Running costs / the watts that never clock out

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.

I measure the boring hours, not the label

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

I measure at the boundary I might actually change. 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.

“Nothing is happening” can still be the job

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.

Three changes, in the order I consider them

1. Remove hours that truly serve no purpose. This is the cheapest change, but only for equipment that shuts down and returns cleanly.
2. Remove duplicate jobs. Consolidating two lightly used machines can save more than replacing one of them with a slightly better version.
3. Replace hardware only after measuring payback. A lower wattage is not enough; the purchase has to recover its complete cost within a useful period.

A 10-watt reduction saves $17.52 per year at the example rate. Spending $100 to get it produces a simple electricity-only payback of about 5.71 years. If the replacement was needed anyway, the decision is different. If the existing equipment works, that is a long time to wait for a small efficiency claim to become money.

The upgrade payback calculator compares purchase cost with the measured wattage difference and daily operating hours.

Where a smart plug helps—and where it lies

A smart plug can remove a genuine standby load on a reliable schedule. It also consumes power itself. If the load already drops to one or two watts, the plug can erase much of the savings it was bought to create.

I also avoid hard-cutting power to storage, computers, network equipment, or anything with a compressor or heater without checking how it handles the interruption. A plug can be useful for control or a deliberate reset even when it never pays for itself through electricity. That is a convenience purchase, not an energy-saving result.

The smart-plug payback calculator subtracts the plug’s own draw before showing a saving.

What this number can and cannot decide

The annual cost answers one question: what the measured electricity draw costs at the rate entered. It does not put a price on reliability, local control, noise, maintenance, privacy, or the time spent rebuilding a working setup.

That is why I keep the number visible but do not let it make the whole decision. A few dollars a year may be cheap for a bridge that keeps the lights understandable. A forgotten 80-watt computer doing work that moved elsewhere is a different case.

How I calculate the cost

The watts-to-kWh conversion follows the US Energy Information Administration’s explanation of electricity measurement. Every dollar figure on this page uses the stated $0.20/kWh example rate. Substitute the variable per-kWh rate from your own bill.

Disclosure: I use the smart-home systems named here, but this page does not claim a measured total for them and contains no affiliate links. If I add a commissioned meter or hardware link later, I will label it beside the link.