TrueNAS is free and the old PC was free, but the meter runs every hour of every day. This page does the electricity maths honestly - how to measure your build's real appetite, what a typical old desktop costs per year next to a small efficient box, and the two settings people reach for that help less than they hope.
A NAS is the only computer in the house that never sleeps, so its idle power draw - not its peak - is its personality. The good news: you do not have to guess. One cheap plug-in power meter and ten minutes give you a number, and the rest of this page is one multiplication.
Example figures only - your tariff and your meter reading decide your real bill.
Borrow or buy a plug-in power meter, plug the finished NAS through it, let the system settle after boot, and read the watts while it does nothing. That is the number that runs 24 hours a day. An old desktop commonly idles somewhere between 35 and 80 watts; a modest modern small PC can idle near 10 to 15 watts. Your reading, not a review site's, is the one that bills you.
Watts × 24 hours × 365 days ÷ 1000 = units (kilowatt-hours) per year. Multiply by your electricity rate and you have the NAS's yearly rent. Run it for your own machine before judging anyone else's: plenty of 'free' builds are cheap to run, and a few are quietly expensive. The worked example above uses an illustrative rate - your bill's rate is the real one.
Letting hard drives sleep when idle can cut a few watts, and on a home NAS that is idle most of the day it feels obvious. The honest counterweight: drives are worn by starting and stopping, and a folder that takes ten seconds to wake annoys everyone. A reasonable middle path used by many home builds is a long idle timeout - or none at all, accepting the small cost for instant, gentle-on-drives service. Decide knowingly; there is no universally right answer.
A scheduled shutdown saves the most power of all, and breaks the most promises: backups that run at night, a family member in another time zone, the photo upload that happens when a phone charges at 2 a.m. If your NAS is purely a daytime office box, a schedule is legitimate. If it is the house's always-there archive, its whole value is being awake - pay the measured bill and skip the guilt.
Every step shown live on screen - installation, pools, shares and the settings that matter.
▶️ Watch Full VideoIt depends entirely on its idle watts, which is why measuring beats worrying. At 15 watts idle the yearly cost is small change; at 70 watts idle it becomes a real line in the budget. Your meter reading answers this in ten minutes.
Usually the PC platform itself - processor, board, power supply losses - dominates. Each spinning drive adds only a few watts. That is why an efficient small PC with two drives beats a big old tower with the same drives, every hour of the year.
Where power cuts are common, yes - not for the electricity, but for the shutdown. A small UPS gives TrueNAS the minutes it needs to close files cleanly instead of losing power mid-write. It protects data, not the bill.