The Russian Service Module (SM) and Functional Cargo Block (FGB) also have solar arrays projecting from their sides, although the FGB arrays are currently retracted.
The power from the American solar arrays is at a voltage of around 160 volts DC. A box called a DDCU (DC-DC Converter Unit) steps that voltage down to 124 volts for use in the US Segment. It is stepped down again for use in international partner modules, as required.
During insolation (daylight), the solar arrays continually gather power and distribute it to the vehicle for use, but a small amount of that power is routed into a box called a BCDU (Battery Charge/Discharge Unit). The BCDU trickle charges nickel hydrogen (NiH2) batteries. Each solar blanket has six batteries. When the ISS moves into eclipse (darkness) and the solar arrays stop gathering power, the batteries begin to discharge to maintain the electrical loads required by the ISS, until daybreak when the arrays take over, again.