Solar battery size calculator for Melbourne homes

How the battery size calculator works

Enter the power you use after dark, the power you use while the sun is up, and your solar size. The calculator works through an average day in each month and shows how much of your evening use each battery size covers in summer and winter, how many full cycles it does a year, and the federal rebate on each size. Only know your daily total? Enter it with the share you use in daylight and it splits it for you.

Don’t know your evening use? Your smart meter data has it: the Essential Services Commission says your distributor can give you that data, and AusNet, for one, records use every 5 or 30 minutes. Our smart meter guide shows where to find it.

1. Solar. The monthly output PVGIS (European Commission Joint Research Centre) models for panels in Mount Waverley, Melbourne, facing north at 20° with 14% system losses: 9,319 kWh a year for 6.6 kW on a 5 kW inverter, as on our 6.6kW system page, and about 1,419 kWh per kW a year for other sizes, before any inverter limit. The same model runs our solar calculator.
2. Spare solar. For an average day in each month: the solar made, less what you use while the sun is up.
3. What the battery moves. The smallest of three: the battery’s usable size, the spare solar and your evening use. It is counted before charging and discharging losses, so a real battery delivers a little less.
4. Suggested size. The smallest size in the list (5 kWh to 25 kWh) that holds your evening use, or the spare solar of an average spring or autumn day if that is less. Summer fills almost any battery and winter fills few, so the months in between set the size. Below half the smallest size (2.5 kWh) no battery is suggested, because it would sit mostly empty.
5. Rebate. Usable kWh × the STC factor for the install date, tapering above 14 kWh, from the federal Cheaper Home Batteries Program rules (Clean Energy Regulator), valued at $38 per STC, the top of the $33–$38 the Clean Energy Regulator says agents usually pay.

What it leaves out: charging from the grid, on a cheap overnight rate or in the free hours of the Midday Power Saver, runs of cloudy days, roofs that don’t face north, shade and an inverter too small for the panels. It works on daily totals, not hour by hour, so use it to narrow the choice; we recommend the size from your bills and your roof at the inspection.

Once you know the size, the battery cost guide shows how to work out the payback from the kWh a battery moves.

Solar battery sizes compared: 5 to 50 kWh

kWh is energy: how much a battery stores. kW is power: how fast it charges and discharges, which sets how much it can run at once. The two are separate figures: Alpha ESS’s single phase SMILE-G3-S5 holds 9.3 kWh to 37.2 kWh, depending on how many modules it has, and delivers 5 kW whatever its size. So a “10 kW battery” or a “20 kW solar battery” usually means 10 or 20 kWh of storage, and storage (usable kWh) is what this chart compares. More terms are in our solar glossary.

The second column uses the Essential Services Commission’s reference consumption for a home, 4,000 kWh a year (about 11.0 kWh a day): the use it treats as typical when it sets the Victorian Default Offer, not a measured Melbourne average. The other two columns run the calculator for two example homes with north-facing solar.

What each size runs overnight in Melbourne

• 5 kWh. The smallest size here: the rebate needs at least 5 kWh of nominal capacity. Full, it holds about 0.5 of a day’s use at the reference consumption. In the example home it covers 42% of the evening in summer and 42% in winter, with about 365 full cycles a year.
• 10 kWh. In the example home it covers 83% of the evening in summer and 66% in winter. A home that uses 10 kWh or less after dark has its evenings covered on any day its solar has that much to spare.
• 12.8 kWh and 16 kWh. The battery sizes in our packages. 12.8 kWh is the smallest size here that holds all 12 kWh of the example home’s evening on an average summer day; in winter it covers 71%, because the spare solar runs out first (about 8.5 kWh a day).
• 20 kWh. In the example home it stores no more than 12.8 kWh does, so the extra capacity sits unused and it does about 198 full cycles a year. It suits a home that uses around 20 kWh after dark: in the big-use home it covers 67% in summer and 67% in winter.
• 30 kWh, 40 kWh and 50 kWh. In the example home none stores more than 12.8 kWh. In the big-use home, 30 kWh covers 100% in summer and 90% in winter, and 40 kWh and 50 kWh add nothing. Batteries this big fill only where the evening use and the solar are both big: see big batteries below.

Find your evening use

Worked example by season

The example home uses 20 kWh a day, 40% of it while the sun is up: 8 kWh in the day and 12 kWh after dark. That split is an example, not an average. It has 6.6 kW of north-facing solar, which leaves about 26.0 kWh a day to spare on an average summer day, 17.8 kWh in spring and autumn, and 8.5 kWh in winter.

In summer, every size from 12.8 kWh up stores all 12 kWh, so the extra capacity of a bigger battery sits unused. In winter the spare solar runs out first: no size moves more than about 8.5 kWh a day, and every size from 12.8 kWh up moves that much. A 10 kWh battery works harder: about 342 full cycles a year. The suggested 12.8 kWh covers 90% of the year’s evening use.

How many cycles a year?

An equivalent full cycle is one battery’s worth of energy out: a 12.8 kWh battery delivering 12.8 kWh. Two half discharges count as one. The calculator adds up the energy the battery delivers in a year and divides it by the battery’s size. A battery charged only from solar fills at most once a day, so it does at most about 365.

In the example, the 12.8 kWh battery does about 309 full cycles and delivers about 3,960 kWh a year: about 39.6 MWh over ten years at that rate, before losses and the capacity a battery loses with age. A 20 kWh battery in the same home does about 198, because the capacity above its evening use is rarely used.

Warranties by battery maker

How big and heavy is a home battery?

