Your option was likely much less expensive, but aeotec does make a clamp-on energy sensor[0]. It's made for reading your whole-home energy but it should work for your purposes. But like I said, your solution was likely much cheaper (these are about $99).
And then I was back at square 1 for the washer, which turns out uses little enough energy in a lot of cases to be hard to tell when it's "done" vs when it's just pausing between cycles or swapping water out.
I also wanted to have the notifications continue until you actually empty it, and that required a door sensor anyway!
I really want to get one of those for the oven though, my wife tends to worry that she left the oven on (she never has!) and something like this connected to it might help ease that worry. And I REALLY don't want to do anything invasive to the oven, as that's one thing that i REALLY do not want accidentally turning on or shorting out.
But until I get around to replacing it, I also have normal IP cameras out front. So i combined the API from Ring with the IP cameras that let you access the video stream to take a snapshot on motion or a doorbell ring, and use HTML5 notifications to send it to all my devices.
They are battery powered, but i've had the oldest ones for almost 2 years now and haven't had to replace the batteries yet, and when I do I can get a pack of like 10 for $15 so it's not bad.
All of the light switches are zwave as well, and they go in the wall behind the toggle switches, so from the outside you can't see any difference.
I get most of them from amazon, and i've used a bunch of vendors over the years. Aeotec, ecolink, linear, and a few no-name brands.
It's not sold any more, and it's not zwave plus, but I have like 12 of them and they are by far the best i've used. They are small, run consistently 100% of the time, have good range, and more.
I have 2 of the aeotec nano switches which are zwave plus, but they are also $60 so it's almost double the money...
I've also used fibaro but ended up removing it because there was a delay between when i flipped the toggle and when it would turn the light on, and that was not okay in my book.
I am looking to replace the light switches with zwave-plus modules simply for the extra security, but i'm not in a massive rush.
In the sensors world, i use ecolink's door window sensors. They are cheap and work well enough, although I did have 2 out of about 20 that were bad and needed to be replaced shortly after installing them.
I am also trying out zooz 4-in-1 sensors for motion right now. They work pretty damn well with a lot of control over sensitivity and range, so I'm happy.
For the garage doors I use linear/gocontrol's GD00Z-4 modules, and my door locks are kwikset with a zwave plugin board, and there are a few plugs around the house from various companies that all have worked perfectly so I don't really have anything bad about them (ge, aeotec, r-tech)
You could hook into the buzzer circuit for this notification.
So where possible, I shy away from any modifications to the device itself that aren't extremely simple to undo.
A vibration sensor and hall sensor on an esp8266 is cheap, works consistently, and is VERY easy to take off if I don't want it any more.
I did hook into a humidifier once kind of like this, I hooked the esp8266 up to the on-off button, but I try to avoid it if I can.
* zwave motion sensors that can turn on the stairs lights at night when I start to walk down
* door/window sensors that will automatically turn the AC off if they are open for more than a minute.
* several methods of presence detection for anyone in the house, and ensuring the doors are locked, lights out, and garage doors down if nobody is home (after an announcement on the google homes just in case anyone somehow is still home)
* automatic notifications to my wife's phone when i'm on my way home after work so she would know when to start dinner (we don't use this one any more as I work from home, but it beat getting phone calls and me guessing how long I was from home every day!)
* notifications that show an image of who is at the front door when there is motion there
* an alert if a door in the house is left open for more than 1 minute
* a vibration sensor combined with a tiny hall sensor on the door of the washer/dryer to notify me when laundry is done (trying to measure power usage on 220v wiring was looking to be more annoyance than it was worth, so i went a different route!)
And one of my favorites that i'm working on right now:
* a system to measure rainfall amount and only turn on the reclaimed water sprinklers when it's not enough, but adjust the runtime accordingly.