Hi all just need some help understanding where my battery power goes. I’ve got a 5.2kw battery but have now been told that you can only use something like 4.5kw of that.Then when my house is all off the system is still drawing 80w, so over a 24 hr period that is roughly 2kw so i only really have a 2kw battery or am i looking at this wrong. 
Yes and no. I don’t know if this will help but…
When I ordered mine it wasn’t clear that the 5.2 had a non-usable reserve (the literature variously suggested between 0% and possibly 10%) and I would probably have gone for the 8 - which unconfirmed rumour suggests is really a 10 already de-rated - but too late now.
It does seem that the 5.2kWh battery is as you say 4.5kWh as it reserves around 14% to protect the cells [12% at the discharge end, and 2% at the fully charged end]. Complicated chaps batteries when trying to balance power and life time. On mine, with the battery having reached 4% and NOT delivering power, there is some self-discharge of around 1% every 7 hours. During normal solar operation there will be a mix of power for the inverter and battery system electronics depending upon the source and sink of the power.
Assuming the house is still connected to the solar system then the background usage of ‘just’ 80 W sounds reasonable. All those standby devices, DECT phones, fridge/freezer cycling, Router, etc.
If our house runs overnight from just the battery then it looks like around 80W reported for each half hour period so say 650 W for 8 hours. However, the power taken from the battery is around 1kW reported so a measure of the (in)efficiency.
I have no information on the accuracy and consistency of the measurements delivered by the system - there will be tolerances and no doubt mild discrepancies between what is reported here, and the bill received from the grid supplier. The £ charge will be definitive.
Also just to confuse the issue it depends whether the percentage figures are for the System [showing power as 4%-100%] or the Battery Management System BMS [ showing figures typically 10%-98%] All done to confuse the already confused.
Not to add to your woes, but there is around 10% or so loss on the round trip between charge and discharge using an inverter; the AC charge cycle is less efficient. And of course, the battery capacity will gradually degrade - from memory I think the prediction is to fall by no more than 20% after 10 years.
Entropy - we are all getting older.
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The_Engineer answered this already. But to add…
I had a similar ‘learning experience’ and ended up adding a 9.5kWh Battery which gives you the full 9.5kWh capacity.
I also added a Hybrid car which opened up the world of extremely cheap night time electricity (Intelligent Octopus Go). You can then set the ‘Timed Charge’ on the batteries for the full period of the night time tariff, your home will then consume power at the cheap rate leaving the battery to take over once the cheap period is over.
Saves alot of money on power and Diesel!
Win win!
Don’t forget that the battery will also supply whatever else is being used at any time, not just yr base load, (ours is around 300w, 2 freezers, a fridge, router TV box, various clocks integrated into appliances, heating hot water controller, battery controller, inverter, EV charger etc, etc.
We have a 13.5kWh battery that mostly lasts the day but does sometimes run out late evening, (like tonight after Mrs worked from home and the washing machine went on for a hot wash for wife’s work uniforms, health care, so can get a bit dirty.)