iBoost (and other diverters) are tricky to make work well with home batteries.  

If you have a home battery, you are likely to use a smart tariff - Octopus go/agile/flux/tesla with it.  I'm with the installer - heat your DHW at a cheap tariff time with the heatpump.  Let the solar charge the battery, and just avoid that battery-iboost clash.  If you need the water hotter than the heatpump can do, get an immersion timer, use it at cheap elec time after the heatpump has done most of the job.

I agree with Rob.The battery will clash with with the solar diverter. They're great if you don't have a battery, but a battery is far more useful and offers you a lot more flexibility.

Im in a similar position to you with an Eddi. I think it may depend on the size of your solar array and battery and hot water use. If you are still exporting a lot even after your battery is fully charged, then it might make sense to shove it in the cylinder instead. This will depend of course on how much you might get for exporting your excess. If we assume a COP of 2.5 on a hot water cycle then you would need to be getting less than unit price/2.5 for your export for it to make sense. In my case I havent fitted a heat exchanger yet so will be relying on the water being heated by immersion. I have put the plumbing in ready though should I decide to get one in the future.

As an aside, we've recently installed an Immersun which has been much better than the iBoost – review coming soon. If anyone's interested, use the discount code MYHOMEFARM10% at checkout to get 10% off your next purchase from Immersun: https://www.immersun.co.uk/products

Posted by: @davesoa

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In mid April I’m having a ASHP installed. Full installation, complete change of all radiators and so on. I have, on FIT, solar panels, a Powerwall and a iBoost+ solar diverter fitted to the immersion heater. The iBoost has worked very well and has recouped about 75% of the cost of connection and, of course, works best in the summer months. My installer says the iBoost will be largely redundant with the ASHP and it would be logical to heat water with a considerably more efficient heat pump during the summer months. However they will fit it if I want within the quoted cost. 
Any views? I see iBoosts do sell on eBay (if I must use that site) so will probably be able to find a new home for it. 

On the face of it, it may seen illogical to heat your hot water using a power diverter, when a heat pump can do so more efficiently, but you also need to consider the energy source, your hot water consumption, the weather conditions and the effect upon the electricity distribution system.

Let's consider each of these aspects in turn, and then the possible effects when they are combined.

Based upon the information provided, it would appear that you have three individual sources of energy supply, direct from the grid, from the solar PV system, or from the battery storage system. It is possible that these energy sources can be used individually, or in combination with each other to provide your total energy needs.

The next factor to consider is the amount of hot water consumption, the storage capacity of the hot water cylinder, and probably to a lesser extent, the time of day when peak hot water demand is likely to occur.

Weather conditions are a further factor that may need to be considered, since a cold, but sunny Spring day, as against a warmer, but overcast Autumn day, could provide quite different results.

Most people don't consider the electricity supply to their home, until it is no longer available, but the collective use by all of those consumers can have both a beneficial, and detrimental effect upon how the electricity supply system operates.

None of the above has answered your question, so let's look at some different scenarios.

Consider a bright, but cold, Spring day, the outside temperature is 3C and your heat pump is working reasonably hard to keep your home warm. As the Sun rises, your solar PV system starts to generate electrical energy, which initially can be used to help power your heat pump to keep your home warm. If the base load of your home, and the heat pump, require 2kW to operate, then if your solar PV system is generating less than this value, you will still be importing power from the grid. When your solar PV is producing 2kW, supply and demand will be balanced, and if solar PV generation is greater than 2kW, your system will start to export power to the grid. In this situation you could consider running the heat pump to produce hot water rather than central heating, but if the heat pump requires 3kW when in DHW mode, and your solar PV is only generating 2.5kW, then it would be necessary to import 0.5kW from the grid. Alternatively, your heat pump could continue supplying the central heating, whilst the excess 0.5kW of PV generation could be used for heating hot water via your iBoost+.

Now consider the Autumn day, warm but overcast. The outside temperature is 12C and solar PV is generating 1kW. Let's assume that the heat pump requires 1kW when in CH mode, but 2kW when operating in DHW mode. Because the weather is milder, the heat pump does not need to operate continuously, and in fact only runs for 50% of the time. You could therefore have the situation where your heat pump is providing CH utilising the free solar power, but also exporting up to 1kW during the periods when the heat pump is stopped. A power diverter could therefore be used to heat your hot water when the heat pump is not actually operating. If instead, the heat pump is now switched to DHW mode, it may be powered by 1kW from the solar PV and 1kW from the grid supply.

