Why Bigger Batteries Demand Big Inverters

Why your inverter matters as much as your battery

The right battery size for your inverter is the one your inverter can actually fill and empty, at the speed your home uses power. For most Australian homes, that means a ratio of roughly 2 to 3 kWh of battery for every 1 kW of inverter, adjusted for your evening peak, your solar and your tariff.

Since the federal battery rebate began, a new kind of deal has appeared: 30, 40, even 50 kWh of storage paired with a 5 kW inverter, at a headline price that looks too good to pass up. Independent solar commentators have been warning about it, and so has the federal government, whose own consumer advice says a battery that is too large for your solar or inverter limits the benefit to your home and the grid.

They're right to raise it. A big battery behind a small inverter is like a huge water tank drained through a drinking straw: the water is all there, but you can only get so much out at once. Turn on the oven, the kettle and the air conditioner together, and the shortfall comes from the grid at peak rates, even with a full battery.

The good news is that this is a design problem, and design problems have solutions. As Australia's largest solar battery installer, with more than 40,000 installations, Solar Battery Group has matched batteries and inverters to almost every kind of home, roof and grid connection in the country. This guide shares what our designers look at, so you can read any quote with confidence, including ours.

In this guide:

• The difference between kW and kWh, and why a quote needs both
• How to calculate battery size for any inverter, step by step
• An inverter battery size chart from 600 W portable units to 15 kW home systems
• How single phase and three phase supply change the answer
• Which appliances drive your peak load, and how to read it from your bill
• How free-power plans and the 2026 rebate tiers affect sizing
• The questions to ask any installer before you sign

Inverter size vs battery size: kW is the tap, kWh is the tank

Every home battery system has two separate ratings, and a quote that only shows one of them is incomplete.

• Battery capacity (kWh) is how much energy is stored. Think of it as the size of the tank.
• Inverter power (kW) is how fast that energy can move in or out. Think of it as the tap, or the straw.

A 5 kW inverter delivers 5 kW whether the battery behind it holds 10 kWh or 50 kWh. A bigger tank lets you run for longer; it never lets you run more appliances at once.

The inverter also limits how quickly the battery can be filled. The time to fill or empty a battery is roughly its capacity divided by the inverter's power:

Hours to fill or empty ≈ Battery capacity (kWh) ÷ Inverter power (kW)

So a 30 kWh battery behind a 5 kW inverter needs at least six hours of full-power charging, before losses. In winter, few homes get six hours of strong surplus sun.

Same battery, different inverter

Hold the battery at 16 kWh and change only the inverter, and the stored energy never changes. Only the rate it can be delivered does.

If your home draws 7 kW in the evening peak, the 5 kW inverter covers 5 kW and the other 2 kW is bought from the grid. An 8 kW inverter could cover all of it, provided the battery itself can discharge that fast.

Peak load vs continuous load

Two more terms explain most sizing mistakes:

• Continuous load is what your home draws steadily over a period, such as the fridge, lights and a split system running through the evening.
• Peak load is the highest total at any one moment, such as dinner time when the oven, cooktop, kettle and dishwasher overlap. Motors in pumps and air conditioners also draw a short surge on start-up, often several times their running power.

Your inverter needs to cover your realistic peak; your battery needs to cover your total overnight energy. Get both right and the grid becomes a backup rather than a habit.

Three kW limits to check, not one

The headline inverter size isn't always the number that limits your battery. Our designers check all three on every datasheet:

1. Inverter AC output. The most the system can deliver to your home at once.
2. Battery charge and discharge power. Some hybrid inverters accept 10 kW of solar but move only 5 kW in or out of the battery. Look for "max charge/discharge power".
3. The battery's own limit. Battery chemistry, the number of modules and stack voltage set a ceiling. A small stack on a large inverter can be capped well below the inverter's rating.

The lowest of the three is the real speed of your system.

How to calculate battery size for an inverter

To calculate battery size for an inverter, work out how much energy you need (kWh) and how fast you need it (kW), then check the battery can deliver both. The method is the same whether you're asking "what size battery do I need for a 2000W inverter" in a caravan or sizing a whole-home solar battery.

Step 1: List what runs at the same time

Add up the wattage of the appliances you'll run together. This is your peak load, and the inverter must be at least this big. Check appliance labels or manuals; for anything with a motor or compressor, allow for start-up surge.

Step 2: Work out the energy you need

Multiply the average load by the hours you need it:

Energy (Wh) = Average load (W) × Hours of use

Note that this uses your average load, not the inverter's maximum. A 2000 W inverter powering a 500 W load for four hours needs 2,000 Wh, not 8,000 Wh.

