The most widespread confusion is reading the power supply's wattage as the machine's consumption. It is not. An 850 W unit states what it can deliver, not what the system asks for. A PC with that supply spends most of its life drawing a fraction of the figure.
The three variables that decide the bill
- Real draw under your usage. Browsing and gaming are separated by a large factor. The weighted average across your hours matters more than the peak.
- The hours. A machine on for eight hours a day is nothing like one on for two, and the difference is linear and unforgiving.
- Your rate. The variable with the widest spread between countries, and the one that makes generic advice worthless. The same machine costs wildly different amounts depending on where it is plugged in.
Where efficiency genuinely matters
On a moderately used machine, moving up an efficiency tier rarely pays for itself in electricity alone. On one that works long hours — rendering, home servers, local inference — the arithmetic changes, because the waste is paid for every day and heats the room on top.
Heat is the part people forget: every watt that does not reach the components ends up in the air, and in summer you pay for it twice.
Work the number out with your rate and your hours rather than a rule of thumb.
