What Size Solar Battery Do I Need? A Practical Sizing Guide

Choosing the right solar battery size is less about how many people live in your home and more about when you use electricity, how much excess solar you generate and what you want the battery to do.

For many homes, the aim is to store excess solar generated during the day and use that energy in the evening and overnight. But the right capacity can change significantly if you also want blackout backup, have high overnight consumption, plan to buy an EV or expect your electricity use to increase.

That is why there isn't one battery size that suits every three-bedroom or four-bedroom home.

If you're considering a battery in Albury, Wodonga or the surrounding region, the most useful starting point is your actual electricity usage and solar generation data.

The short answer: how big should my solar battery be?

The right battery should generally be large enough to store a useful amount of your excess solar generation and supply the electricity you actually need after your solar panels stop producing.

That means looking at two important numbers:

1. How much excess solar energy you have available to charge the battery.
2. How much electricity you use in the evening and overnight.

You then need to consider whether you want backup power and whether your electricity use is likely to increase.

For example, installing a larger battery doesn't automatically make a system better. If your solar system rarely produces enough surplus energy to charge it, you may be paying for storage capacity that you don't regularly use.

Likewise, a battery that is too small may regularly run out early in the evening and leave you importing more electricity from the grid than expected.

The goal is to find the capacity that fits the way your property actually uses and generates electricity.

Battery capacity is measured in kWh

Solar battery capacity is normally expressed in kilowatt-hours (kWh).

This tells you how much energy the battery can store.

It is different from kilowatts (kW), which measure power. Battery power affects how much electrical load the system can supply at one time, while battery capacity affects how long that stored energy can last.

Both can matter when designing a system.

A household might have enough battery capacity to cover its total evening energy consumption, for example, but the battery and inverter still need sufficient power output if several high-demand appliances are operating at the same time.

Usable capacity vs nominal capacity

When comparing batteries, pay attention to usable capacity, not just the largest capacity number shown on the specification sheet.

Nominal capacity refers to the total amount of energy contained within the battery.

Usable capacity is the portion that the system allows you to use under its normal operating parameters.

The difference matters because two batteries advertised with similar overall capacities may not necessarily provide exactly the same amount of usable stored energy.

The Clean Energy Council's approved battery listings include both nominal and usable capacity information for listed products, making usable capacity an important specification when comparing systems.

Start with your electricity usage

Before choosing between a 10 kWh, 15 kWh or 20 kWh battery, it helps to understand your home's electricity consumption throughout the day.

A quarterly electricity bill can provide your total consumption, but interval data from a smart meter is considerably more useful for battery sizing.

Interval data can show when you are actually using electricity.

For battery sizing, we're particularly interested in what happens after solar production begins to fall.

Consider things such as:

• cooking and evening appliance use
• heating and cooling
• electric hot water
• pool pumps
• clothes dryers and washing machines
• overnight appliances
• home offices
• EV charging
• other significant electrical loads.

Two homes consuming the same total number of kilowatt-hours each day could require very different battery systems.

One household might use most of its electricity between 9 am and 4 pm, when its solar panels are generating.

Another might have an empty house during the day but substantial electricity demand between 5 pm and midnight.

The second household potentially has much more energy use that a battery could cover.

Work out how much excess solar you actually have

Your evening consumption is only half of the equation.

You also need energy available to charge the battery.

In a typical grid-connected solar system without a battery, solar electricity is first used by the home. Surplus generation can then be exported to the grid.

When a suitable battery is added, that excess solar can instead be used to charge the battery before stored energy is used later when household demand exceeds solar production.

So when we're assessing battery capacity, one of the questions we look at is:

How much solar are you currently exporting during the day?

If a home regularly exports substantial amounts of solar electricity, there may be plenty of surplus generation available for battery charging.

If almost all solar production is already being consumed directly by the home, the situation is different.

A large battery has limited value as solar storage if there isn't enough surplus solar available to charge it regularly.

If you're still planning your solar system, rather than adding a battery to an existing system, solar panel and battery sizing can be considered together. Cross Border Power's solar and battery installation service is based on assessing the property's energy usage, goals and site conditions before recommending a system.

How evening and overnight consumption affects battery size

For households primarily interested in increasing their use of their own solar energy, evening and overnight consumption is one of the most important sizing considerations.

Imagine two households that each consume the same amount of electricity over a full day.

Household A: high daytime consumption

This household runs appliances, heating or cooling and other electrical equipment while solar is being generated.

