Before a hybrid inverter takes load onto the battery it works through a stack of checks in the order its manufacturer wrote into the firmware: whether the grid is present and inside the voltage and frequency windows the standard permits, what the battery's management system is reporting about state of charge and available current, whether the load sitting on the backup circuits will fit under the inverter's own continuous backup output rating, what the priority order and time schedule say it should be doing at this hour, and whether it has the thermal headroom to keep doing it. Those checks — their sequence, their thresholds, and what happens when one of them comes back no — are the product. It is why two quotes with identical panel and battery figures can describe two very different machines.
The moment I stopped treating the inverter as a box on the garage wall and started treating it as something that makes decisions was a Tuesday afternoon with cloud coming in from the south-west. I was standing in the passage outside the garage with a cup of tea going cold, watching the screen next to the DB. Pool pump running, fridges cycling, the usual background draw of a house that's home. Half an hour earlier the screen had said the array was making more than the house was using. Now it said less.
Nobody in the house noticed a thing, which is the trick. In the time it took me to walk in from the kitchen, the inverter had run a sequence of checks and quietly changed its mind. Grid voltage steady around 230 V. Frequency hovering on 50 Hz. PV input sliding down. Battery at 78 percent. House load flat.
Three separate questions were being asked at once. Is the grid still there and still worth having. Should the shortfall come from the grid or the battery. And can the battery actually give it.
That third one is the question most people miss, because it's answered by a different device. The battery has its own management system, and it talks to the inverter over a data cable. What it sends isn't a kWh figure. It's a permission slip — how many amps the battery is prepared to release right now, given its temperature, its cell balance and its state of charge. A battery sitting at 95 percent can still decline the request. The inverter doesn't argue with it. It changes mode.
The two jobs in one box
Underneath, a hybrid inverter contains a bidirectional conversion stage. The same bridge that turns DC from the panels or the battery into AC can run the other way and rectify AC back into DC to charge the battery. Which direction it runs, and how hard, is the decision. The hardware is a single set of switches being told what to do.
When the grid is up, those switches are told to follow. The inverter watches the grid's voltage and frequency and pushes current in step with them. It has no real choice — it's paralleled with a machine thousands of times its size. Follower mode is where self-consumption happens, where surplus leaves for the municipal meter if your tariff pays for it, and where the battery gets charged when the schedule says so.
Follower mode has a boundary, and in South Africa it's set by NRS 097-2-1. The standard defines voltage and frequency windows and a disconnect time for small-scale embedded generation. Step outside those windows — a dip, a fault, a dead line — and the inverter has to stop feeding the grid, quickly. This is anti-islanding, and it exists for the electrician working on a line that's supposed to be dead. Even a hybrid that's about to run your whole house on battery has to disconnect from the grid first.
So the first real check isn't "am I losing the grid". It's "is the grid gone, or merely bad". A grid that's present but out of window gets treated, for those few seconds, almost like a grid that's absent.
When the grid goes, the inverter changes job. Now it forms — it creates its own voltage and frequency reference, and the backup circuits follow it instead of following the municipality. Same silicon, different role. This is the mode that runs the house at 20:00 on a stage 4 evening.
In between those two jobs there's a set of working modes the inverter slides through without any grid event at all: self-consumption, forced charge, timed discharge, export, hold. On a decent day it will move through three of them before lunch. Those transitions never show up on a bill, but they're exactly what the priority order and the schedule are controlling.
The switch you only notice when the clock resets
Between those two worlds sits the transfer mechanism — a relay or a static switch, sometimes inside the inverter, sometimes a separate box beside it. It moves the backup circuits from grid supply to inverter supply. How fast it moves is a physical property of the part, and it's the single most underestimated line on a solar quote.
A quick one transfers in the region of a few milliseconds to a few tens of milliseconds. Your router stays up. The microwave clock holds its time. The desktop doesn't reboot and lose the document you hadn't saved yet. A slow one takes a second or more, and every time the grid drops the house goes dark for a beat before it comes back. During heavy load shedding that's four or five resets a day, every day, for months.
None of that appears in the panel kWp or the battery kWh. It might appear on the quote as a single word — "backup" — or not at all.
The ceiling nobody reads twice
Total rated output and maximum continuous backup output are two different numbers, and the second one is almost always the smaller. A machine sold as 8 kW might carry 8 kW alongside the grid but only around 5 kW continuously on battery, and only at 25 °C. Push past that and the firmware picks from a short menu: trip, throttle, or hand the overload back to the grid. In an outage the last option doesn't exist.
Which is why the essential load list does more work than the inverter's headline figure. Build it in watts, not appliances. A kettle sits around 2 kW. A geyser element is 3 to 4 kW. A microwave is around 1.2 kW. A fridge compressor pulls roughly three times its running current for half a second every time it starts, and the inverter sees that half-second. It is doing the same arithmetic you are, every second, and it never averages anything out.
The check that runs on a clock
Not every decision comes from a sensor. Some come from the schedule. Priority order — solar to load, then battery, then grid, or solar to load, then grid, then battery. A discharge floor, so the battery doesn't fall below a set percentage before a set hour. A charge window. These are choices made on time, not on measurement, and they're why identical hardware behaves completely differently in two houses.
If your neighbour's system and yours came off the same pallet with the same firmware, and hers holds the battery back until 20:00 while yours starts discharging at 16:00, nothing is broken. It's a schedule difference. Over a year, against a tariff with a peak window and a feed-in credit, that one setting can decide whether the system pays for itself or merely looks like it should.
What I listen for now
I've stopped reading the kW number on a quote first. I read the backup output, then the transfer time, then whether the priority order is even adjustable — because that tells me what the machine will decide when nobody's watching. At 20:00 on a dark evening, with the geyser off and the fridges running, it's still asking its questions. Grid present? Battery willing? Load fits? Schedule allows? Temperature fine?
Everything else on the quote is what the machine has. Those answers are what it does with it.
