Hi all, new to solar and inverter details here. Been reading up on inverters and PV arrays to recharge my (24V) battery bank (2 x 218Ah lithiums) during the day.
Seems the “off-grid” variety inverter is what I need. But different sources say different things. Some say when solar is not enough to support the load on the system then it switches over completely to mains. Others say the inverter will first draw on both solar and battery before switching over to mains.
Which one is correct?
There are two definitions for what “hybrid” means. In English the term means something that mixes properties of two other distinct types, but what those types are is what is in question.
The original meaning of the term was a device that mixes properties of a PV-inverter (one that ties to the grid, without batteries, and simply pushes power into the grid), and a battery inverter (one that purely powers things form the battery). When you mix these properties, you get an inverter that can mix/blend power from the battery and the grid (and sometimes also solar power).
The newer meaning, used by some of the lower end manufacturers, is a device that is both a battery inverter AND has a solar charger in the same device. So it mixes properties of a battery inverter and a solar charger/MPPT.
Of course there are also devices that combine all three.
That means the term is effectively useless for comparative purposes. You will have to find out what it means by looking at the spec sheet.
Off-grid, by the archaic definition, means there is no grid at all. Nothing. Maybe a Diesel generator. Inverters that cannot tie with the grid (they either power everything from the grid, or everything from the batteries, they cannot mix/blend) are sometimes called off-grid, and in that sense it is almost correct, but if you want to be pedantic even that is an abuse of the term.
What you said is pretty much what I expected 
The inverter I am looking at has a spec sheet on the web page. I would like to use solar / battery blending but don’t have the technical nous to assess whether this is possible.
If someone could have a peek I would be eternally grateful!
Looking at the photo of the label on the side, that device is made by Voltronic Power (in China).
It takes the PV at around 240VDC, that means the MPPT feeds directly into the high voltage DC bus from where it is switched to the AC side. That is precisely what a PV-inverter (grid tied, without batteries) does. This one then additionally also include a buck/boost stage to a 24V battery, which makes it a true Hybrid inverter.
That aside… check the certification. If it is not certified for NRS097 (the South African grid code related to embedded generation), I would not buy it. Also, it is cheap for a reason. Sorry for urinating on your battery (as we say in these parts)… and feel welcome to ignore me if your use case is different and it doesn’t matter as much.
If you’re using this somewhere off the beaten path, on a farm maybe, and the low price is worth replacing it in 3-5 years, then go ahead
Thanks for this - much appreciated 
I’ll follow up on the certification side. Don’t want to invalidate insurance.
Looking at this as a temporary solution until I can afford full hybrid solar at 48V…
If you won’t be fitting solar panels, then NRS097 doesn’t apply. If it is a temporary solution, you may already be leaning towards a backup-only setup.
Also, if the system is not connected to the grid at all, NRS097 also doesn’t apply.
Insurance may require that it complies with certain other SANS regulations though.