All costed ~41k with installation.
I decided to do the installation on my own and learned a few things that might help others.
If you bought a multi-battery inverter, then it's a good idea to charge up each battery separately before connecting it to the inverter. This will take about a day per battery. You might need to buy/rent/borrow a battery charger for this or if you have a vehicle, then buy a car battery charger as investment and use that here. If you don't do this, then the batteries might degrade faster over time. Make sure the charger outputs the voltage specified in the battery's manual (13.5/13.8 or 14.4 depending on the battery type).
Ensure your batteries are as close as possible to manufacturing date and/or have sequential serial numbers if you're buying new. This ensures they'll age at the same rate.
I didn't do any of this and two years later, one of the batteries has degraded significantly more than other three. Also, serial number for this battery ends in 4, the others are 6,7,8. So this was either a straggler, a forgotten piece or a customer return. I never checked at the time of installation (I was way excited and impatient to have everything up and running):
https://techenclave.com/threads/wtb-sinewave-ups-inverter-so-need-suggestions.198174/#post-2269678
It's the last voltage reading in the list, it drops to under 11v (basically dead) after three minutes of load. All four batteries are 28 months old, so this should normally not have happened.
Lastly, test the voltage drop across the wires connecting the batteries together under load, it should be well under 50mV (mine are 20mV). If it's higher, then the wire isn't thick enough. Also, all of the series connecting wires should be the same length and size.