A home battery is a cabinet or a stack of modules, and its weight grows with its capacity: across the datasheets below, about 7.6 to 17.8 kg per usable kWh. Whole stacks of about 10 kWh weigh from 85 kg (Fox ESS EQ5500-L2, 10.92 kWh) to 167 kg (BYD HVS 10.2, 10.24 kWh), and the tallest here, BYD HVM 22.1, stands 2,160 mm high.

The Sungrow, Fox ESS and BYD stacks stand on the floor; Tesla and Sigenergy batteries can go on the floor or a wall, and the wall has to carry the weight. Tesla’s Powerwall 3 has its inverter built in, so its weight includes one; the other figures are the battery alone. Alpha ESS’s figures leave out its SMILE-G3-S5/B5 inverter: 19.5 kg, 610 × 212 × 366 mm. The battery standard, AS/NZS 5139:2019, sets where it can go. It can’t go inside a habitable room: a bedroom, living room, kitchen, dining room, study or any room that could become one; in a ceiling space or wall cavity; on a roof, unless there is a permanent ladder or stairway to it; under a stairway or access walkway, or in an escape route (Solar Victoria).

Where a battery can go

Big batteries (20 to 50 kWh): who needs one?

A big battery is only used in full when the home empties it most evenings and the solar refills it most days. Take the big-use home: 50 kWh a day, 40% of it while the sun is up, so 30 kWh after dark, with 20 kW of north-facing solar. A 30 kWh battery covers 100% of its evening use on an average summer day and 90% in winter, with about 356 full cycles a year. 40 kWh and 50 kWh store no more in any season, and at 50 kWh the cycles fall to about 213.

That much solar needs a big inverter, and the inverter is what the networks limit: every Victorian network pre-approves up to 10 kW of inverter on a single phase home and 30 kW on three phase, counting solar and battery inverters together. Check which supply you have.

The rebate tapers as the battery grows: each kWh from 14 to 28 kWh earns 60% of the full rate, each kWh from 28 to 50 kWh 15%, and usable capacity above 50 kWh earns nothing, although a battery of up to 100 kWh nominal can still claim. Installed in May–December 2026, going from 20 to 30 kWh adds 35 STCs (about $1,300), and from 30 to 50 kWh only 20 STCs (about $800).

• Evening use, measured. Around 20 kWh or more after dark, from your smart meter data.
• Solar to refill it. In the big-use home, 20 kW of panels.
• Or long backup, if its inverter can run the load: see home battery backup.

When a bigger battery doesn’t help

• Winter spare solar. In the example, the spare solar on an average day runs from about 6.4 kWh in June to 11.7 kWh in August. Every size from 12.8 kWh up stores the same from June to August, about 8.5 kWh a day. More panels raise winter output: size the solar first, or see what a 6.6kW system makes by season.
• One rebate claim per property. Only one battery per property can claim the rebate (one per grid connection, or NMI). Modules added later don’t get the rebate if the first battery already claimed it: extra capacity can claim only if the existing battery never claimed STCs, and must add at least 5 kWh of nominal capacity. Choose the size you need at the start.
• Usable, not nominal. Spec sheets give two figures. Nominal capacity is the maximum amount of energy a battery can store at full charge, usually the figure on the product label; usable capacity is the amount of energy that can be discharged, which is what your home can draw from the battery. This calculator and the rebate count usable kWh; the 5 kWh minimum to qualify is nominal.
• Sizes come in steps. Batteries are built from modules, and the module size differs by brand, so the size you buy is the step at or just above the suggestion.

What 20 to 50 kWh batteries cost

Solar calculator

The federal rebate by battery size

The federal Cheaper Home Batteries Program aims for a discount of around 30% on a home battery, paid as small-scale technology certificates (STCs). The number of STCs depends on the battery’s usable capacity and the install date, not your postcode. The rate tapers with size: the full STC factor on the first 14 kWh, 60% from 14 to 28 kWh and 15% from 28 to 50 kWh, with nothing above 50 kWh. So each kWh above 14 earns less: installed in May–December 2026, a 10 kWh battery gets about $258 per kWh, a 50 kWh battery about $132.

The factor steps down on set dates until the end of 2030, so the same battery earns fewer STCs later. Dollar figures use $38 per STC, the top of the range the CER says agents usually pay, for every period. The number of STCs is set by the program; their dollar value in later years depends on the STC price then.

What each size costs

The battery rebate in detail

Battery prices by size

Battery payback calculator

Sizing for blackout backup

A grid-connected solar system switches off in a blackout to protect line workers, so backup needs an inverter with a backup output and a backup circuit for the loads you choose.

Backup storage = the load you keep on (kW) × the hours you want it. For example, 1.5 kW for 10 hours is 15 kWh. Add up the watts on the rating labels of what you’d keep running and divide by 1,000; the calculator’s backup option takes your own figures, because homes differ too much for a typical one.

• Power, not just storage. The battery’s power rating (kW) caps what it can run at once, whatever its size.
• It may not be full. A blackout can start on a winter morning, after the battery has run the evening and before the sun has refilled it.
• Backup or daily use. If the backup kWh is bigger than the size your solar fills, a bigger battery buys backup hours, not daily savings.

Home battery backup in Melbourne

The battery sizes in our packages

Our single phase packages (6.6, 15 and 19.8 kW of solar) include a 12.8 kWh battery; our three phase packages (15, 21 and 28 kW) include 16 kWh or more. In the example home the calculator suggests 12.8 kWh of usable capacity for 6.6 kW of solar, the same figure as the single phase package battery. Spec sheets also give a nominal figure, so check the usable capacity of the model your quote names.

Compare our packages

Battery sizing: your questions