Step 3: Allow for efficiency and usable capacity

Inverters lose roughly 5–15% converting DC to AC, and batteries shouldn't be emptied completely. Lithium iron phosphate (LFP) batteries typically allow 80–100% of their capacity to be used; lead-acid batteries around 50%.

For home batteries rated in kWh:

Battery (kWh) = Energy needed (kWh) ÷ (Inverter efficiency × Usable fraction)

For 12 V, 24 V or 48 V batteries rated in amp-hours (Ah):

Battery (Ah) = Energy needed (Wh) ÷ (Battery voltage (V) × Inverter efficiency × Usable fraction)

Step 4: Check the battery can supply the current

This is the step most online calculators skip. At full output, an inverter draws a large current from the battery:

Battery current (A) ≈ Inverter power (W) ÷ (Battery voltage (V) × Inverter efficiency)

A 2000 W inverter on a 12 V battery draws about 185 A at full load. The battery's management system, cables and fuses must all be rated for that, which is why larger inverters usually move to 24 V or 48 V.

Step 5: Check you can refill it

A battery only earns its keep if it's refilled regularly. Divide its capacity by the charging power available (solar surplus, a charger, or the inverter's charge rating) to see how many hours it takes. If you can't fill it on a typical winter day or within a cheap charging window, it's bigger than you need.

Worked example: what size battery for a 3000 W inverter?

A 3000 W inverter runs a 1,200 W average load for 3 hours on a 24 V LFP battery, at 90% efficiency and 90% usable capacity.

1. Energy needed: 1,200 W × 3 h = 3,600 Wh
2. Battery size: 3,600 ÷ (24 × 0.9 × 0.9) ≈ 185 Ah at 24 V (about 4.4 kWh)
3. Current at full output: 3,000 ÷ (24 × 0.9) ≈ 139 A, so choose a battery rated for at least 150 A continuous

The answer is a 24 V battery of about 200 Ah, or two 12 V 200 Ah batteries in series.

Inverter battery size chart: 600 W to 15 kW

Use this chart as a starting point: small inverters are sized by how long you want to run them, while home solar inverters are sized by a battery-to-inverter ratio of about 2–3 kWh per kW.

The portable and off-grid rows assume a lithium (LFP) battery, 90% inverter efficiency and 90% usable capacity. The home rows assume a grid-connected hybrid or battery inverter.

Two things the chart can't tell you:

• How long you need to run. For 600 W to 3000 W inverters, multiply the 1-hour figure by the hours you need at full output, or scale it down if your real load is lower.
• Whether you can refill it. For home systems, the battery must be refillable from your solar or a cheap charging window. A 5 kW inverter on 40 kWh is outside this chart for good reason.

What size battery for a 5 kW inverter?

For a typical home, 10–15 kWh is the sweet spot for a 5 kW inverter. It covers most evenings, refills from a 6.6 kW solar system on most days, and keeps the battery working hard enough to pay for itself. Some light-use homes run larger batteries on 5 kW happily, but only when they rarely draw more than 4–5 kW at once.

Sizing batteries and inverters for a solar PV system

In a home solar system, the panels, inverter and battery have to be sized as a set: the panels decide how much energy you can harvest, the inverter decides how fast it moves, and the battery decides how much you keep for later.

One inverter or two? Hybrid vs AC-coupled

You don't always need two inverters. It depends on how the system is built.

If you already have solar, an AC-coupled battery lets you keep a working inverter. If you're starting fresh, a hybrid is usually simpler and more efficient. Our team covers both in our inverter range, including micro inverters for shaded or complex roofs.

What about the "regulator"?

In older off-grid systems, a separate charge controller (regulator) managed the flow from panels to batteries. In a modern grid-connected home system, that job is built into the hybrid inverter's solar inputs (called MPPT trackers) and the battery's management system. When sizing a domestic system today, you're choosing an inverter and a battery; there's rarely a separate regulator to buy.

What size inverter for a 10 kW solar system with a battery?

For a 10 kW solar array with a battery, a 8–10 kW hybrid inverter is the usual match, paired with roughly 20–28 kWh of storage. Australian rules generally allow solar panels up to 133% of the inverter's rating, so a 10 kW array can sit on an inverter as small as about 7.5 kW. With a hybrid, some of that extra solar can go straight into the battery.

Your network matters too. Many distributors cap single-phase inverters or exports at around 5 kW per phase by default, with more allowed on application or with export limiting. Our designers check your local rules before quoting.

Rules of thumb, and when each applies

You'll find different sizing rules online because each one solves a different problem.

In practice, our designers use all three and let your real usage decide.

