In combining both battery storage and water heating from surplus PV output I was keen to ensure that the two solutions worked together well. I consider that there are potentially two issues with such an arrangement:

1. Firstly, the system may go unstable. With two independent systems each trying to absorb the same PV output, they may overreact both claiming the excess and putting the house into import, then both back-out in response to the input restoring the system to export, and then repeat the cycle endlessly etc. In control system terms you might consider that it had too much gain.
2. Secondly, even if stable, the resulting split of diversion into two systems (battery and immersion heater) may not be optimal. In my case a kWh of stored electricity is worth more than four times a kWh of gas saved – so I’d clearly want to prioritise battery charging.

I considered various relatively complex solutions and then thought of one of great simplicity which required no additional parts.

My solution is simply to install the current clamp for the high priority device around both the incoming power cable and the cable to the lower priority device. The nature of a current clamp is that the current it reports is the sum of all the cables that it encircles. Thus, by placing the battery clamp around both the incoming supply and immersion heater feed, it doesn’t matter how much of the potential export has been diverted to the immerSUN, the battery clamp still sees what would have been exported were the immerSUN not active and gradually takes that power. The immerSUN in turn sees the reducing export / increasing import on the meter tail and backs off to restore the balance.

My installer had previously combined immerSUN and battery without such an arrangement and advised that the slow response of the battery compared to the immerSUN remained stable, but as the immerSUN responded most quickly it was prioritised.

However the above graph from the immerSUN shows my battery successfully tracking the PV output, so my arrangement is working successfully.

The only downside, which for me isn’t an issue, is that the immersion heater becomes invisible to the battery. Even if the immersion heater is operated via an immerSUN boost when no PV output is available, the incoming mains current is entirely matched by current in the immersion heater feeder in the opposite direction, and the battery’s current clamp sees nothing. Thus the battery cannot support the immersion heater because it doesn’t know that the immersion heater is drawing any current. I however don’t see this an issue since we use gas to heat the water when the ImmerSUN’s diversion is insufficient. Even if we did use electricity the battery is not able to deliver sufficient power to run the immersion heater at 3kW.

In my installation the battery current clamp is actually inside the consumer unit to minimise the additional wiring to allow the clamp to encircle both the incoming mains and the immersion heater feed.

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