Mixed battery sizes

We have 2 x 5.2kWh and 2 x 2.6kWh batteries - they were installed during/just after Covid 19, hence the mixture of sizes which were the only ones available at the time and plus, we added to the original set up later.
We have only just realised that we only actually achieve around 11-12 kWh capacity: using Octopus Intelligent charging overnight ( although GivEnergy app stated all batteries are 100% full !).
Is there anything that can be done to improve this ?
We are contemplating adding a further battery to help with the running of a 5kW heat pump, to be installed soon.
We currently have 2 PV systems totalling 6.7 kWp - which produce around 5.0MWh per annum - should we try and add more panels ?
Any thoughts suggestions appreciated.
Many thanks

keithpiper 5.2 & 2.6 only have 80% Depth of Discharge, so 12kWh is about all you’ll get I’m afraid.

ChrisRowe thank you. if we add ,say, a 5.2kw new generation battery will that help - someone suggested it would improve the inbalance ?? and would a gen 2 suffice ?

keithpiper The other problem you will suffer from is that when one battery is full the whole chain stops charging, and when empty again the whole chain stops discharging.

GivEnergy recommend the largest battery is first in the chain which helps mitigate this but it is a limiting factor.

With 5 batteries in the chain and two tiny 2.6’s this could be a real issue.

In terms of payback you would get far more for your money by adding more solar panels if you can, even an AC coupled one, exporting as much as you can in the summer to build up a credit on your account that pays for the winter heat pump consumption.

Assume you are on Octopus Intelligent Go rather than Flux? You can add another battery (or replace the 2x 2.6’s with a 9.5) but as well as the issue I mention above you will get into the inverter throughput limit, potentially stopping you charging fully in the cheap rate.

Battery payback is rubbish, a 5.2 costing say £2,500 fitted (finger in the air price):

Let’s say you save 15p charging overnight vs using grid during the day (and that 15p includes the circa 15-20% inverter/battery round trip losses)

Per night you will save 15x5.2 =78p
6 months of winter 78x365/2=£142.35
Payback of 2500/142.35=17.562 years ☹️

Buy solar