Do smart plugs save electricity?
Short answer: Smart plugs save electricity only when they switch off a device that really draws power while idle, or enforce a schedule you would not keep by hand. The plug itself uses power: Shelly lists its Plug US at under 1 W, so at most about $1.58 a year at 18¢/kWh. Cutting a 10 W standby load for 16 hours a day nets about $8.94 a year.
How much power does a smart plug use by itself?
A smart plug stays connected to Wi-Fi or another network all day, so it always draws a little power. Shelly’s spec sheet for the Shelly Plug US lists self-consumption as “less than 1 W.” Other brands vary, so check the spec sheet for yours.
The plug’s own draw runs all 8,760 hours of the year, whether the outlet is on or off.
| Plug’s own draw | kWh per year | Cost at 18¢/kWh |
|---|---|---|
| 0.5 W | 4.38 | $0.79 |
| 1 W | 8.76 | $1.58 |
Formula: watts × 8,760 h ÷ 1,000 × price per kWh.
That is the bar a smart plug has to clear before it saves anything.
When does a smart plug actually save money?
Use this formula:
Net savings per year = (standby watts × hours cut per day × 365 ÷ 1,000 × price) − (plug watts × 8,760 ÷ 1,000 × price)
The table below assumes the plug cuts power 16 hours a day, draws 1 W itself, and electricity costs 18¢/kWh.
| Device standby load | Saved by cutting 16 h/day | Plug’s own cost | Net savings per year |
|---|---|---|---|
| 1 W | $1.05 | $1.58 | −$0.53 (loses money) |
| 5 W | $5.26 | $1.58 | $3.68 |
| 10 W | $10.51 | $1.58 | $8.94 |
| 20 W | $21.02 | $1.58 | $19.45 |
Worked example for 10 W: 10 × 16 × 365 ÷ 1,000 = 58.4 kWh. At 18¢, that is $10.51. Subtract $1.58 for the plug, and you net $8.94.
If a plug costs $15, it would pay for itself in about 1.7 years on a 10 W load, about 4 years on a 5 W load, and never on a 1 W load.
Modern phone chargers and many small gadgets often idle well under 1 W. Measure first. Many smart plugs report power use in their app, or you can use a plug-in watt meter. See which devices waste the most standby power for typical culprits.
How do electricity rates change the result?
Here is the same 10 W load cut 16 hours a day, with a 1 W plug, at several rates.
| Rate | Standby cost avoided | Plug’s own cost | Net savings per year |
|---|---|---|---|
| 12¢/kWh | $7.01 | $1.05 | $5.96 |
| 15¢/kWh | $8.76 | $1.31 | $7.45 |
| 18¢/kWh | $10.51 | $1.58 | $8.94 |
| 25¢/kWh | $14.60 | $2.19 | $12.41 |
| 30¢/kWh | $17.52 | $2.63 | $14.89 |
The EIA expects the 2026 U.S. average residential price to be about 18.2¢/kWh. Your bill shows your actual rate.
Where smart plugs help most
- Entertainment centers. TVs, soundbars, and game consoles in instant-on modes may draw several watts while “off.” Cutting them overnight is the classic use.
- Forgotten loads. A schedule can switch off holiday lights or a fan you would otherwise leave running. Savings here come from the schedule, not from standby. See what Christmas lights cost to run.
A smart plug cannot make an appliance more efficient while it runs. It only changes when the device has power.
Smart plug safety: what not to plug in
- Check the amp rating. The Shelly Plug US is rated for 15 A at 110 V, about 1,650 W. Some plugs are rated lower. Never exceed the rating on the plug or its label.
- Never use one with a space heater. The Massachusetts Department of Fire Services says to always plug a space heater directly into a wall outlet, not a power strip or extension cord. Treat a smart plug the same way. See what a space heater costs to run.
- Do not cut power to refrigerators or freezers. Food safety depends on steady cooling. See how many watts a refrigerator uses.
- Do not cut power to medical devices, such as CPAP machines or oxygen concentrators.
- Leave routers, modems, and devices that install updates overnight on, or schedule a window when they can stay powered.
A surge protector solves a different problem. See whether a surge protector is worth it.
Quick answers
Do smart plugs use electricity when off? Yes. The plug’s radio and electronics stay powered. The Shelly Plug US lists under 1 W, at most about $1.58 a year at 18¢/kWh.
Is it worth using a smart plug on a phone charger? Often not. An idle charger may draw less than the plug itself. Measure it first.
How much can a smart plug save a year? It depends on the load. Cutting 10 W for 16 hours a day nets about $8.94 a year at 18¢/kWh. Cutting 20 W nets about $19.45.
Can I plug a space heater into a smart plug? No. Fire officials advise plugging space heaters directly into a wall outlet.
Can I put a refrigerator on a smart plug? Do not use one to switch a refrigerator off. Interrupted cooling can spoil food.