Example pairings

These are illustrations, not quotes. Your own usage data should override them.

Modular batteries make it easier to get this right over time. Systems such as Alpha ESS and Sigenergy SigenStor let you start at the right size and add capacity later, provided the inverter was chosen with that growth in mind. You can compare batteries side by side or browse our full solar battery range.

Single phase and three phase: why it changes the answer

On a three-phase supply, an inverter's rating is split across the three phases, so each phase gets roughly a third. A 15 kW three-phase inverter supplies about 5 kW per phase, not 15 kW to any single appliance.

Most Australian homes are single phase: one live supply carries everything. Larger homes, rural properties and homes with big pumps, ducted air conditioning or fast EV chargers are often three phase. You can usually tell by your switchboard (three linked main switches) or by asking your retailer.

Surge loads catch people out on three phase. A small pool pump that runs at 0.4 kW can draw several kilowatts for a moment as it starts. A 10 kW single-phase inverter shrugs that off; a 10 kW three-phase unit, with about 3.3 kW per phase, may not.

If you're on three phase, ask any installer to show you the per-phase output, where your biggest loads sit, and whether the battery backs up one phase or all three in a blackout. Our blackout protection guide explains the options.

Appliance energy usage: find your real peak

The single most useful number for sizing an inverter is your evening peak: the most power your home draws at once between about 4 pm and 9 pm. High energy use appliances overlap at exactly that time, and that overlap, not your daily total, decides whether a 5 kW inverter is enough.

High energy use appliances

The figures below are typical running power. Check your own appliance labels, as models vary widely.

Now picture a winter weeknight: ducted heating at 4 kW, the oven at 2.5 kW, and the kettle at 2 kW. That's 8.5 kW for a few minutes. On a 5 kW inverter, 3.5 kW comes from the grid even with a full battery. On a 10 kW inverter, the battery covers all of it.

How to read household energy use from your bill

Your bill shows how much energy you use (kWh), but not how fast. To size properly, you need both.

1. Daily usage (kWh per day). Printed on most bills. This guides your battery size: aim to cover your typical dusk-to-dawn use, not your whole day.
2. Interval data. If you have a smart meter, your retailer or distributor can provide your usage in 30-minute (or shorter) blocks. This shows your real evening peaks and how they change with the seasons.
3. Seasonal swing. Compare a summer and a winter bill. Solar output falls sharply in winter while heating loads rise, so a battery that's easy to fill in January may be half-empty in July.

If you're planning to electrify (an EV, heat pump hot water, or replacing gas heating), add those loads now. Each one adds kilowatts to your peak, and it's far cheaper to choose the right inverter once than to replace it later. Our heat pump and HVAC solutions are designed with this in mind, and our Solar Savings page shows where the biggest savings usually come from.

Free power hours and the 2026 rebate: two new reasons to size carefully

Free midday power and the tiered battery rebate both reward the same thing: an inverter big enough to fill your battery quickly, and a battery no bigger than you can use.

Sizing for three free hours of power

The regulated Solar Sharer Offer started on 1 July 2026 in NSW, South Australia and south-east Queensland, and Victoria's equivalent began on 1 October 2026. Both give three free hours of grid power in the middle of the day, capped at 24 kWh per day. Several retailers offer their own free windows with different hours and caps.

On these plans, the limit isn't your solar. It's how much your inverter can pull into the battery in three hours.

These figures assume no losses and that the battery accepts the inverter's full rating. The cap also counts everything you import in the window, so a 1 kW household load uses about 3 kWh of it.

For most single homes, 8–10 kW of charging power is the practical target. Free-power plans can change, and rates outside the window are sometimes higher, so we size for flexibility rather than one tariff.

How the battery rebate now rewards right-sizing

Since 1 May 2026, the federal Cheaper Home Batteries Program pays progressively less for each kWh above 14 kWh. The dollar figures move with the certificate (STC) market, so treat them as approximate.

Based on installations from May to December 2026. The rate steps down every six months until 2030.

Three things follow for your system:

• The rebate no longer subsidises a giant tank. Going from 28 kWh to 50 kWh adds only about $830 of rebate, so capacity you can't fill or empty through a small inverter is now mostly paid for by you.
• You can claim once. Choose an inverter that suits the battery you may grow into, because a later inverter upgrade isn't rebated.
• Equipment and installer rules apply. The battery and inverter must be on the Clean Energy Council's approved lists, installed by an accredited installer, and capable of joining a virtual power plant.

Our team handles the rebate paperwork for every eligible install. See how much you could claim on our National Battery Rebate page.

