Energy measurement
How much does an always-on device cost to run?
At $0.20 per kWh, every watt left on for a full year costs about $1.75. That makes a 10-watt device roughly $17.52 a year and a 50-watt device about $87.60.
My house has a Caséta bridge, a Eufy HomeBase 3, and a Home Assistant host that are supposed to be available when nobody is touching them. Calling all of that draw “idle waste” misses the point. Those boxes receive switch events, store camera recordings, run automations, and keep local control available.
I still want to know what they cost. I just do the arithmetic before deciding that a schedule, smart plug, or replacement computer is an improvement.
Calculate a device with your watts and electricity rate
The one conversion I remember
A one-watt load running continuously uses 8.76 kWh in a 365-day year:
1 watt × 24 hours × 365 days ÷ 1,000 = 8.76 kWh/year
Multiply 8.76 by the variable price per kWh from the electric bill. At $0.20/kWh, one continuous watt costs $1.752 per year. The table below scales that same calculation; it is not a claim about the normal consumption of any named product.
| Average draw | Energy per year | Cost per year |
|---|---|---|
| 5 W | 43.8 kWh | $8.76 |
| 10 W | 87.6 kWh | $17.52 |
| 25 W | 219 kWh | $43.80 |
| 50 W | 438 kWh | $87.60 |
| 100 W | 876 kWh | $175.20 |
Measure a representative period
A 300-watt power supply does not mean a computer draws 300 watts all day. The number I need is average wall power across the period I am considering. For a plug-in device, that usually means leaving a suitable electricity meter connected long enough to catch sleep, backups, updates, disk activity, charging, and the quiet hours in between.
If a meter reports 0.36 kWh across 24 hours, the average draw was 15 watts:
0.36 kWh × 1,000 ÷ 24 hours = 15 W average
Include everything that would change with the proposed shutdown or replacement. A mini PC with an external drive is not a 10-watt system if the drive enclosure pulls another 18 watts. A network stack is not only the router if the modem, switch, access points, and controller all have to remain powered with it.
Check what depends on the device
The Caséta bridge spends most of its life looking uneventful. That is what I want. It is waiting for a physical control or an automation. HomeBase is supposed to be ready for security events. Home Assistant is supposed to notice that the vacuum needs help or that a light changed state.
I do not put infrastructure like that on a schedule merely because midnight looks idle on a dashboard. Before cutting hours, I write down what disappears with the power:
- Do wall controls or automations stop responding?
- Does recording, monitoring, or backup stop?
- Does the device return to the correct state after power is restored?
- Does another always-on box now have to replace the missing job?
The answer may still be “turn it off.” It just needs to be a system decision, not a reaction to the word idle.
Keep a measurement record
Record the meter’s starting and ending kWh readings, elapsed hours, connected equipment, and anything unusual during the run. Note a missed backup, a long update, or an unusually busy day. If the period leaves out a weekly job, measure longer rather than treating one quiet day as typical.
Compare before and after over similar workloads. Use a meter rated for the connected load and check its stated accuracy at low power; a tiny displayed difference is not necessarily a real saving. The figures here are arithmetic examples, not measurements of my bridge, HomeBase, or Home Assistant host.
Choose the next calculation
For a shutdown schedule, include the controller’s own consumption in the smart-plug calculator. Before cutting power, check the equipment’s shutdown and restart requirements; a timer is not a substitute for a clean shutdown.
For replacement hardware, compare the measured difference with the full purchase cost in the payback calculator. A 10-watt reduction saves $17.52 per year at this page’s example rate, before any purchase cost.
Use your electricity rate
The unit conversion follows the US Energy Information Administration’s electricity measurement explanation. Use the charges that vary with consumption, not the entire bill divided by kWh if that includes fixed charges. With time-of-use rates, calculate each period separately. These examples assume a flat $0.20/kWh and 365 days.