O yes they can…
I use this very successfully: https://sonoff.tech/product/diy-smart-switch/powr3/
Give them a try - you will be a changed man 
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I think Richard is referring to the Sonoff Basic, the most popular and common one (I have several of them myself). It has a 10A capacity, so it’s only really food for 2000W or so. The Sonoff POW supposedly has a 15A relay, and should be good for 3kW, but again, in practice people have discovered that it’s a bit of a marketing number. But as you point out, you simply need to buy up into the higher spec ranges…
I use a contactor with a Sonoff Basic to switch my one geyser. But in that particular case I am not measuring the energy so it doesn’t matter.
I think it’s food for anything > 2000W 
Not to pick a fight with you IT manne but do you trust WiFi?
In my industrial automation life we would never trust a flaky wireless signal for control…
Are you building and industrial home or residential home?
You do get wired network MCUs, but then the investment is much higher and you need to wire everything up as well.
Wifi works pretty good if you make sure you have good coverage where you need it. If you are trying to strech it to the very last signal bar you are going to be disapointed.
Well, if you need to op for LAN, then go for the shelly Pro series. They can do WIFI, BT and LAN. I got the Pro 4PM
There are some pretty nifty security in and around Wifi. At least, that’s what I’m told with Ubiquity Wifi routers and firewalls and goeters.
UTP is secure, I prefer that, but in industrial vs home automation, bah, most people don’t even change their router password.
And if you have done that, also not foolproof, as my Microtik was hacked twice, and there was no Wifi involved.
IT security is super next level today.
on the older and basic sonoffs the connectors are probably the biggest concern with bigger loads.
apologies to those sensitive to gratuitous use of non-finger based thermal measurement…
this is an older sonoff TH16 rated for 15A. Irony is I was deliberately trying to have some strands missing form the connectors on the right - the left most connector (Live out) I was doing “normal”. Load was 3kW (kettle and electric heater) and total time from temp in the mid twenties to +80°C was about 3 minutes.
Full disclosure, the wire was only 1mm2 but that exact same connector, same load, etc. did not reach such high temperatures earlier - the high temp was after re-doing the connections to get the board in the housing/cover. The 79°C is the relay btw. The blade/push connectors seem very hit/miss.
Think on the newer sonoffs the connectors are improved though.
What exactly happens if a light doesn’t turn off or on at a particular time.
For a geyser, what happens if it doesn’t turn off, it’s meant to be permanently on anyway.
When automating something or making it “smart”, you don’t remove any existing protection but rather add additional protection where necessary.
Although I can add my alarm to homeassistant and Alexa, I have chosen not to. Yes it’s convenient, but I need some inconvenience at times.
I was on the no-Wifi bandwagon for a long time, even going as far as to prototype a CAN-bus network of sensors and relays (multiple controllers broadcasting is wonderful), but it’s just too damn hard.
RS485 doesn’t work as a bus unless you use Modbus for everything, but then you need to poll, so it’s slow, especially when mixing device types. Not geared for whole-home.
Ethernet-based MCU things are much more flakey than ESP chips (although some are actually based on ESPs, which is nice). And in some homes wiring things up is easy, but in others it is insane. Living in one of those now.
Z-wave isn’t a thing outside of the US. Zigbee works pretty well and doesn’t know what “internet” even means, but there’s some learning there about repeaters vs routers, opening all you switches to press the (re-)join button and network healing which seems like a whole new stick to hit yourself with.
So Wifi it is: if you invest in your Access Point infrastructure, it works well. And good equipment makes it easy to do multiple SSIDs to split IoT from everything else. VLANning is another new thing to learn, but it gets easier with Mikrotik and especially Unifi. Mesh networks (Deco / others) also work well for coverage, but security is more tricky if they can’t do VLANs. Even ethernet isn’t more secure than Wifi if you have Wifi for anything else, unless you do VLANs, so they’re on par there. (Edit: POE security cameras literally put a wired connection outside your house, so you actually already need VLANs if you’re using them.)
Wifi is typically cheap & easy to get started and forgiving of changes and upgrades. I’ve got the first Withings Wifi scale which is no longer configurable, but still works fine – only issue is that my IoT network is now named after a house I haven’t lived in for 10+ years. I just keep using the same network name on new equipment and it keeps connecting. Even this isn’t really an issue, since IoT needs to be on 2.4GHz while computers & phones like 5GHz better anyways.
So you can continue fighting it, or get stuff done. I choose the latter.
Aside: For mission-critical stuff the Shelly Pro @Gman mentioned seems perfect, but for me it’s a stretch to say that my switch(es) will always remain working, but the Wifi AP plugged into a POE port will not. What is super interesting is that those can be programmed to work offline, if you can get the right signals to its input, but that’s even more wiring…
Now local vs cloud-based control is a different story altogether, but both Shelly and Sonoff have good stories there and even Tuya has something.
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I don’t mean to be too snarky, but everyone I’ve ever spoken to who were worried about Wifi because of stability / security / whatever concerns had no alternative. They didn’t use Control4 / ethernet / anything else, they just had nothing. (Including myself.)
However, owner of my previous company did rip out a whole-home Control4 system after months of technicians tweaking it, replacing everything with Philips Hue (Zigbee) & Shellys, just skipping the things were an immediate replacement wasn’t available.
So it very much feels like “get stuff done” choice.
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The least dependable part of my HomeAssistant installation was not the Wi-Fi connected switches. They work perfectly. The least dependable part was the Raspberry Pi running HA. After replacing that with something else, I’ve never had a problem again.
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The thermostat is better left in place, or like I’ve seen, a geyser becomes a rather big “kettle” spewing boiling water out of the release valve.
The thermostat goes, the geyser is permanetly off, far as I know. Stand to be corrected on that though.
GW, with all the stuff, still has a thermostat in place. 95deg cutoff if memory serves, as last resort protection.
I had a thermostat fail in the ON position. But in the 17 years of being a home owner, that happened exactly once. Once too many though, and we picked up on it rather quickly…
Let to a very humourous exchange between my wife and someone else at Church. “I’m sorry, got to go, my husband needs to fix the geyser thermostat”… “What? Your husband can do that?”… I love it when you can make the wife proud.
Or when they say … My husband says.
Priceless.
I agree. I meant the sonoff is just switching power to the thermostat which otherwise has power permanently if there’s no sonoff.
When I created my own contraption using the sonoff th16, I still used a geyserwise thermostat but replaced the geyserwise sensor with a ds18b20 but the thermostat has the mechanical cutoff. My sonoff turns off at 80 degrees and I’ve never experienced any issues this far. My geyser is outside though so even if it needs to vent, I don’t have any ceilings that will be damaged.
Honestly, I’m running a Ubiquity setup with 3 APs and a LOT of IOT Wifi devices, and I still get Wifi problems.
Pain is I still get devices which decides to connect to the furthest possible frikken AP and then “go offline” often.
I am planning, for example, to move my VenusGX for example to hardwired ethernet. Yes, lights = not so bad. But remember, Wifi doesn’t just break when you try and switch on something - but also when you want to switch off something. I’ve had a Sonoff (driving a contactor, driving a geyser) switch on perfectly but failing to switch off because it dropped from the wifi at that point. I eventually added Tasmota rules to automatically switch it off after 15mins locally (not dependent on Wifi).
Edit: yes the geyser’s thermostat, set to 65, remained. But still, I’d prefer not to kill batteries SOC driving the geyser all the way to that point.
Could you elaborate on how you modified the GW thermostat to work with the sonoff?
You can go onto unifi portal and force it to connect to a certain AP.
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