How to over think monitoring a kettle 🙂

So we have a shared 5L electric kettle at work and I got fed up of getting downstairs to find out someone had filled it up and it was cold. So for fun (sad I know) I set up a HA instance and used Tasmota Energy plug to monitor the power the kettle is taking. The kettle takes 800w when heating from cold and about 100w when warming up. So basically if the kettle is taking power it is cold (not hot enough) and if it stops it is hot (apart from when it goes off totally overnight). An Icon on the HA front end states whether the kettle is heating, keeping warm, hot or off.

Problem is now that when I get down stairs to a hot kettle, it has not been filled up enough to make a tea!

So I thought it might be possible to estimate the amount of water in the kettle by measuring the energy taken by the kettle each time it heats up.

e.g. It supposedly takes 0.091kWh to heat up 1L of water from 20C to 100C
in the morning when it is cold it takes about 470Wh to heat up about 5.1L of water from about 18C to 100C which is quite consistent with the 0.091kWh for 1L.

So I guess it is possible to estimate the amount of water in the kettle by measuring the energy during warm up periods.
Problem is I have no idea how to get HA to do this.

Any ideas or help is welcome, and yes I know this is a bit OTT but I consider it a learning process.

I also considered weighing the kettle but did not want to add any more hardware.

That would certainly be the most accurate method and possible the only ‘live’ method.

One problem (maybe) with measuring the power is that you’ll get ‘drift’ over time as the power needed will depend on the accuracy of your 20℃ assumption so that error could build up over time. It’s also possible that it will average itself out.

I guess, if you can use history_stats to measure how long the kettle has been on for the last time giving a power usage and therefore an estimate of how much water was in it.

However, another problem is that you will know neither how much water was subsequently used nor subsequently refilled i.e. you will only ever be able to calculate what it ‘was’ not what it ‘is’. …I think (!?).

Yep drift is something I was worried about. Theoretically at the beginning of each day the drift could be reset. I know in the morning the kettle is at approx 16C to 20C and I could measure the energy it takes to get to 100% and get a good estimate of the amount of water in it at the beginning of the day.

I should also mention that the kettle goes into a warming cycle at 92C and stops at 100C. So I could theoretically work out how much energy was used during the warmup cycle to again get a good guess at the amount of water in it when it warmed up. Maybe a Watt/hours required Per DegC (wH/degC) value could be derived from this information.

I think the biggest cause of drift would probably be when someone fills up the kettle with cold water. It then goes into a heat cycle but I have no idea how hot the water in the kettle was to start with not if the person fully filled the kettle. Even then though if I know the wH/DegC and I know the kettle stops at 100C this might also be able to be used as a ‘calibration’ point.
I guess using a derived wH/DegC also assumes that water heats up in a linear fashion given a constant amount of power being is used to heat the water.