Always disconnect from any power source before working inside a computer due to risks of electrocution and stored power in capacitors. Never connect yourself to a building's electrical system, including ground wires, as they can be energized. Maintain a healthy respect for electricity and always double-check disconnection from the main power source.

Desktop computers contain a power supply that converts AC (Alternating Current) from wall outlets to DC (Direct Current) needed by the motherboard and other components. Power supplies convert 120V or 240V AC to specific DC voltages: 3.3V, 5V, and 12V. This conversion is crucial as all computer components rely on the power supply's output.

Amperes (amps or A) describe the number of electrons flowing per second, analogous to water flow in a hose. Voltage (volts or V) represents the electrical pressure, similar to water pressure. Wattage (watts or W) measures real power use and is calculated by multiplying volts by amps. For example, 120 volts and 0.5 amps result in 60 watts.

AC (Alternating Current) constantly reverses direction, typically seen as a wave. It is used to transmit power from plants to outlets. In the US/Canada, AC is 110-120V at 60Hz, while in Europe, it's 220-240V at 50Hz. DC (Direct Current) flows in a single direction with constant voltage, and is the output of a computer's power supply.

Power needs vary by region (e.g., 120V/60Hz in US/Canada, 230V/50Hz in Europe). Older power supplies might have a manual switch to select between 120V and 230V. Modern power supplies automatically detect and adjust to the input voltage. incorrect manual settings and plugging into a wrong voltage can cause spectacular failure.

Power supplies output various DC voltages, often referenced as positive and negative values. Common outputs include +12V for PCI Express adapters and hard drives, +5V for some motherboard components, +3.3V for newer motherboards, M.2 slots, and RAM, and +5V SB for standby power. Some motherboards use -12V for integrated LAN, and older supplies might offer -5V for legacy adapter cards. Documentation or the power supply label details specific outputs.

The main power connection to the motherboard is a 24-pin connector, providing 3.3V, 5V, and 12V DC. Originally 20-pin, most modern motherboards use the 24-pin. A 24-pin connector can be used with a 20-pin motherboard by leaving four pins unused. The connector is keyed to prevent incorrect orientation.

Servers often use redundant power supplies, where each can support 100% of the system's power needs. This ensures continuous operation if one fails; if one goes out, the other takes on the full load. These are typically hot-swappable, allowing replacement while the system is running, maximizing uptime.

Less expensive power supplies have fixed cables, leading to clutter within the computer case. Higher-end power supplies are modular, allowing users to connect only the necessary cables, improving cable management and airflow. Hybrid power supplies also exist, offering a mix of fixed and modular connections.

Power supply wattage dictates capacity, with higher wattage models being more expensive but not improving computer speed. To determine required wattage, sum the maximum needs of all components (CPU, storage, video card, etc.). A good rule of thumb is to choose a power supply that can support 50% more than the current total load, allowing for future growth. Energy efficiency, ranging from 80% to 96%, is crucial as lost power converts to heat, impacting cooling. The 80 Plus certification (bronze, silver, gold, platinum, titanium) indicates efficiency levels, with Titanium being the most efficient, reducing heat and saving power costs.