How it is calculated
Multiply the average power in watts by the 8760 hours in a year and divide by 1000 to get kWh; then multiply by the price per kWh on your bill. What matters is real average power, not the rating of the power supply: a PC with a 500 W supply may draw only 40 W at idle.
| Device | Average power | kWh per year | Cost per year (€0.20/kWh) |
|---|---|---|---|
| Raspberry Pi style board | 5 W | 43.8 | €8.76 |
| Low-power mini PC | 10 W | 87.6 | €17.52 |
| 2-bay NAS | 20 W | 175.2 | €35.04 |
| Reused old PC | 60 W | 525.6 | €105.12 |
| Tower server | 120 W | 1,051.2 | €210.24 |
Calculated examples; rounded values.
Which device pays off
The gap between a mini PC and an old PC can exceed €80 a year, so a mini PC pays for itself in two or three years. If you only need file sharing and backups, a low-power NAS is the most efficient; for many services or virtual machines, a current mini PC with an efficient processor offers a good balance.
Measure real power use
The most reliable way is a plug-in power meter: put it between the device and the wall and let it measure for at least a full day, because power changes between idle and busy periods. Spec sheet figures are usually maximums.
Also measure accessories that stay on with the server: external drives, switch, UPS or router. Each adds a few watts that add up over a year.
How to cut the bill
Set hard drives to spin down after a period of inactivity, disable what you do not need in the BIOS and use the most efficient power profile. If the server is only needed during the day, schedule it to shut down at night and start in the morning.
Replacing hard drives with SSDs lowers power use and noise, and grouping services on one machine with containers or virtual machines is usually more efficient than running several small devices.
What about a VPS?
A basic VPS costs €4 to €10 a month, similar to the electricity an old PC uses, and includes a fast connection, a fixed IP and hardware you do not have to maintain. In exchange your data lives elsewhere and large storage is expensive. To keep many TB at home a NAS is still cheaper; to publish a website or service, a VPS often makes sense.
Example: small board versus old PC
Say you want a server for home automation, phone backups and file sharing. An old PC averaging 60 W costs about €105 a year; a Raspberry Pi or a 5 to 10 W mini PC, between €9 and €18. With a gap of more than €85 a year, the new device pays for itself quickly, makes less noise and heats the room less.
Before buying, check that the small device has enough memory and connections for what you want to run.
Idle versus load power
A home server spends most of the day waiting, so idle power weighs far more on the bill than peak power. Look at that figure when choosing processor and motherboard, and disable dedicated graphics cards, lights and peripherals you do not need.
Remember the other costs
Electricity is not everything: drives are replaced every few years, a UPS needs new batteries and the time you spend maintaining the system counts too. Include those figures when comparing with a paid service.
Frequently asked questions
How much does a Raspberry Pi cost to run per year?
Between 3 and 7 W on average depending on model and accessories: at €0.20 per kWh, between €5 and €12 a year.
Does a NAS use a lot of power?
A two-bay NAS with drives spinning draws about 15-25 W; with drives asleep, under 10 W.
Which price per kWh should I use?
The average price on your bill, including network charges and taxes. The electricity consumption calculator lets you enter your own.
Does switching the server off at night save much?
If it is off 8 hours a day you save a third of its consumption: about €35 a year for a 60 W machine. It is worth it if nobody uses it at night and no scheduled jobs run then.