How to Size Your Solar Inverter Correctly

kW versus kWh: the distinction that fixes half the confusion

Before sizing anything, separate two numbers that people constantly conflate.

Kilowatts (kW) measure power — the rate at which you draw energy at a given instant. This is your inverter's rating. A 5kW inverter can deliver up to 5kW of power at once. Kilowatt-hours (kWh) measure energy — power over time — and that is your battery's job. A 5kWh battery holds five kilowatt-hours of stored energy.

An analogy helps. Think of the inverter as the width of a pipe and the battery as the size of the tank. The pipe determines how much can flow at once; the tank determines how long the flow can last. You need both correctly sized, but they answer different questions. This guide is about the pipe.

Step one: list what you run at once

The heart of inverter sizing is honest arithmetic about simultaneous demand. Not everything you own, but what realistically runs together during the times you care about — especially load-shedding.

Start with the low-draw essentials that hum along constantly: LED lights, Wi-Fi and networking, a TV, phone and laptop charging, and the fridge and freezer. Added up, these are modest and rarely trouble even a small inverter. The picture changes the moment you introduce high-draw appliances, which is where the real sizing happens.

The power-hungry culprits

A handful of appliances dominate any home's peak demand. An electric kettle is a notorious one — small, innocent-looking, and a heavy draw for the minute or two it runs. A geyser element, an electric stove or oven, a microwave, a tumble dryer, an iron, a pool pump and larger power tools all pull hard. The question is never whether you own these, but how many might run at the same instant.

If you can stagger them — boil the kettle, then microwave, rather than both together — a smaller inverter copes. If your household routinely runs several heavy items at once, you need more headroom. This behavioural reality is why two homes with identical appliance lists can need different inverters.

Step two: add sensible headroom

Once you have a realistic peak, you do not size the inverter to sit exactly at that number. You leave headroom for two reasons.

First, surge. Many appliances — anything with a motor or compressor, like a fridge, pump or power tool — briefly draw far more than their running power at the moment they switch on. A good inverter tolerates short surges above its continuous rating, but your continuous sizing should still leave comfortable margin so normal start-ups do not nudge you toward a trip.

Second, life changes. You might add an appliance, a home office, or more battery later. A little headroom now is cheaper than replacing an inverter that turned out to be exactly, and therefore inadequately, sized. The balance to strike is real, though: too much headroom is money spent on capacity you never use. This is precisely the judgement a professional assessment is good at.

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Step three: match the inverter to the battery and panels

An inverter is never sized in isolation. It has to be balanced against the battery that feeds it and the panels that recharge that battery.

A large inverter married to a tiny battery can deliver plenty of power but only for a very short time — a wide pipe on a thimble of a tank. A large battery behind an undersized inverter stores lots of energy you cannot draw quickly enough to run your appliances. And too few panels behind a big battery means it never fully recharges between load-shedding slots. The three components form a system, and the art is in balancing them for how you actually live. Getting that balance right is the single biggest reason to have a proper site assessment rather than guessing from an online calculator.

What the packages tell you about sizing

Alumo's tiered packages are a useful illustration of sizing in practice, because each pairs an inverter rating to a matched battery and panel count for a particular kind of home.

The entry Bronze "Watt-a-Deal" uses a Sunsynk 3.6kW inverter with a 3.6kWh battery and four panels, from around R1,911/month. That 3.6kW rating suits a smaller home or flat that mainly wants to keep lights, connectivity, electronics and a fridge going through an outage, staggering any heavy appliance use.

Step up to the Gold "The Power Bae" with a Sunsynk 5kW inverter, a 5.32kWh battery and six panels, from about R2,423/month, and you gain the headroom to run more simultaneously — a genuinely more comfortable fit for a busy family home that does not want to think carefully every time it switches something on. The jump from 3.6kW to 5kW is exactly the kind of sizing decision this guide is about: not "more is always better," but "match the pipe to how much you really draw at once."

For larger homes or businesses, the ratings climb further — Alumo's business systems run to 80kW and 125kW inverters — but the principle is identical at every scale: size the inverter to realistic simultaneous demand, leave sensible headroom, and balance it against battery and panels.

Getting it right without guesswork

You can absolutely reason your way to a rough inverter size using the steps above, and doing so helps you understand your own home. But the final figure deserves a professional eye, because a good installer does more than add up appliance labels. They consider surge behaviour, your daily and load-shedding patterns, future plans, and how the whole system balances — then recommend a size that neither trips nor wastes your money. That is the value of a site assessment, and it is why the smartest first step is simply to get one.

Frequently asked questions

What size inverter do I need for my house?

It depends on how much power you run at the same time, not on your house's size. Add up your realistic simultaneous load — essentials plus any high-draw appliances that run together — and leave headroom for surges. A small home staggering heavy use might need around 3.6kW, while a busy family home often suits 5kW or more.

What is the difference between inverter kW and battery kWh?

Kilowatts (kW) measure power — how much you can draw at once — and that is the inverter's rating. Kilowatt-hours (kWh) measure energy over time — how long you can run — and that is the battery's job. Think of the inverter as the width of the pipe and the battery as the size of the tank.

What happens if my inverter is too small?

It will trip or overload when you try to draw more power than its rating allows — for example, running a kettle, microwave and geyser element together — even if the battery is full. You then have to reset it and stagger your appliance use. Correct sizing avoids this frustration.

Can I run high-draw appliances like a kettle or geyser on solar?

Yes, provided the inverter is rated to handle them, ideally not all at once. High-draw items dominate sizing, so if you want to run several together you need a larger inverter. A site assessment establishes which appliances your chosen system can comfortably support.

Who works out the correct size for my system?

SolarGuide is an independent referral partner. Alumo carries out the site assessment and supplies, installs, finances and warrants the system, sizing the inverter to your real demand. All pricing here is indicative and subject to that assessment and credit approval. We simply help you compare and get a free quote. Get a free quote →