The inverter size comes out of the essential-loads list, not out of the inverter catalogue. That list, the set of circuits a household refuses to lose while the grid is down, sets the inverter's continuous rating, because only what is on the list draws anything at all during an outage. The battery then converts that rating into hours. Anything left off contributes nothing, which is why two houses can buy the same 5 kW hybrid and have completely different experiences of it. Build the list first and the number produces itself. Buy the inverter first and the list quietly turns into a defence of the purchase.
I do my first pass with a torch, at night, during an actual outage. Not because I enjoy it. It is the only hour when the list tells the truth.
You walk the passage and count what is still alive. The alarm keypad is glowing, because a small sealed battery inside the panel is doing exactly what it was installed for. The gate motor's receiver is awake on its own standby battery, waiting for a remote nobody will press until morning. The fibre box and the router sit dark on the shelf. The phones are stacked on the counter next to a power bank. None of that walk feels like a load. That is precisely why it gets missed.
The next pass happens in daylight, with a plug-in energy meter. You put it between the wall socket and the fridge plug and leave it there for a full day, not ten minutes. Ten minutes tells you almost nothing about a fridge. The compressor runs, shuts off, sits warm and quiet, then starts again, and the honest number is the average over a cycle rather than whatever the label claims. The average is small. The moment it starts is not. That start is where undersized inverters go to die.
Treat the wattages in this piece as typical shapes rather than specifications. Measure your own. What matters is the shape of the arithmetic, not the exact figure.
Simultaneous demand, then duration
Two different questions get answered by the list. The first is how much has to run at the same instant, and that sets the inverter rating. The second is how long it has to keep running, and that sets the battery. People collapse the two and then wonder why a big inverter with a modest battery only carries the house for an hour.
The instantaneous question is the awkward one, because a house is not a sum. The fridge, the router, the gate motor and the alarm panel can all sit comfortably under a kilowatt together. Add one resistive load and the picture changes completely. A kettle, a geyser element, an oven, a toaster or an iron will each, on their own, ask for more than that entire group. That is not a reason to do anything in particular. It is simply why the composition of the list moves the number so violently. Whether a kettle's circuit sits on the list is a household decision, and it is the single decision that swings the inverter rating most.
Standby loads are the other half of the surprise. A router idling is a handful of watts. The fibre box is a few more. A decoder on standby, an alarm panel topping up its battery, a gate motor's transformer, chargers left in sockets, the microwave's clock, the inverter's own electronics, all of it adds up to a background hum that never stops. The inverter's self-consumption deserves its own line on the page, because it is the one load that runs for every second of the outage and cannot be switched off. A small inverter with a fan and a display can burn through a meaningful slice of a small battery overnight while powering almost nothing.
Why a fridge is not a lamp
An incandescent lamp is a resistor. You switch it on, it draws what it says, and it behaves. A compressor-driven appliance is a motor, and a motor at the instant of starting pulls several times its running current for a fraction of a second. A fridge, a chest freezer, a pool pump and a borehole pump all share that habit. An inverter with a healthy surge allowance rides it. An inverter already carrying a heavy load when the compressor kicks in may not, and the symptom is usually a shutdown with no explanation and a house that comes back on five minutes later by itself.
Electronically controlled appliances add a second wrinkle. Modern fridges with variable-speed compressors start softly and are easy on an inverter. Older ones are not. Some power supplies for small electronics behave differently on a stepped or modified waveform, and the tell-tales are tiny: a clock that drifts, a charger that runs warm, a motor that sounds slightly wrong. None of it is dramatic. All of it is worth writing down.
The practical consequence is staging. If the gate motor runs at the same moment the fridge compressor starts and the kettle goes on, an inverter that looked generous on paper is suddenly at its limit. The list is not just a total. It is a timeline.
What the bill is actually for
The municipal bill is the right tool for a different job. It tells you how much energy the house uses over a month, which is what you need when you size the solar array and estimate self-consumption. It tells you nothing useful about an outage, because an outage is a power question and the bill is an energy question. I have watched people size backup from a monthly total, then act surprised when the inverter trips on a cold evening.
The essential-loads list has a second use that rarely gets mentioned. It is the only fair way to compare two quotes that carry the same inverter model number. Supplier A backs up six circuits; Supplier B backs up two. Same inverter, same price bracket, same paperwork, and completely different products. When you write the list down and hand it to both, the conversation changes from what the unit can do to what the house will actually get. Anything that cannot be compared cannot be priced.
Where the list goes wrong
A list built from memory is usually short by one thing, and it is normally the gate motor or the alarm. A list built at noon on a sunny day will always miss the evening peak, when the geyser has run, the oven is on and the freezer is cycling hard. A list that ignores the inverter's own draw overstates the battery. A list that forgets to mark which items overlap in time overstates the inverter.
I keep mine in a notebook, in three columns: circuit, running watts, and whether it can be delayed. The delayed column is the honest one. It is where I admit that not everything needs to start at the same instant, and it is usually where the sizing problem dissolves without buying a bigger machine.
There is a tradeoff worth naming, because it cuts the other way too. A larger inverter is not automatically safer. It costs more, it idles heavier, and it needs a battery big enough to make the rating meaningful. An oversized inverter on a small battery is an expensive clock: it tells you the time until the lights go out. A right-sized inverter with a well-made list runs quieter, lasts longer and leaves money for the battery, which is the part that actually carries the house.
The list is also not permanent. A freezer arrives, a work-from-home desk appears, a baby monitor goes in, someone starts charging an e-bike in the garage. Each of those quietly rewrites the specification. A battery-ready inverter buys you time on that, but only if the original list was honest to begin with.
So the walk through the dark passage is not sentiment. It is measurement. When the power eventually comes back and the fridge compressor thumps into life, you hear exactly how large the thing you refuse to lose really is, and you have the number you needed before you ever opened a catalogue.
