Homelab power calculator: what your lab really costs

Monthly cost = average watts × 730 hours ÷ 1,000 × your price per kWh. A lab that averages 200 W uses 146 kWh a month: about €44 at €0.30/kWh or $25 at $0.17/kWh. Idle draw is most of the bill, because a homelab sits idle most of the day.

The short answer

cost per month = average watts × 730 ÷ 1,000 × price per kWh

730 is the number of hours in an average month (8,760 ÷ 12). A lab averaging 200 W uses 146 kWh a month: about €44 at €0.30/kWh or $25 at $0.17/kWh. Use the price per kWh from your own bill, including network charges and taxes; regional averages hide large differences.

The calculator

One row per device type, with:

Column	Meaning
Device	Pick a preset (table below) or type a name
Qty	How many
Idle W	Watts at the wall when idle
Load W	Watts at the wall when busy
Busy h/day	Hours per day at load (0–24)

Below the rows: price per kWh with a currency picker, and an optional “add UPS losses” percentage (default 0; a line-interactive UPS typically adds a few percent).

Outputs: average watts, kWh per day / month / year, cost per month and year, heat in BTU/h, and a ranked bar of which device costs the most (share of the total). One button: Copy as template rows, which copies the list in the Devices sheet format (model and name columns) for importing into Homelab Twin.

Typical starting values

Use these only until you’ve measured. Real draw varies a lot with CPU, BIOS settings, disks and load.

Device	Idle W	Load W
Mini PC, low-power quad-core	8	25
Mini PC, 6–8 cores	12	50
Raspberry Pi / SBC	4	8
NAS, 4 bays, HDDs spinning	30	45
1U server, older dual-socket	80	160
2U server, older dual-socket	110	220
Desktop tower used as a server	35	120
AI box with one 24 GB GPU	60	400
Switch, 8-port unmanaged	4	5
Switch, 24-port managed (without PoE load)	20	25
Router / firewall appliance	12	18
Wi-Fi access point (not on PoE)	6	12

Example lab: one 2U server busy 4 h a day, plus an idle 4-bay NAS, two low-power mini PCs, a 24-port switch and a router, averages about 206 W. That’s ≈ 151 kWh, or ≈ €45 a month at €0.30/kWh, and the 2U server alone is 62 % of it.

Measure, don’t guess

• Whole lab: a plug-in meter or smart plug at the strip that feeds the rack, or your UPS’s output watts (if the UPS reports only a load percentage, multiply it by the rated watts).
• Per device: a smart plug per device for a week, or the server’s management controller (ipmitool dcmi power reading on most rack servers).
• Over time: log readings to see idle versus busy hours. That’s what turns the “Busy h/day” column from a guess into a number.

Where the money goes

1. Idle draw, around the clock. A server idling at 110 W costs about €290 a year at €0.30/kWh even if it never does anything.
2. Old enterprise servers. Dual-socket boxes with many fans and drives idle high. Replacing one with a mini PC often pays for itself in electricity within a year or two; work it out with the payback calculator in the Proxmox power guide.
3. Spinning disks. Each 3.5” drive spinning idle adds a few watts; a 12-bay array adds up.
4. The GPU you forgot. An AI box that idles at 60 W costs more per month than the four mini PCs next to it.

See it on your rack

The calculator tells you the total. Homelab Twin shows where it goes: watts and cost per device on a 3D model of your rack, the power chain from the wall through the UPS and strips to each power supply, the load on every strip, and UPS runtime. Import the same device list (the Copy as template rows button gets you halfway) and add readings to replace the estimates.

Sources

• Hours per month and kWh arithmetic: 8,760 h per year ÷ 12 = 730 h
• 1 W = 3.412 BTU/h (standard conversion)
• ipmitool dcmi power reading: DCMI power management on IPMI-capable servers

Questions

How do I calculate my homelab's electricity cost?

Add up the average watts of every device, multiply by 730 (hours in an average month), divide by 1,000 for kWh, and multiply by your price per kWh. For a device that idles most of the day, average watts = (idle watts × idle hours + load watts × load hours) ÷ 24.

Why is my measured power lower or higher than the power supply rating?

A power supply's rating is the maximum it can deliver, not what the device draws. Real draw is usually a fraction of it at idle. Measure at the wall with a smart plug, a plug-in meter or your UPS's readout.

Do PoE devices count twice?

Only if you add them twice. Access points and cameras powered over Ethernet draw their power through the switch, so a wall reading of the switch already includes them. Add them as separate rows only if you measure the switch without PoE load.

How much heat does my homelab make?

Almost all the power ends up as heat: 1 watt ≈ 3.41 BTU per hour. A 200 W lab adds about 680 BTU/h to the room, roughly a small space heater on its lowest setting.