Build a power budget and size a UPS for the lab

short · 40 min · Objective 1.1

Task

Work out how much power your lab equipment draws, convert it into the VA rating a UPS is sold by, and size a UPS with headroom and a target runtime. Then check the one thing that decides whether redundant power supplies are actually redundant: whether they are fed from different sources.

Steps

  1. List each device in lab/power/budget.csv with header device,source,watts,power_factor, where source says whether the figure came from a label, a specification sheet or a meter. Use a power factor of 0.9 where none is published.
  2. Add a row for each device's display or other peripherals that would share the same UPS.
  3. Compute the total watts and the total VA (watts divided by power factor) and write them to lab/power/totals.txt as watts: <n> and va: <n>.
  4. Size a UPS so the load is no more than 80 per cent of its rating, and choose a runtime target long enough for a clean shutdown. Record ups_va: <n>, ups_watts: <n> and runtime_minutes: <n> in lab/power/ups.txt, with the model you would buy.
  5. For each device with two power supplies, or for the pair of switches you would buy for a real rack, write in lab/power/feeds.csv with header device,psu,feed which circuit or PDU each supply would use.

Verify

These checks run in a POSIX shell: Terminal on macOS or Linux, and on Windows Git Bash (it comes with Git for Windows) or WSL. A stock Windows PowerShell or Command Prompt has no awk or grep, so there the first line fails.

awk -F, 'NR>1 && $3>0 {w+=$3; va+=$3/$4} END {printf "%d W, %.0f VA%s", w, va, ORS}' lab/power/budget.csv
grep -E '^(watts|va): [0-9]+' lab/power/totals.txt
awk -F': ' '/^ups_va/ {u=$2} END {print u" VA UPS"}' lab/power/ups.txt
awk -F, 'NR>1 {f[$1]=f[$1]" "$3; n[$1]++} END {for (d in f) print d":"f[d]}' lab/power/feeds.csv

Compare the first line with your totals file: they must agree. The UPS rating must be at least 1.25 times the VA total, which is the 80 per cent rule. In the last output every device with two supplies must list two different feeds. The same feed twice means a single PDU or circuit failure takes both supplies out, and the redundancy is only on paper.

Notes

Nameplate figures are maximums. A plug-in meter usually shows far less, which is why real data centres size from measured load plus a margin. A UPS's runtime at a given load is in its specification sheet as a curve, not a single number.

This is an independent study companion for CompTIA Server+ SK0-005 and is not produced by or endorsed by CompTIA.