The battery and the inverter in a hybrid system have to be sized as one two-way power channel, not as two separate purchases. The inverter carries a specified maximum battery charge power and a separate maximum battery discharge power, and the battery carries its own C-rate ceiling in each direction. Whichever of those two numbers is smaller, in that direction, is the real limit on your system. It is very rarely the number printed largest on the quote.
I was standing in the garage at twenty past six on a July morning, phone in hand, watching the charge power climb by a hundred watts at a time. The battery was sitting on 34 percent. Load shedding for my block was scheduled for eight o'clock. The sun was still behind the neighbour's roof and the array was producing close to nothing, so the charging figure on the app was small enough to be slightly insulting. By quarter to eight the battery had crawled to 51 percent, the power dropped, and I spent the next two hours doing the arithmetic I should have done before I signed anything.
Nobody had lied to me. I had simply read the wrong line.
The number everyone quotes is the least useful one
A hybrid inverter's headline rating is its AC output. That is the figure that gets said out loud in the kitchen: a 5 kW inverter, an 8 kW inverter. It tells you how much the inverter can push into your house at once. It tells you almost nothing about how it behaves with a battery.
On the datasheet there are two other lines, usually further down, sometimes in a table with a different unit. Maximum battery charge power. Maximum battery discharge power. They are frequently different from each other, and both are frequently lower than the AC output. Someone designed the charging stage to a certain size and the discharge stage to another, and neither was sized to match the marketing number on the box.
So the first thing I do now, before I look at panel counts or roof angles, is find those two lines and write them down. Then I find the battery's continuous charge current and continuous discharge current, and write those down next to them.
What the C-rate is really doing to your number
C-rate is just how fast a battery can move energy relative to how much it holds. One C means a full battery's worth of energy per hour. Half C means half of that.
Here is the part that catches people. C-rate is a rate, so the power ceiling it produces depends on capacity. A 10 kWh battery at 0.5C will give you about 5 kW of continuous power. A 5 kWh battery at 1C gives you the same 5 kW. And a 10 kWh battery with a manufacturer-specified continuous discharge of, say, 0.3C gives you 3 kW no matter what inverter you bolted to it.
Illustrative numbers, not a claim about any particular product: you could have an inverter rated at 8 kW of AC output sitting on a battery whose continuous discharge works out to roughly 2.4 kW. During load shedding, that system holds 2.4 kW. The kettle and the microwave together will find that out for you.
The inverter is not going to save you. It will simply stop delivering when the battery says stop. Most of them do it quietly, which is why people blame the battery, or blame the installer, when the truth is that the two components were never matched.
Charge and discharge are two different roads
The other thing I got wrong was assuming that if a system could deliver a lot, it could also absorb a lot. It cannot. Those are separate paths through the inverter with separate limits.
Plenty of hybrids are generous one way and stingy the other. It is common to see an inverter that can discharge a battery hard but charges it at a fraction of that, because the charging side has to share its budget with everything else and because the designers assumed the grid would do the heavy lifting when it is available.
Which brings me to the path that actually matters in this country.
Your array size is not your charging speed
Solar charging goes through its own channel: PV in, DC-DC conversion, battery. That channel is capped, and the cap is often well below both the AC output rating and the maximum discharge power. You can cover the whole north-facing roof and still watch the battery fill at the same leisurely pace, because the bottleneck is not the panels.
There is a second filter on top of that. During the day, the house eats first. The geyser, the fridge, the pool pump, whatever is running, all draw from the array before anything reaches the battery. So the hours you think you have for charging are not the hours you actually get.
If you can charge from the grid overnight, this matters less, though on a prepaid meter it is a cost decision as much as a technical one. If you cannot, or you have deliberately turned grid charging off, then you are living entirely on the solar window, and that window is short and unfriendly.
Highveld winter gives you a clear sky and a narrow band of useful generation. Cape Town winter gives you rain for three days and a battery that never quite gets there. Over a run of bad weather, total battery capacity stops being the interesting number. The length of the useful generation window, and how much of it survives the house's own daytime load, is the interesting number.
This is also why "how many hours of backup will I get" is not a question anyone can answer honestly from a spec sheet. It depends on the load, on how much sun the day gave you, and on how far down you are willing to run the cells.
Two lines on the datasheet that decide what you can even buy
Inverters specify a supported battery voltage range, and they specify a communication protocol for talking to the battery's management system. That pair of facts restricts what you can pair up in practice, quite apart from the power numbers.
Get the voltage window right and the protocol wrong, and you are in the awkward middle ground: the system may run, but the state of charge is guessed rather than known, and everything downstream of that guess gets softer. Your app percentages drift. Your cut-offs are approximate. The inverter stops being able to manage the battery it is charging and starts merely feeding it.
I am not going to tell you which combinations work. That is between your installer, the manufacturer's supported list and whoever is signing your paperwork off. What I will say is that it belongs on your comparison sheet right next to the power figures, because a power match on paper is worth very little if the two devices cannot hold a conversation.
What I check now, in order
The inverter's maximum battery charge power. The inverter's maximum battery discharge power. The battery's continuous charge current. The battery's continuous discharge current. The PV-to-battery path limit. The supported voltage range and protocol.
Then, and only then, the battery's total capacity in kWh.
That last one is the number that sells systems, and it is the one that tells you the least about how a system will behave on a Tuesday in July. Big capacity with a slow charging path is a battery that spends winter permanently half full. Big capacity with a weak discharge path is a battery that cannot run the house at the moment you need it most.
The system is a two-way channel, and a channel is only as wide as its narrowest point. In each direction, one of the two components is the narrow point. Find out which one, in each direction, before you sign.
