Can a smart plug pay for itself?
Sometimes. But switching off a tiny standby load with another tiny load is not the slam dunk the box makes it sound like.
Enable JavaScript to calculate. The formula and worked example below are available without it.
Calculations run in your browser. Your entered values are not sent to me. A copied link includes your values after the # sign.
Here is the whole calculation
Power avoided by the schedule
minus power used by the smart plug
times your electricity price
minus what you paid for the plug
The calculator does the unit conversions, but that is really all it is doing. No invented average household. No assumption that every sleeping device is secretly ruining you.
A decent use case
Say something pulls 40 watts and genuinely does nothing useful for 12 hours a day. Cutting those hours avoids 175.2 kWh a year. If the plug itself uses one watt around the clock, it gives 8.76 kWh back. At $0.20/kWh, the net saving is $33.29 a year. A $15 plug pays for itself in about five and a half months.
Change that 40-watt load to two watts and the story falls apart. That is why this page exists.
Measure the boring state
The important number is not the biggest wattage printed on the device. It is what the device actually draws during the hours you plan to kill. If it already sleeps properly, there may be very little left to save. Let a suitable meter run long enough to catch idle periods, updates, charging cycles, and whatever the device does at 3 a.m.
Do not automate yourself into an outage
I would not hard-cut power to storage, computers, network gear, or anything with a compressor or heater just because the payback number looks good. First ask what the device does while idle, how it handles sudden power loss, and whether it comes back cleanly.
A smart plug can still be worth buying for convenience, presence simulation, or a reliable way to reset something awkward. Those are real benefits. They just are not electricity savings, so I keep them out of the result.
Buying one for Home Assistant?
Check the exact model, not merely the logo on the front. On/off control does not guarantee power monitoring. Local can mean different things. And a plug that forgets its state after an outage can turn a clever automation into a scavenger hunt.
My broader Home Assistant setup explains how I divide jobs between Lutron Caséta, ESPHome, Matter, and Eufy gear.
The source, if you want to check me
The watts-to-kWh conversion follows the US Energy Information Administration’s explanation. The subtraction and payback are plain arithmetic. If my math is wrong, tell me.