Standout: About $11 with voltage and current readings
Watch out: No Apple Home or Matter
Not for: Apple Home households
The SONOFF S31 is a 15A Wi-Fi smart plug that measures real-time power, current and voltage and keeps 100 days of history in the eWeLink app. It costs about $10 to $15.
What you need before you buy
2.4GHz Wi-Fi.
The eWeLink app, plus Alexa or Google Home for voice control.
An appliance under 15A.
What you can do with it
Find out what a space heater, dehumidifier, freezer or TV setup actually costs to run, cut standby power on a schedule, and get alerts when an appliance finishes. Want the bigger picture? Our Whole-Home Energy Calculator shows how plugs fit with your other upgrades.
How I researched this: SONOFF’s S31 product page and manual, the Amazon listing, and a video review from Open Source Home Automation. Prices checked October 30, 2026.
SONOFF S31 Smart Plug
The SONOFF S31 is an $11 smart plug with real-time power, current and voltage readings. Here is who it suits and who should skip it.
Product Brand:
SONOFF
Editor's Rating: 4.2
Pros
About $11, the cheapest energy-monitoring plug we review
Shows real-time power, current and voltage
Stores 100 days of usage history
Works with Alexa and Google Home
Popular with Home Assistant users
Cons
No Apple Home or Matter support out of the box
2.4GHz Wi-Fi only
eWeLink app is basic
Bulky body can crowd the second outlet
Specs
Model
SONOFF S31 (US)
Rating
15A
Energy monitoring
Real-time power, current, voltage; 100 days of history
Connectivity
2.4GHz Wi-Fi, no hub
App
eWeLink (schedules, timers, remote control)
Works with
Alexa, Google Home
Certification
ETL, FCC, RoHS
Verdict
The S31 is the cheapest way to see what an appliance uses, with detailed readings and 100 days of history. Apple Home users should spend a little more on the Tapo P110M or Shelly Plug US Gen4.
