Plugs · Head to head

Sonoff S31 WiFi Smart Plug vs Sonoff WiFi Smart Plug S40

Full Smart Plugs comparison in 2026

Updated November 2026
Data from the official Sonoff site.
2026 verdict
The Sonoff WiFi Smart Plug S40 is the better all-round pick of these two energy-monitoring plugs.
Sonoff WiFi Smart Plug S40
Winner
Sonoff WiFi Smart Plug S40
$10.90
Buy on Amazon USA
It adds overheat and overload protection and Bluetooth for pairing, and it is the newer model, while keeping the same 1800W and 15A rating, energy monitoring and Google Home, Alexa, SmartThings and Home Assistant support. The Sonoff S31 WiFi Smart Plug is the tinkerer's favourite: owners describe years of uninterrupted use, it stores 100 days of power, current and voltage data with a running cost, and it is widely flashed with ESPHome or Tasmota for fully local control. Choose the Sonoff WiFi Smart Plug S40 for a safer everyday plug, and the Sonoff S31 WiFi Smart Plug if you want long-term reliability reports and the option to run custom firmware.
Works with
Connectivity
  • Wi-Fi 2.4 GHz
vs
Works with
Connectivity
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FeatureSonoff S31 WiFi Smart PlugSonoff WiFi Smart Plug S40Winner
Rating4.14.2Sonoff WiFi Smart Plug S40
Connectivity2.4 GHz WiFi2.4 GHz WiFi, BluetoothSonoff WiFi Smart Plug S40
Weather ResistanceNONOTie
Plug FormatType BType BTie
Maximum Load Power1800W1800WTie
Energy MonitoringYESYESTie
IntegrationsGoogle Home, Alexa, Apple HomeKit, Samsung SmartThings, Home Assistant, MatterGoogle Home, Alexa, Apple HomeKit, Samsung SmartThings, Home Assistant, MatterTie
On/Off ButtonYESYESTie
Zigbee RouterNONOTie
Local ControlYESYESTie

What the differences mean

Overheat and overload protection

The Sonoff WiFi Smart Plug S40 lists overheat and overload protection, which is the safety feature to want on a space heater or any heavy load. The Sonoff S31 WiFi Smart Plug does not document it, so a buyer running a high-wattage appliance should treat the S40 as the safer choice.

Bluetooth pairing

The Sonoff WiFi Smart Plug S40 uses Bluetooth to pair before joining Wi-Fi, which shortens setup. The Sonoff S31 WiFi Smart Plug has no Bluetooth radio, and owners report that 2.4 GHz pairing through the app is where first-time setup often stumbles.

Energy history

The Sonoff S31 WiFi Smart Plug stores 100 days of power, current and voltage data with a total cost, which suits tracking an appliance over a season. The Sonoff WiFi Smart Plug S40 shows real-time readings plus daily, weekly and monthly history, though owners note the graphs come only at a monthly resolution with no minimum, maximum or average.

Firmware and local control

The Sonoff S31 WiFi Smart Plug is the model the DIY community flashes with ESPHome or Tasmota for local control with no cloud in the path. The Sonoff WiFi Smart Plug S40 has upgradeable firmware and configurable power-on behavior, but it is not the one buyers reach for when they want to run custom firmware.

Age, size and warranty

The Sonoff WiFi Smart Plug S40 is the newer plug and carries a one-year warranty, with a compact 76 x 40 x 32 mm body. The Sonoff S31 WiFi Smart Plug dates from 2019 and has no warranty period documented in this catalog, and neither model is weather resistant, so both belong indoors.

Which one to buy if

What owners say

Owners of both Sonoff plugs buy them for the same reason, real energy monitoring at a low price, and both groups use it to watch standby draw, appliances and charging. Sonoff S31 WiFi Smart Plug owners describe the opposite experience from a short-lived gadget: many run a dozen units for years without a failure, praise the 100 days of power, current and voltage history and the option to flash ESPHome or Tasmota for local control, and their main complaint is 2.4 GHz pairing friction with band-steering routers. Sonoff WiFi Smart Plug S40 owners like the real-time readings, the overload protection and the upgradeable firmware, but a notable share report units dropping off Wi-Fi or failing after two to three months, alongside readings that read zero at very low draws and app history limited to a monthly view.

Frequently Asked Questions

Which is better overall: Sonoff S31 WiFi Smart Plug or Sonoff WiFi Smart Plug S40?

The Sonoff WiFi Smart Plug S40 is the better all-round plug because it adds overheat and overload protection and Bluetooth pairing on top of the same 1800W rating, energy monitoring and ecosystem support. The Sonoff S31 WiFi Smart Plug is the better choice for tinkerers, with a longer documented energy history and firmware that owners flash for local control.

What is the biggest difference between the Sonoff S31 WiFi Smart Plug and the Sonoff WiFi Smart Plug S40?

Safety and setup. The Sonoff WiFi Smart Plug S40 lists overheat and overload protection and pairs over Bluetooth, while the Sonoff S31 WiFi Smart Plug does not document overload protection and has no Bluetooth, though it stores far more energy history.

Do the Sonoff S31 WiFi Smart Plug and the Sonoff WiFi Smart Plug S40 monitor energy?

Yes, both monitor energy. The Sonoff S31 WiFi Smart Plug tracks real-time power, current and voltage and keeps 100 days of history with a total cost, while the Sonoff WiFi Smart Plug S40 shows real-time plus daily, weekly and monthly history that owners describe as monthly-resolution only.

Which plug works with Home Assistant and can run custom firmware?

Both work with Home Assistant. The Sonoff S31 WiFi Smart Plug is the one the DIY community flashes with ESPHome or Tasmota for local control, while the Sonoff WiFi Smart Plug S40 uses its own upgradeable firmware. Neither reaches Apple Home or Matter without a SONOFF hub.

Who should buy the Sonoff S31 WiFi Smart Plug?

Tinkerers and Home Assistant users who want to run ESPHome or Tasmota, owners who value a long energy history and reports of years of reliable service, and buyers who do not mind a fiddlier Wi-Fi pairing step.

Who should buy the Sonoff WiFi Smart Plug S40?

Buyers who want a safer plug for heavier appliances thanks to overheat and overload protection, an easier Bluetooth pairing, and a newer model with a one-year warranty for everyday energy monitoring.

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