What size home battery do I need? Sizing from your evening use, spare solar and the 14 kWh tier

Size a home battery to the smallest of three numbers: the energy your home uses from late afternoon until the sun comes up, the solar energy you typically export during the day, and the capacity that still gives a sensible net price after the discount. For many Australian households with a 6–10 kW solar system, that lands somewhere between 8 and 16 kWh usable. Since 1 May 2026 the federal discount applies in full only to the first 14 kWh, at 60% from 14 to 28 kWh and 15% from 28 to 50 kWh, so capacity beyond about 14 kWh needs a clear reason — an EV, an all-electric home with high evening loads, or backup needs. Check the battery's power (kW) as well as its capacity: it must be able to run your evening loads at the same time.

This is the pillar guide for the sizing and quotes section. It links to the worked examples and quote guides that follow.

Step 1: get your interval data

If you have a smart meter, your retailer or distributor can provide interval data (usage in 5- or 30-minute blocks). Energy Made Easy lets you upload it for plan comparison in many states, and Victorian Energy Compare does the same in Victoria. From that data, work out for a typical day in summer and winter:

No smart meter? Your bill's daily average and your inverter app's export figure give a rough start, but ask the installer to base the design on measured data.

Step 2: find the overlap

The useful battery size is roughly the overlap between evening use and spare solar.

Worked example (illustrative household, 6.6 kW solar):

A 13.5 kWh usable battery covers the summer evening fully. In winter it will only fill to about 6 kWh from solar unless the household uses a free-midday or cheap off-peak plan to top it up. A 20 kWh battery would add little in summer (use is the limit) and nothing in winter (solar is the limit). For this household, about 10–14 kWh usable is the efficient range.

Step 3: check power, not just capacity

A battery has two key ratings:

• Capacity (kWh): how much energy it stores
• Continuous power (kW): how fast it can deliver energy

If your evening peak is 6 kW (oven, air-con and hot water at once) and the battery delivers 5 kW, you will still draw about 1 kW from the grid at peak. Ask for continuous and peak power ratings, and whether the backup output differs from normal operation. See battery backup in blackouts.

Step 4: test sizes against the discount tiers

The jump from 14 to 28 kWh doubles the capacity but adds only 60% more certificates. For households whose overlap is 12 kWh, a 28 kWh battery mostly stores energy they will never have to shift. The STC factor guide explains the tiers.

Step 5: allow for the future, carefully

Reasons to size up:

• Electric vehicle charged in the evening
• Switching from gas to heat pump hot water or reverse-cycle heating that runs in the evening
• Growing family or working from home changes
• Backup for medical equipment or frequent outages

Reasons not to size up:

• The extra capacity sits above your overlap most days
• Battery prices and the discount both change; adding modules later may be possible (the CER allows expansions of at least 5 kWh, with only new modules claimed)
• The federal rules require the battery to stay at the property until the end of 2030 or end of warranty, whichever is later

The CER requires retailers to size batteries for the premises' energy use and for the solar system. If a quote proposes a battery much larger than your measured overlap, ask the installer to explain the sizing in writing.

Step 6: run the numbers

Once you have a size, price it and test payback:

1. Estimate the price range with the installed cost estimator.
2. Estimate the discount with the discount estimator.
3. Calculate payback with the payback calculator and follow the payback worked example.

The other guides in this section cover specific situations:

• Adding a battery to existing solar — inverter compatibility and AC- versus DC-coupling
• Solar and battery package worked example — pricing both together with both sets of STCs
• Comparing home battery quotes — a line-by-line method
• Battery backup in blackouts — essential circuits or whole home
• Battery finance options — loans, interest-free offers and state loans

Common sizing mistakes

• Sizing to the discount instead of your overlap
• Ignoring winter, when solar may not fill the battery
• Forgetting the power rating, so the battery cannot cover evening peaks
• Not accounting for a VPP taking some stored energy (see virtual power plants)
• Using the nominal rather than usable capacity

FAQ

Is a 10 kWh battery big enough?

For many households with evening use around 8–12 kWh and enough spare solar, 10 kWh usable covers most evenings. Check your interval data for summer and winter.

Should I get a bigger solar system first?

If your daytime exports are smaller than your evening use, more solar may be needed to fill a battery. Check roof space and your network's export limit.

Does the battery need to cover overnight use?

Ideally it covers the expensive peak period first. Overnight use on an off-peak rate is less valuable to shift.

Sources

• Clean Energy Regulator — retailer sizing obligations, expansion rules, STC tiers
• Energy Made Easy — upload interval data and compare plans
• Victorian Energy Compare — Victorian plan comparison with usage data
• NSW Climate and Energy Action — guide to owning a battery