A significant portion of its electricity may already come directly from the solar panels.

Its overnight consumption is relatively low.

A very large battery may therefore provide limited additional benefit.

Household B: high evening consumption

This household is relatively quiet during the day but becomes busy from late afternoon.

Cooking, heating or cooling, entertainment, hot water and other appliances are predominantly used after solar production drops.

If the solar system produces enough surplus energy during the day, this household could potentially make greater use of battery storage.

These are simplified examples, but they demonstrate why choosing a battery based purely on household size can be misleading.

Is a 10 kWh battery big enough?

A battery around this size may suit some households, but there is no reliable rule that says a 10 kWh battery is right for a particular number of bedrooms or occupants.

The important question is how that usable capacity compares with your actual energy profile.

If your home only needs a moderate amount of stored energy between sunset and the following morning, a larger system could be unnecessary.

If your evening and overnight consumption is substantially higher, or you have particular backup requirements, more storage may be worth considering.

The decision should also account for how consistently your solar system can recharge the battery.

When would you consider a larger battery?

There are several situations where additional storage capacity may make sense.

Higher evening and overnight electricity use

Homes running significant electrical loads after sunset may have more use for stored energy.

A larger solar system with substantial exports

If your solar system regularly produces considerably more electricity than the home consumes during daylight hours, you may have more energy available for battery charging.

More extensive blackout backup requirements

If backup power is important, the system needs to be designed around the circuits and appliances you want to operate and the duration for which you want to operate them.

Future electrification

Your electricity usage today may not be the same as your usage in five years.

A household planning to add an EV, electric hot water, induction cooking or additional heating and cooling should consider those future loads when designing a solar and battery system.

Some battery systems also allow capacity to be expanded later, although the options and requirements depend on the particular battery and system configuration.

Don't size a battery from summer solar production alone

Solar production changes throughout the year.

A system that produces plenty of excess electricity on a long, clear summer day may have considerably less surplus generation available during shorter winter days or poor weather.

Household electricity consumption can change seasonally as well, particularly where electric heating or cooling is used heavily.

This doesn't necessarily mean you should install a battery large enough to cover every possible winter scenario.

Instead, seasonal production and consumption should form part of the overall assessment.

Sizing around an unusually good solar day can leave you with capacity that is harder to fill during other parts of the year.

Battery sizing for blackout backup is different

If your main reason for buying a battery is backup power, capacity needs to be considered differently.

First, it is important to understand that having solar panels and a battery does not automatically mean every appliance in your home will continue operating during a blackout.

Backup capability depends on the battery, inverter, system design, switchboard configuration and the circuits connected to the backup supply.

You therefore need to decide what you actually want running during an outage.

That might include:

• refrigerators and freezers
• lighting
• internet equipment
• selected power points
• medical or other essential equipment
• heating or cooling
• water pumps or other property-specific equipment.

Keeping a fridge, lights and internet operating is very different from trying to run an entire electrically intensive home normally during a blackout.

The battery's power output also matters here. Even if there is enough stored energy available, the system needs to be capable of supplying the loads connected at that particular moment.

Backup requirements should therefore be discussed during the system design rather than assumed from the battery's kWh capacity alone.

What about three-phase homes?

Three-phase supply adds another consideration.

A three-phase home doesn't automatically require a larger battery, but the way the battery, inverter and backup system interact with the property's phases needs to be assessed.

This becomes particularly important where the property has significant three-phase loads or the owner expects particular equipment to operate during a blackout.

Rather than choosing a battery from capacity alone, the installer should look at the property's electrical configuration and what the battery is expected to supply.

Should you size your battery for an EV?

An EV can significantly change a home's electricity profile, but that doesn't necessarily mean you should buy a battery large enough to charge the vehicle entirely from stored energy.

Where possible, charging an EV directly from available solar generation during the day can avoid unnecessary energy conversion and reduce the amount of stationary battery capacity needed solely for vehicle charging.

But every household is different.

If the car is away from home during solar production hours and is usually charged in the evening, its effect on the household's load profile may be much greater.

Consider:

• how far you normally drive
• how frequently the car needs charging
• whether it is home during daylight hours
• your charger capacity
• your solar system size
• your electricity tariff
• other household loads.

If an EV is planned within the next few years, tell your solar installer before the battery system is designed.

Think about electric hot water, heating and cooking too

EVs aren't the only reason electricity consumption can increase.

Australian homes are increasingly using electricity for appliances that may previously have relied on gas or other fuels.

