A three-phase house does not need a three-phase inverter. It needs an inverter matched to the circuits you want to keep alive when the grid drops, and whether one unit can carry the whole house or only part of it is decided by your phase configuration and how your distribution board is arranged, not by a list of appliances.
I worked that out the slow way, standing in a passage with the board door open.
The torch was mine. The clipboard was his. He ran a finger along the row of breakers, stopped at the main switch, and counted three poles where I had expected one. Three-phase supply, he said. So before we talk about inverters, we talk about which circuits you actually want on backup.
For the two weeks before that, I had been doing what most people do. I listed appliances. Fridge, freezer, lights, TV, Wi-Fi, the garage door, the borehole pump, and the geyser if I was feeling brave. I added up watts and multiplied by hours and arrived at a number that felt like a decision. It wasn't. That number told me how much power I wanted. It told me nothing about which circuits those appliances sit on, and in a three-phase house that is the only thing worth knowing first.
Your appliance list measures watts. The board measures phases.
Open a three-phase board with me and look at what is actually there. The single-phase circuits in the house are spread across three phases. The kitchen plugs might sit on one. The lights and bedrooms on another. The geyser and the outbuilding on the third. Nothing about that split is visible from your appliance list, and you cannot infer it from your municipal bill either. The only place it exists in any reliable form is inside the board, and someone qualified has to open it and read it properly.
That is why the same house, with the same appliances, can end up with two completely different solar designs depending on nothing more than how the electrician who wired it years ago distributed the circuits across the phases.
What a single-phase inverter does on a three-phase supply
Put a single-phase inverter on a three-phase supply, wired in the ordinary way, and it will energise only the circuits on the phase it is connected to. Lights might come back on. The garage might not. The geyser almost certainly won't. It looks like a fault. It isn't. It's topology, and it is entirely predictable if you look at the board first.
The alternative is to have the contractor rearrange the board so that the circuits you care about end up on the phase the inverter serves. That is possible, but understand what you have bought at that point. You have not bought a bigger inverter. You have bought electrical work inside the board, done by a qualified person, signed off with a Certificate of Compliance, and limited by how loaded each phase already is.
The two shapes of the decision, and what each one costs you
There are really two shapes to this. One is a single three-phase inverter serving all three phases. The other is separate inverters on individual phases, each covering its own slice of the house.
They differ in cost and in coverage, and the difference is not subtle. A three-phase inverter keeps three-phase loads running. If you have a three-phase borehole pump, a three-phase aircon, or three-phase machinery in a workshop, that is the whole argument, and it is a strong one.
The separate-inverter route is cheaper per unit and lets you build up in stages, but you end up with more equipment on the wall, more than one point of failure, and coverage that stops at the edge of each phase. You also need somewhere to put it all, which in a suburban garage is a real constraint rather than a theoretical one.
Here is the part that gets lost in all of this. Most three-phase houses in South Africa have nothing in them that actually runs on three phases. The supply is three-phase. The house is not. Everything is single-phase, spread across the three legs, and it has been like that since the day it was built. If that is your house, you are paying for a three-phase inverter to preserve a capability you never use.
The size trap on the three-phase side
Three-phase hybrid inverters are sold in a narrower range of power ratings than single-phase models. That sounds like a small detail. It changes the quote.
On the single-phase side the range is wide and the steps between sizes are fine, so it is possible to land close to what your essential loads actually need. On the three-phase side, the smallest unit available may already be larger than your essentials require. You do not get to buy the small one, because it doesn't exist in that category. So the design either grows to justify the hardware, or you pay for headroom you will never draw.
That is not an argument against three-phase inverters. It is an argument for knowing what you are buying before the number arrives on a quote.
Unbalanced phases are what actually limit you
Three-phase supplies in houses are almost never balanced. The geyser sits alone on one phase and pulls hard. The kitchen sits on another. The lights and plugs spread across the rest. This matters because when you decide what goes on backup, you are not moving a list of appliances. You are moving circuits that belong to specific phases, with specific existing loads already sitting on them.
If the phase you want to back up is already carrying a lot, the inverter on that phase has to have the headroom for everything you add. On a three-phase inverter that headroom is shared, but the load on one leg is still load, and it still counts. That is the real constraint, and it is invisible from the outside. It is also why two neighbours with identical-looking houses can be told different things and both be right.
The registration side changes too
The phase configuration does not stop at the inverter. Connection capacity and metering arrangements differ between single-phase and three-phase supplies, and that difference flows straight into the SSEG application. The questions are not the same and the answers are not interchangeable. If you plan to register the system, the supply type is part of the submission, not a footnote to it.
I would rather know that at the start than halfway through.
Find out before you sign
Most owners only discover their phase configuration when a contractor opens the board. By then the quote is written, and if the answer goes the wrong way, the quote gets rewritten. Sometimes the whole conversation restarts.
It costs nothing to find out earlier. A qualified person can tell you from the board. On some properties the main switch and the meter make it obvious at a glance. On others it takes a proper look. Either way, it is a short conversation before the quoting starts, not a discovery after the deposit.
The board is the project. The inverter is a box.
This is what I wish someone had said to me at the beginning. Any work inside the distribution board has to be done by a qualified person and covered by a Certificate of Compliance, and that is true whether you are adding one circuit or changing how the phases are loaded. It changes the scope of the job. It does not change the inverter.
So when someone asks whether a three-phase house needs a three-phase inverter, the question is framed wrong. The house does not need anything. You need a decision about which circuits stay alive, and that decision has to be made while looking at the board, with the phases in front of you, before anyone starts talking about models and kilowatts.
Get that part right and the inverter choice almost makes itself. Get it wrong and you will spend the next five years explaining to your family why the kitchen works during load shedding and the bedrooms don't.
