Electric hot water (Solar divert or overnight)

[Bam Bam]

I thought I was being clever getting a Tesla (UK) T-Smart immersion device to get hot water on a timer before we got our solar installed in 2023.

Timer vs diversion

The T-Smart was a lot cheaper than getting an eddi (or the more basic alternatives like the solar iboost). Around £150 vs. £300 or £400.

It does not detect the solar input, but I didn't think was a deal-breaker.

Essentially, we have a battery which acts as a buffer anyway. We wouldn't be taking from the grid during the day because the battery would cover the difference and the battery would get topped up later on.

Best laid plans, though, because installing the device was problematic, causing a leak, and we had to get a plumber in so that was extra cost.

Failure
Unfortunately, the Tesla device failed in August. We had a power cut and it seems that blew up the device.

Inevitably this was right before we switched to the cheap overnight tariff for the first time.

Alternatives
Now I see that there are very cheap Wi-Fi devices out there

So my options are:
1. Get a new T-Smart even though I've lost faith in the device after the previous one blew. Tesla will send us the replacement bits (the valuable stuff) for £82.
2. Get a basic immersion heater (£14 [update that one doesn't fit so it would be £30 apparently according to Tesla]) and a WiFi-enabled wall switch from Amazon (£23).

The better option would be to just get a heat pump which would mean a new water cylinder and much more efficient electric water heating. However, Pebbles doesn't want to invest too much in the house as she wants to move.

Does anyone have any experience with these (no doubt Chinese made) switches? to be honest the British made product has let me down.

 

[Everest]

What does your existing device do exactly? I have immersion diverter that runs off excess solar to heat all our hot water for about 9 months on the year for free.

Guess the best solution will depend on the amount of PV you have, size of battery and, more importantly, output power of the inverter. You don't want to be drawing from grid at peak times if you are running the washing machine and kettle, for example at the same time a dumb timer is trying to heat your water.

 

[Bam Bam]

What does your existing device do exactly? I have immersion diverter that runs off excess solar to heat all our hot water for about 9 months on the year for free.

Guess the best solution will depend on the amount of PV you have, size of battery and, more importantly, output power of the inverter. You don't want to be drawing from grid at peak times if you are running the washing machine and kettle, for example at the same time a dumb timer is trying to heat your water.

Cheers for the reply.

It has an app so you have a lot of control.

You can request a boost for an hour.

But mainly it is to set off/on schedules. I was running four schedules during the day to top up the hot water.

Load times
I was running it at times there would not be other loads (before and after lunchtime). Good point about the washing machine. I've got the bread machine on now as well, though I don't think that peaks too high.

The issue doesn't apply in the middle of the night as it will be using the grid not the battery then.

System details
Our inverter has a 5kw output, though the battery on its own can only provide about 4kw. Battery plus solar gets it up to 5kw.

In fact we might get extra generation in the summer if there is some battery capacity available (as with Rolfe's similar/identical hybrid system) so using a little battery before midday is a good idea.

Features?
I don't feel we are missing out from not having the ramping up and down functionality on this (unlike the car power of course which is twice as much so it would need to be limited when running from solar).

 

[Everest]

Food for thought! How do you control your car charging from solar & do you have any IT skills or use something like Home Assistant? IME the inverter power limit will be the main issue. So if you can get data regarding solar production, house-usage and grid import-export you could dynamically turn on or off the heater.

We looked into getting a 2kW immersion heater to replace the 3kW one when we had a lower-power inverter, but at the end of the day they were very expensive and non-trivial to fit (I'm more comfortable with electrons and software than water and gaskets!) - so built a system to control the diverted power instead.

 

[Bam Bam]

Food for thought! How do you control your car charging from solar & do you have any IT skills or use something like Home Assistant? IME the inverter power limit will be the main issue. So if you can get data regarding solar production, house-usage and grid import-export you could dynamically turn on or off the heater.

We looked into getting a 2kW immersion heater to replace the 3kW one when we had a lower-power inverter, but at the end of the day they were very expensive and non-trivial to fit (I'm more comfortable with electrons and software than water and gaskets!) - so built a system to control the diverted power instead.

For the car we have the Givenergy EVSE, which is the same brand/app as our solar inverter and battery. It can ramp up and down.

The Givenergy App is great - loads of data and very user friendly. Also a web portal where you can dig further into the data.

I'm not too up on the IT side and haven't investigated home assistant or anything else.

 

[Everest]

That sounds promising then If there is any integration between Givenergy and HA, then I think it would be quite straightforward (*) to have HA send a signal to a relay + contactor when there is spare energy, or spare SOC in your home batteries to divert some electrons to the immersion heater. Then add a temp sensor on your water tank and Bob's your Uncle

(*) I say "I think it would be quite straightforward" from what I have read and seen about HA when used by non-programmers - I haven't used it myself. I have programming skills, so tend to avoid doing things 'the easy way' and build something much more complex and custom myself

 

[gazza487]

I have one of the Amazon immersion heater switches that are WiFi. It’s been working daily for 15months was around £20, works well only thing is the back box supplied is pants. You need a deeper one. It’s in a cupboard with painful access moving stuff so having remote ability is great (for clock changes/switching as it pulls the time when it connects to the net).

It made sense for me (before and after solar) using the EV rate of 7.9p for hot water. Using solar for this doesn’t. SEG at 15p vs buying at night for near half that.