Wattage Ratings & PSU Sizing

Wattage Rating

• Power supply unit (PSU) label shows its maximum output capacity in $\text{watts (W)}$.
• Typical office desktop: $200!\text{–}!300\,\text{W}$.
• High-end / gaming systems: $500!\text{–}!900\,\text{W}$ or more.

Estimating System Power Needs

• Sum the wattage of every component:
  • CPU: $\approx17\,\text{W}$ (low-power) to $240!\text{–}!250\,\text{W}$ (high-end).
  • Motherboard: $50!\text{–}!80\,\text{W}$.
  • GPU example: $230\,\text{W}$ (e.g.
    $6700\,\text{XT}$).
  • Optical drive: $30\,\text{W}$.
  • Hard-disk drive: $9\,\text{W}$ each.
  • SSD (M.2): $9\,\text{W}$.
  • Case fan: $6\,\text{W}$ each.
• Sample gaming build: $80+250+230+9+9+(6\times6)=614\,\text{W}$.
• Select a PSU larger than the total (e.g.
  $614\,\text{W}\rightarrow750!\text{–}!850\,\text{W}$) to allow future upgrades.

Converting Amps to Watts

• Use $P=I\times V$ (amps $\times$ volts).

Efficiency & Wall-Outlet Draw

• PSUs are not $100\%$ efficient.
• Wall draw $=\dfrac{\text{PSU rated W}}{\text{Efficiency}}$.
  • $500\,\text{W}/0.70\approx714\,\text{W}$ ((70\%) efficient).
  • $500\,\text{W}/0.80=625\,\text{W}$ ((80\%) efficient).
• Lost power becomes heat; higher efficiency saves energy and cooling costs.

Exam Focus

• Identify that more components → higher PSU wattage required.
• Understand efficiency: PSU draws more power than it delivers, difference = heat.
• Higher-efficiency units (e.g.
  $80\%$) reduce operating costs; important in gaming rigs & server farms.