The exact model matters. SONOFF sells an S31 Lite that does not include energy monitoring. For this use, verify that the listing says S31, not merely S31 Lite or a Zigbee variant.
Verify the energy variant before the return window closes
The official store lists S31 and S31 Lite together while assigning power monitoring and history to S31 only. Save the ordered variant and check the received label before connecting an appliance. A family-level product title can conceal the wrong selection.
After setup with a harmless, supported load, check that the stock app exposes the measurement you purchased, that the device name identifies the correct outlet and that accumulated energy covers the intended period. Record the firmware and app version if an expected function is missing. Resolve the discrepancy through normal seller or manufacturer support; do not open the enclosure to make a different variant perform the job.
For optional integrations, write down the exact supported route and any plan requirement. A community project using alternative firmware is not evidence that the shipping unit has that feature. If your essential requirement is a documented local API, choose hardware sold with that capability rather than treating a modification as part of this review’s recommendation.
This acceptance checklist is based on the official S31/S31 Lite distinction , not a hands-on test or a guarantee of any seller’s stock.
What the S31 can tell you
The S31 measures electricity used through its own outlet. That makes it useful for questions such as:
- How much energy does this dehumidifier use over a day?
- Is an older refrigerator cycling unusually often?
- What is the standby draw of a television setup?
- How much does a window fan add during hot weather?
It cannot explain the entire electric bill. Hardwired heating, central air, electric water heating, ranges, dryers, and every other circuit bypass the plug. Use the result as appliance-level evidence, not as a substitute for a panel monitor or utility meter.
Verified limits at a glance
| Specification | SONOFF S31 |
|---|---|
| Input | 120V AC, 60Hz |
| Maximum current | 15A |
| Network | 2.4 GHz Wi-Fi, 802.11 b/g/n |
| Stock app | eWeLink |
| Voice platforms named by SONOFF | Alexa and Google Home |
| Monitoring | S31 only; not S31 Lite |
The 15A label is a ceiling, not a recommendation to run every 15A load through a smart plug. Check the appliance nameplate, startup behavior, outlet condition, and both manufacturers’ instructions. Do not use a plug-in controller where unattended switching could create heat, fire, flooding, food-safety, or mechanical hazards.
Setup has two separate compatibility checks
First, the physical plug must fit without blocking something you need. Wall outlets, surge protectors, and crowded power strips vary. Keep the S31 directly visible during setup and make sure its body does not force a neighboring plug loose.
Second, the network must offer 2.4 GHz Wi-Fi. Many modern routers combine 2.4 and 5 GHz under one network name, which can work, but the S31 itself does not join a 5 GHz-only network. Guest-network isolation, enterprise authentication, and aggressive client controls can also complicate setup.
The normal consumer path uses the eWeLink app. Account, cloud, notification, and remote-control behavior should be reviewed in the current app privacy and service terms before putting the plug on a sensitive household device. SONOFF’s official store says core switching and monitoring use eWeLink, while integrations such as IFTTT can require an optional paid eWeLink plan. Verify the exact integration before treating it as part of the purchase.
Energy data is useful when the question is specific
A smart plug becomes valuable when it changes a decision. Measuring a coffee maker for an hour out of curiosity is different from tracking a dehumidifier across humid and dry days to choose a schedule.
Write down the question before collecting data. Then keep the operating conditions comparable: same appliance mode, similar room conditions, and a long enough period to capture normal cycling. One brief reading can miss defrost cycles, compressor starts, thermostatic behavior, or standby periods.
For a broader plan, the site’s guide to smart-home energy monitoring versus automation separates measurement from control. The S31 can do both at one outlet, but measuring first is often the safer route.
Remote switching is not appropriate for every appliance
The ability to turn something on from a phone is convenient only when remote operation is safe. Lamps and some fans are straightforward examples when their makers permit switched-outlet use. Portable heaters, cooking appliances, pumps, medical equipment, and motorized devices need much more caution and may explicitly prohibit this setup.
An “off” command also may not create the same isolation as unplugging a device for service. Disconnect equipment physically before cleaning, opening, or repairing it.
Schedules can reduce unnecessary runtime, but a schedule should not defeat an appliance’s internal controls. If a refrigerator, sump pump, aquarium device, or network router is essential, accidental switching can matter more than the energy data.
How it differs from a Matter plug or panel monitor
A Matter-compatible plug may be the cleaner choice for a household standardizing on a particular smart-home ecosystem. Verify whether energy data is exposed in the chosen platform, because basic on/off interoperability does not guarantee that every monitoring feature appears everywhere.
The site’s Meross smart plug review covers a different ecosystem and buyer fit. Compare current model numbers rather than treating every rectangular Wi-Fi plug as equivalent.
A panel monitor answers the broader question when the goal is to understand central HVAC, water heating, or total household demand. It costs more and may require electrical-panel installation. The S31 addresses one compatible plug-in device instead.
About hobbyist firmware discussions
Search results for the S31 often focus on opening the enclosure or installing alternative firmware. That is not the normal stock-product experience covered here. Opening a mains-voltage device can damage its enclosure, defeat safety protections, expose hazardous components, and affect warranty or certification status.
Buy the S31 only if the shipping product’s documented eWeLink functions meet the need. Do not purchase it on the assumption that a modification will be easy, safe, or supported on the hardware revision received.
Who should buy it
The S31 is a reasonable fit if all of these are true:
- one plug-in appliance is the measurement target;
- its normal current and startup behavior fit the device instructions;
- 2.4 GHz Wi-Fi is available at the outlet;
- eWeLink is acceptable;
- remote switching would be safe, or can remain unused;
- the exact listing is the energy-monitoring S31.
It can be especially useful for renters because it does not require work inside an electrical panel. The site’s guide for renters monitoring energy without new wiring adds other low-impact approaches.
Who should skip it
Choose something else for whole-home measurement, hardwired equipment, 240V loads, 5 GHz-only networks, or an ecosystem that must work without the stock app. Skip any appliance whose maker says not to use a smart plug, extension device, timer, or remotely controlled outlet.
Also skip it if the only goal is switching. A simpler plug may meet that need without paying for monitoring that will never be used.
Final recommendation
The SONOFF S31 fits a focused diagnostic job: plug in one compatible appliance, gather a representative period of energy data, and decide whether a schedule or equipment change is justified. Its limitations are equally clear: one outlet, 120V/15A labeling, 2.4 GHz Wi-Fi, and a stock experience tied to eWeLink.
Check the Amazon listing for the exact S31 energy-monitoring model before buying. If the page says S31 Lite, Zigbee, or a different current rating, stop and verify the variant rather than assuming the features carry across.