The Cheapest Battery Kilowatt-Hour Is the One You Do Not Buy

The federal battery rebate is generous, and it pays for capacity. It does not pay you to cycle that capacity. Which means the smartest thing you can do before sizing a battery is reduce the load it has to carry.

August 2026 · 11 min read · All states · By Mark Riley, Cutter Electronics

The Cheaper Home Batteries Program has been running since July 2025 and the funding went from $2.3 billion to $7.2 billion. It is the largest consumer energy subsidy in the country right now. If you are quoting residential solar, batteries are in every second conversation.

Here is the part that gets skipped. The rebate is calculated per kilowatt-hour of usable capacity. It rewards you for buying storage. It is completely indifferent to whether you ever fill that storage or empty it. A battery you cycle at 40% depth every night earns the same subsidy as the identical battery cycled at 90%, and returns a lot less.

So the useful question is not how big a battery the rebate will help you buy. It is how small a battery you can get away with. And the answer to that is set almost entirely by your evening load.

What the scheme actually pays, as of now

From 1 May 2026 the rebate is tiered by battery size and steps down every six months through to the scheme ending in 2030.[1][2]

Two things follow. The first 14 kWh is the best value capacity you will ever buy under this scheme. And because the rate steps down every six months, a battery bought in 2029 attracts roughly a quarter of the support a battery bought today.

None of which tells you what size to buy. For that you need to know what the battery is actually being asked to do.

A battery only ever carries the evening

This is obvious once said and routinely ignored in quoting. A home battery charges from surplus solar during the day and discharges after the sun goes down. Daytime load is served directly by the panels. The battery exists to move energy across the hours when the panels produce nothing.

So the capacity you need is not a function of your annual consumption. It is a function of what you draw between roughly 4pm and 8am.

AEMO looked at 20,000 detached Australian homes in the second quarter of 2026. Across the 4pm to 9pm window, homes with a battery drew about 0.3 kW from the grid on average. Homes with solar but no battery drew about 1.0 kW.[3] That gap is the battery doing its job, and the size of the job is set by what those homes are running after dark.

Lighting is the one load solar can never touch

Every other major household load has at least some daytime component you can shift. Pool pumps can run at noon. Dishwashers and washing machines can be timed. Hot water can be scheduled into the solar window. Air conditioning peaks in the afternoon when there is still sun.

Lighting cannot be shifted, because lighting is a function of darkness. You cannot run your lights at 11am to take advantage of the solar. Every kilowatt-hour of lighting load falls squarely in the window the battery has to cover.

Lighting is around 5% to 10% of the average Australian household electricity budget, and that share has fallen because of LED uptake.[4] But the average conceals the point. Within the evening window specifically, lighting takes a much larger share than it does across the day, because that is the only time it runs.

What that looks like in a real house

Take a home built or renovated between about 2000 and 2012, which is when halogen downlights were fitted almost universally. Twenty 50W MR16 or GU10 downlights is a modest count for a house of that vintage. Many have thirty or forty.

These are worked examples with stated assumptions, not survey figures. Substitute your own numbers.

DCCEEW puts the LED saving against halogen at around 75%.[4] The table above is slightly better than that because a 7W LED downlight is a like-for-like replacement for a 50W halogen on light output, which is an 86% reduction in connected load. Use 75% if you want to be conservative. The conclusion does not change.

4.3 kilowatt-hours a night is not a rounding error. On a battery sized to cover the evening, it is 4.3 kWh of capacity you no longer have to buy.

The money, both ways

Say installed battery capacity runs $1,000 per kWh before rebate. At roughly $252 per kWh of subsidy, your net cost is about $748 per usable kWh inside the first 14 kWh band.

Cut 4.3 kWh off the requirement and the arithmetic goes like this:

You forgo about $1,084 of rebate by buying less battery. You avoid about $4,300 of capital to get it. The lighting work costs somewhere around $1,250 and also cuts your bill every day for the next decade, including the daytime hours the battery was never involved in.

Where this argument does not hold

Sizing a battery purely on evening load is a simplification, and it is worth being honest about the cases where it breaks down.

• Backup is a capacity requirement, not a load requirement. If you want the house to ride through a two-day outage, you size for that, and efficiency helps but does not change the objective.
• Virtual power plant participation pays you for capacity made available to the grid, not for capacity you use yourself. If you are joining a VPP, a bigger battery genuinely earns more.
• Tariff arbitrage on a time-of-use plan can justify charging from cheap overnight grid power and discharging at peak, which is a separate business case from solar self-consumption.
• An EV changes everything. Home charging is a large, mostly overnight load that dwarfs lighting. If an EV is coming, size for it.

What does not change in any of those cases is the order of operations. Reducing a load is cheaper than storing it, whatever else the battery is also doing.

The order that makes sense

1. Measure the evening load before anything else. Most smart meters give you interval data through your retailer's portal. Look at 4pm to 8am on a typical weekday. That number, not your annual bill, is what sizes the battery.
2. Fix the lighting. It is the cheapest load reduction available, it lands entirely inside the battery window, and unlike most efficiency work it needs no behaviour change.
3. Deal with any other resistive evening load. Electric hot water on an evening element, a second fridge in the garage, standby loads on old AV gear.
4. Then size the battery, against the load profile you now actually have rather than the one you had before the work.
5. Then check the tier. If the honest answer lands near 14 kWh, understand that capacity above that line earns 60% of the rate and price it accordingly.

Doing it in the other order is common and expensive. Size the battery against a halogen-lit house, retrofit LEDs a year later, and you have bought and subsidised several kilowatt-hours of storage that now sit unused every night.

For electricians quoting this work

There is a straightforward commercial point here. A battery quote that arrives with a lighting assessment attached is a more credible quote, and it is a second job on the same site visit. It also protects you from the conversation eighteen months later when the customer works out their battery is oversized.

The same logic applies at commercial scale, where the loads are larger and the lighting share of after-hours consumption is often higher again. We covered the combined case in selling LED and solar together, and the off-grid version, where there is no grid to fall back on and the arithmetic is brutal, in remote Australia lighting.

Summary

The federal battery rebate pays roughly $252 per usable kilowatt-hour, gives full value only on the first 14 kWh, and steps down every six months to 2030. It subsidises capacity and is indifferent to whether you use it. A battery only ever carries the load between sunset and sunrise, and lighting is the one significant household load that cannot be shifted into the solar window. Replacing twenty halogen downlights removes around 4.3 kWh a night from exactly the window the battery has to cover, which is worth more in avoided capital than the rebate you give up by buying a smaller battery. Measure the evening load, fix the lighting, then size the battery. Not the other way round.