Now the temps have dropped below 8C, the HP is working harder and the battery is not big enough to support the house load.

I calculated adding a second 5kW battery would support the HP at 1.6kW max load Nov-Mch.

That comment is welcome, but can be read in two different ways.

Since others will be reading here in future, I'd like to add some clarity.

If you need more power into the home from your battery storage system, then you add another inverter.
That allows more watts to be delivered.

If you want more stored energy, then you add another battery.
That allows you to supply the home for a longer period of time. 

You might need to do both, because these two concepts interact.

Adding a second inverter will drain the battery faster, potentially leaving you without enough stored energy to last through the night.

Now the temps have dropped below 8C, the HP is working harder and the battery is not big enough to support the house load.

I calculated adding a second 5kW battery would support the HP at 1.6kW max load Nov-Mch. The payback for this extra battery would be twice that of the first battery as it only pays back Nov-Mch, approximately 18 years assuming static electricity charges, 14 years if you assume 2.5% increase per year.

If I can get a second hand battery at half the cost of new and avoid fitting charges by installing it myself, I could justify a second battery. 

Personally, each extra 16kWh costing £1000, when adding battery capacity to the inverter, I am looking to minimise space near the inverter.  

On ROI, by exporting mostly at peak times, return from future PV production can also be improved.  

Now the temps have dropped below 8C, the HP is working harder and the battery is not big enough to support the house load.

I calculated adding a second 5kW battery would support the HP at 1.6kW max load Nov-Mch. The payback for this extra battery would be twice that of the first battery as it only pays back Nov-Mch, approximately 18 years assuming static electricity charges, 14 years if you assume 2.5% increase per year.

If I can get a second hand battery at half the cost of new and avoid fitting charges by installing it myself, I could justify a second battery. 

That gut-feel is crucial in the learning curve to making decisions which are 'safe'.

Hats off to the ones that persist, @Transparent ! 😀

Not saying never again, tempting to think learnt enough and the next battery DIY assembly would be quick..😉

But those pleasant moments packing the cells into the box will not be easily decoupled from the weeks leading up to it... 😃

...

I'm hopeful that the newer Sodium Ion chemistry launched by BYD for their small affordable domestic car market may soon find it's way into stationery battery storage.

...

16 sodium atoms walked into a bar, followed closely by a masked man in a cape.

...

Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Batman!

Sorry; couldn't resist. Taxi's already on its way.

But to add a battery made of sodium cells we would probably need a separate inverter to one already supplying lithium.

I doubt that anyone would want to run batteries made of two different chemistries in parallel,
but if you restricted the voltage range of the inverter to that of the LiFePO4 cells, then it's theoretically possible.
You'd just end up with much lower overall capacity.

There's nothing magic about an inverter claiming to be "compatible" with a particular battery chemistry.
Provided it could be configured to deliver current and voltage in the required range, then what's the problem?

Solar/hybrid inverters do not require a CAN-bus or RS485 connection to a BMS in order to work!

At present two of my batteries are operating without a BMS present at all...
... let alone a data cable!

These may seem strange concepts to you at the moment, but don't let them concern you.

Once you've wrestled with the knowledge-gap to build your first home-battery, your confidence will soar.

The concepts behind running inverter, BMS and battery combinations are just as important as the 'rules'.
You gradually get a feel for the subject matter.

That gut-feel is crucial in the learning curve to making decisions which are 'safe'.

You need to accumulate an idea of what's good practice, and what might be risky.

For example, you'll notice that I've placed my LiFePO4 cells sideways on the support shelving.
No one told me to do that, and I've never seen that orientation mandated in any instructions.

I made that decision because it's less likely that I would accidentally create a short-circuit by dropping a metal tool onto terminals.

I want a metal spanner to fall away from those high-current busbars, not weld itself across them!  😲 

Notice also that I've added in-line fuses to the balance-leads.

They should never be needed.

But... if there ever was a BMS connected which went faulty, how much current would it draw through those 17 balance wires?
When they melted and broke (which they would of course), then might that result in electric arcs?

This level of detail is far beyond what most DIY battery builders would do.
But there's no harm in adding safety features which might never be required.

In general they look very similar. But to add a battery made of sodium cells we would probably need a separate inverter to one already supplying lithium. And I can see the recent Solis inverters do not have that configuration ready made.

On the plus side, perhaps the current transport caffafle with lithium will be avoided .

My personal thoughts...

CfD auction pricing is going up for wind in the next auction

Grid upgrades already baked in and new ones will go ahead to reduce risk 

New reactor strike prices won't be cheap 

The new cap and floor rates for LDES won't be cheap, we will get a better idea from the 77 initial shortlisted projects next year. 

The £7500 boiler upgrade scheme is unviable unless we are happy to see large increses in bills. It is hard to scale the upfront costs financially.... 

Cost of capital is going up for governments and private sector investment in the UK.... Lots of costs linked to inflation and cost of capital... 

Etc. 

CfD, plus LDES, plus short duration storage, plus grid upgrades, plus balancing and curtailment costs, plus extra monitoring etc etc.. It is all going to add up to more than the average price of electricity from a gas turbine.

We are likely to see wholesale gas prices fall, that may put downward pressure on electricity prices. 

The one thing renewables will do is stop the eye watering spikes from major events like the Ukraine war.

A large amount of generation (used and curtailed) and grid storage is going to be locked in at high prices for years. 

Local energy pricing is dead for many years, it won't come back on the table before the next election at the earliest. 

The polical landscape is unstable pushing up costs, and lots may change leading up to the next election. 

I would be very surprised to see lower  average prices, a few homeowners will be able to save if they can invest/change behaviour.