How to spot a well-designed battery quote

A well-designed quote shows both kW and kWh, explains how each was chosen from your usage, and answers the questions below in writing. These are the same questions our designers answer before any Solar Battery Group quote goes out.

Questions to ask any installer

1. What is the inverter's continuous output in kW, and per phase if I'm on three phase?
2. What is the maximum battery charge and discharge power? Is it lower than the inverter rating, and why?
3. Does the number of battery modules limit how fast it can charge or discharge?
4. How long will it take to fill the battery from empty: from my solar in winter, and in a three-hour free window?
5. Which of my evening loads will the battery cover, and which will still draw from the grid?
6. Is blackout backup included, and exactly which circuits are backed up?
7. Where will the system measure grid import and export, and will it see every load, including sheds, pools and sub-boards?
8. Can I schedule charging myself in the app?
9. Will it work with a smart EV charger, a virtual power plant or a wholesale-price retailer if I switch later?
10. What are my network's inverter and export limits?
11. Can the battery or inverter be expanded later, and what would that cost?
12. Can you show me the rebate calculation by tier, and the installation date it assumes?

Red flags

• A quote that lists kWh but no kW, or describes the inverter in "kWh".
• A battery of 20 kWh or more on a 5 kW inverter, with no discussion of your evening loads or EV.
• One standard package for every home, with no request for your bills or usage data.
• Promises of "no more bills" without checking what your solar produces in winter.
• Blackout backup listed as an optional extra after the headline price.
• Full payment upfront to a new or unknown seller.

A big battery isn't automatically a bad choice. Homes with light evening loads, a need for several days of backup, or a wholesale trading plan can make good use of extra capacity. The problem is a big battery chosen for its headline price rather than for how your home uses power.

Your journey to zero bills starts with the right match

Zero bills don't come from the biggest battery on the market. They come from a battery your solar can fill, an inverter that can keep up with your evenings, and a design built around how your household actually lives.

That's the journey we've helped more than 40,000 Australian homes make. Every Solar Battery Group system starts with your usage, not a package: we look at your bills, your evening peak, your roof, your phase, your tariff and your plans for the next ten years. Then we match the kW and the kWh so every part of the system pulls its weight, and we handle the rebate paperwork along the way.

Take the first step today.

• Two minutes: answer a few questions with Find My Fit and see which system suits a home like yours.
• Ready for numbers: get a free, tailored quote, with the inverter kW, battery kWh, charge rate and rebate tiers shown in writing.
• Prefer to talk: call our team on 1300 223 224.

Bring your latest bill. We'll show you what it would take to bring the next one down to zero.

Frequently asked questions

What size battery do I need for a 600W inverter?

A 12 V 100 Ah lithium battery suits most 600 W inverters. At full load it draws about 56 A and runs for roughly 1.5 hours; at a typical camping load of 150–200 W, it lasts most of a night.

What size battery for a 1500W inverter?

Allow about 154 Ah at 12 V for each hour at full output. A 12 V 200 Ah lithium battery, or a 24 V 100 Ah battery, is a practical starting point. Check the battery can supply about 140 A continuously at 12 V.

What size battery for a 2000 watt inverter?

A 24 V battery of 100–200 Ah is the usual choice for a 2000 W inverter. On 12 V it would draw about 185 A at full load, which needs very heavy cabling and a battery rated for that current.

What size battery for a 3000W inverter?

A 48 V 100 Ah (about 5 kWh) or 24 V 200 Ah lithium battery suits most 3000 W inverters. Either gives roughly 1.5 hours at full output and much longer at typical loads.

What size battery for a 5kW inverter?

For a grid-connected home, 10–15 kWh is the sweet spot. Larger batteries can work on a 5 kW inverter for homes with light evening loads, but they take longer to fill and can't power more appliances at once.

Is a bigger battery always better?

No. A battery only saves money when it's regularly filled and emptied. If your solar or inverter can't fill it, or your inverter can't deliver it fast enough, the extra capacity sits idle. Since May 2026, the rebate on capacity above 28 kWh is also much smaller.

Do I need a new inverter to add a battery to my existing solar?

Not always. An AC-coupled battery adds its own inverter and keeps your existing solar inverter in place. A hybrid inverter replaces your old one and handles both. The right choice depends on the age of your inverter, your roof and the battery you choose.

Can I add more battery capacity later?

With a modular battery, yes, as long as the inverter has the headroom. That's why we recommend choosing the inverter for where your home is heading, not just where it is today.

Will a battery power my whole house in a blackout?

Only if the system is designed and wired for it, and only up to the inverter's backup output. Many systems back up selected essential circuits. Ask exactly which circuits are covered.