Interior Lights

Passive solar lighting is the use of natural illumination instead of artificial lighting systems. (See also Daylighting) more...

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In the U.S., about 22% of an average building's electric bill is for lighting. Inefficient lighting systems can use up to 40% of the electricity consumed by a poorly-constructed modern commercial building that does not utilize sunlight for lighting.

Windows

A window is a free daytime passive solar lighting system, used for over two thousand years (since glass was first invented in Rome). Romans recognized the Right to Light as early as the 6th century, and English law supported solar rights in the Prescription Act of 1832.

The problem with windows is that even the best ones have far less resistance to undesirable heat transfer, than a well-insulated wall.

When there is nothing but glass between exterior temperature extremes and interior living space, windows can waste far more energy than leaving efficient lights turned on.

Skylights

Skylights are often used for daylighting. Skylights admit more light per unit area than vertical windows, and distribute it more evenly over a space. The thermal performance of skylights is affected by stratification, i.e. the tendency of warm air to rise and collect in the skylight wells, which in cool climates increases the rate of heat loss. Poorly-constructed or incorrectly-installed skylights may leak, and single-paned ones may weep with condensation. Skylights may also be more prone to breakage (from hail, falling tree limbs, etc.) than vertical windows.

The skylight topic is controversial. Although roof-angled glass provides good daylighting, it can also create a solar furnace in the summer (when the sun is nearly perpendicular), and then lose more solar gain than it captures (when the low winter sun reflects off of it,and warm interior air rises to touch the cold glass on winter nights). Heating and cooling costs for a building are normally greater than the value of skylight daylighting, especially when modern energy-efficient lighting systems are employed.

Skylight Drawbacks:

Significant source of heat loss or heat gain; Can constrain design of building shape and orientation; Difficult\Complicated to specify; Point of condensation; Uncontrolled, uneven illumination; Susceptible to water leakage; Susceptible to ventilation leakage; Not appropriate for low ceilings; Difficult to relocate or reconfigure; Suitable for downlighting only (i.e. N/A to directional lighting or uplighting); Does not maximize the use of available sunlight; A source of light pollution at night; Cannot be easily turned off; Security concerns;

Read more at Wikipedia.org