Immersion boost & Tesla Powerwall

[Andy]

I guess the question is do I push it with the supplier and insist on adding an EDDI booster?

You could just get an electrician to install it after. Its a simple job if you are able to run a cable to the consumer unit for the CT clamp.

[nowty]

Generally with battery inverters, if you can set a minimum export setting on the diverter (immersion boost) it should work. If you cannot set a minimum export, it may or may not work.

[Oldgreybeard]

Generally with battery inverters, if you can set a minimum export setting on the diverter (immersion boost) it should work. If you cannot set a minimum export, it may or may not work.

That works well if you want battery charge priority, but I've found that it doesn't work at all well if you want hot water priority. To get hot water priority you need to be able to set a threshold on the inverter, to stop it charging until a certain export threshold, so allowing the hot water diverter to "win" the race every time. I've not found a foolproof way to do this. I've frigged the CT settings on the inverter, that make it more likely that the Eddi will "win", but the variable time delay with both the inverter switching and the diverter switching seem to throw enough variation into the mix to cause it to fall over from time to time. I often revert to manually boosting the Eddi, just to try and get it to charge the hot water even if the battery is not at 100%.

[openspaceman]

Most days this summer (all but 3 I think) I have been able to fill the battery and heat enough water for a bath so it has hardly mattered what happened first, I manually switch the immersion.

[Oldgreybeard]

Most days this summer (all but 3 I think) I have been able to fill the battery and heat enough water for a bath so it has hardly mattered what happened first, I manually switch the immersion.

Depends how big the battery is and how big the PV system is. It typically takes around 4kWh to heat our hot water. We have a 6kWp PV system and just over 21kWH of battery capacity. The majority of the time we don't generate enough to fully charge the battery, so it's important to use that the hot water heats first.

For those with smaller batteries or larger PV systems the priority may not matter. Today was a good example. We generated 21kWh, but I forgot to manually switch the hot water to charge, so we've ended up with a battery at full capacity by around 5pm, but we only put about 1kWh into the hot water, so it will end up getting heated off the grid tonight.

[Nigel52]

Thanks everyone for replying. As our water is heated by the gas boiler which also heats the water in the underfloor central heating on three floors, I may stay with proposed system to start with and when the Tesla Powerwall finally arrives see how the system works before tweaking it. At least the PV will help subsidise all the other daily electric including multiple heating flow pumps and the battery may help nighttime hours.

[NikoV6]

We have Powerwall and focus on recharging off solar during the day. We also have an iBoost+ which we raised the threshold to 500W to stop it cutting in over the battery. 10Kw of solar here with ASHP.

I read somewhere you are better using your excess solar to drive your ASHP, much more efficient than the immersion

[Stinsy]

I read somewhere you are better using your excess solar to drive your ASHP, much more efficient than the immersion

Pros/cons

HP is more efficient than a resistive heater by a factor of its CoP. However HP is “on or off”, requires a lot of power to get going, and can be damaged if cycled too frequently. Whereas a resistive heater can be turned on and off as much as you like and power can be varied.

Basically, if you have enough power in your batteries to run the HP for a few hours it is better to use that. Otherwise the resistive element and diverter are a better choice.

[nowty]

I read somewhere you are better using your excess solar to drive your ASHP, much more efficient than the immersion

Pros/cons

HP is more efficient than a resistive heater by a factor of its CoP. However HP is “on or off”, requires a lot of power to get going, and can be damaged if cycled too frequently. Whereas a resistive heater can be turned on and of as much as you like and power can be varied.

Basically, if you have enough power in your batteries to run the HP for a few hours it is better to use that. Otherwise the resistive element and diverter are a better choice.

During summer months when I have an excess of generation I heat my water via the resistive immersions for the reasons Stinsy has mentioned. I am only just now starting to resort to my heat pump for water heating as solar generation is now dropping. Heatpump running hours are a finite resource and costly in terms of the capital expense of replacing them compared with immersions.

[NikoV6]

Thanks both! Runs off to lower threshold on iBoost!