A further benefit of having a power diverter is the fact that it attempts to minimise both import and export of electrical energy from and to the grid. Most people fully appreciate the reduction in import, but may not fully understand the benefit of reduced export. The supply of electrical energy has to be largely generated to meet demand, since it cannot yet be stored in vast quantities. There can therefore be situations where renewable energy supply, can in certain areas, exceed demand, so exporting your solar PV generation in those situations can be detrimental to efficient grid operation. Varying import and export by consumers can also lead to phase imbalance, which also can have a detrimental effect. Increased losses on the electrical distribution system lead to higher operating costs, which in turn leads to higher bills for the consumers.

You are in a better position than many, in that you also have battery storage, so may also be able to supply your heat pump and other loads from the battery to help cope with the fluctuations in solar generation. You can also help with grid losses and generation costs by load shifting, from peak periods to low demand periods.

I would therefore recommend that you keep your iBoost+ and try to use it to minimise your demand on the electricity supply system. In the height of Summer you should not have to run your heat pump just for hot water production, and at all times a power diverter can be used to help perform legionella disinfection, rather than solely by the heat pump.

I hope that the above answers your question.

Posted by: @robl

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iBoost (and other diverters) are tricky to make work well with home batteries.  

If you have a home battery, you are likely to use a smart tariff - Octopus go/agile/flux/tesla with it.  I'm with the installer - heat your DHW at a cheap tariff time with the heatpump.  Let the solar charge the battery, and just avoid that battery-iboost clash.  If you need the water hotter than the heatpump can do, get an immersion timer, use it at cheap elec time after the heatpump has done most of the job.

 

I fully agree with Rob that having both a battery storage system and a power diverter could lead to problems where the diverter drains the battery. I don't yet have battery storage, so have not looked at this problem in detail.

If my understanding is correct, a battery storage system can be configured such that it does not export back to the grid, but only supplies the home load. I would assume that a similar method could be utilised with a power diverter, in that it is not powered by the battery storage system.

Does anyone know of any 'off the shelf' devices that could be installed that would achieve this objective?

@robl

One question I have been meaning to ask for some time is how does the charging of batteries from solar PV cope with variations in solar PV output?

Some fascinating points made. My iBoost is playing nicely with the Powerwall. Today, for example, the PW algorithm charged my battery to 75% last night using Go (from 50% when I last looked). It has now charged to 100%, my tank is full of hot water from additional iBoost heating and right now I’m exporting 2.5kWh to the grid. As I’m on FIT payments I am on the deemed scheme and getting 3.95p for the export (plus a substantially higher amount for generation). 

Posted by: @davesoa

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Some fascinating points made. My iBoost is playing nicely with the Powerwall. Today, for example, the PW algorithm charged my battery to 75% last night using Go (from 50% when I last looked). It has now charged to 100%, my tank is full of hot water from additional iBoost heating and right now I’m exporting 2.5kWh to the grid. As I’m on FIT payments I am on the deemed scheme and getting 3.95p for the export. 

I think the point that Rob was making is that when there is no solar, and the water cylinder requires heating, does the power diverter take electrical energy from the battery, to heat the water in the cylinder?

Is there any form of control between the Tesla Powerwall and the iBoost?

@derek-m Sorry misunderstood. No, the power diverter does not take power from the battery at any point. It only takes power when a) the battery is at 100%, b) and/or my EV is not on charge via the Zappi using surplus solar c) there is enough PV output for surplus power to be exported. Technically I don’t know how this is achieved. There is a CT clamp connected to a sender to the iBoost on one of the tails at the main fuse board. I assume this can detect ‘surplus’ after all the other conditions are met. 

Posted by: @davesoa

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@derek-m Sorry misunderstood. No, the power diverter does not take power from the battery at any point. It only takes power when a) the battery is at 100%, b) and/or my EV is not on charge via the Zappi using surplus solar c) there is enough PV output for surplus power to be exported. Technically I don’t know how this is achieved. There is a CT clamp connected to a sender to the iBoost on one of the tails at the main fuse board. I assume this can detect ‘surplus’ after all the other conditions are met. 

It would appear that your present system may have already been configured to cope with the possible interaction between the battery storage and the power diverter, so the installation of an ASHP should have no affect other than to increase your electricity consumption.

How best to operate your system from an energy consumption, and most cost effective manner, point of view, will probably vary with the weather conditions and time of year.

I suppose that the available options would include the following-

Utilise solar PV generation as much as possible, since this reduces energy consumption from the grid and hence can be more cost effective. Obviously solar PV generation has limits, so decisions will need to be made as to which is the better option, run the heat pump, charge the EV and/or battery storage, and/or produce hot water.

How best to use ToU tariffs, particularly when solar PV generation is low. Whilst this option may be cost effective, it may actually increase overall energy consumption, since storing energy in batteries is not 100% efficient.

How to minimise and balance consumption from the grid, which may be not only beneficial to you, but also the energy supply system.