Future changes could include:

• a heat pump hot-water system
• induction cooking
• additional air conditioning
• electric heating
• a swimming pool or spa
• home extensions
• additional occupants.

Solar Victoria specifically recommends considering future changes such as switching from gas hot water to an electric heat pump or adding more solar panels when planning battery capacity.

A good battery design should therefore consider where your home is heading, not only what appeared on your last electricity bill.

Can a solar battery be too big?

Yes.

More battery capacity isn't automatically better.

For a battery installed primarily to store excess solar, there needs to be enough energy available to charge it and enough demand later to make regular use of the stored energy.

Oversizing can mean paying for capacity that frequently sits unused.

The NSW Government's battery guidance similarly recommends considering both excess daytime solar and overnight electricity consumption when working out an appropriate battery size.

There can still be legitimate reasons to install additional capacity, including backup requirements and anticipated future energy use. But those should be deliberate design decisions rather than simply choosing the biggest battery available.

Can a solar battery be too small?

A battery can also be undersized for what you are trying to achieve.

If it regularly reaches its minimum charge early in the evening while the household continues using significant grid electricity, additional storage may have been useful.

But that observation still needs context.

A battery running empty isn't automatically proof that you need a bigger one. The additional capacity also needs a suitable source of energy for charging and should make sense for your goals and budget.

This is why analysing both sides of the equation matters:

Can you fill it, and can you use it?

A practical battery sizing checklist

Before deciding on a battery capacity, gather the following information.

1. Your electricity bills

Look at your total consumption and how it changes throughout the year.

2. Smart-meter interval data

This provides a much clearer picture of when your home consumes electricity.

3. Solar generation and export data

For an existing solar system, monitoring data can help establish how much surplus solar is available.

4. Your evening and overnight load

Identify how much electricity you generally use once solar generation has fallen.

5. Your reason for installing a battery

Are you mainly trying to increase solar self-consumption, reduce grid imports, provide blackout backup or prepare for future electricity use?

6. Backup requirements

Decide which appliances or circuits are genuinely important during an outage.

7. Planned future loads

Include EVs, electric hot water, heating and cooling, renovations or other changes likely to affect electricity consumption.

8. Your existing electrical system

The solar inverter, switchboard, available installation space and single or three-phase supply can all influence the final system design.

Battery size should come before battery brand

It is easy to begin by comparing battery brands, features and specifications.

But before deciding which product is appropriate, you should have a clear idea of what the battery actually needs to achieve.

A system with impressive specifications can still be the wrong system if its capacity, output or backup configuration doesn't match the property.

Cross Border Power works with battery and inverter options including GoodWe ESA, Sigenergy and Sungrow, but there isn't one product that is automatically right for every household.

The better sequence is:

1. understand your energy profile
2. establish how much usable storage you need
3. determine your backup requirements
4. assess the property's electrical and solar system
5. compare suitable battery systems.

That approach makes the product comparison much more meaningful.

How Cross Border Power approaches battery sizing

For homes around Albury Wodonga, we don't recommend a battery simply from the number of people living in the house.

A proper assessment can consider your property, electricity usage, existing or proposed solar system, goals and backup requirements.

Where appropriate, that can include reviewing electricity bills and available usage data before recommending a tailored solar and battery system.

Cross Border Power is locally owned and operates across Albury Wodonga, with local in-house electricians completing solar and battery installations rather than handing the installation from a sales company to subcontractors.

You can learn more about our solar and battery installations in Albury Wodonga or contact Cross Border Power for a free, no-obligation quote.

Frequently asked questions about solar battery size

What size battery does the average home need?

There isn't a single battery size that can reliably be recommended for an average home. The appropriate capacity depends on the home's electricity consumption, when that electricity is used, available solar surplus, backup requirements and future plans.

Is a 10 kWh battery enough to run a house overnight?

It depends on how much usable capacity the battery provides and how much electricity the house consumes overnight. A lower-use home may require considerably less stored energy than a property running heating, cooling, hot water or other large loads.

Should my battery match the size of my solar system?

Not on a simple one-to-one basis. Solar panels are rated in kW while battery storage capacity is measured in kWh, and the appropriate combination depends on generation, household consumption and system goals.

Is a 20 kWh battery better than a 10 kWh battery?

Only if the additional capacity is useful. A larger battery can store more energy, but it may offer limited benefit if the solar system cannot regularly charge it or the household doesn't use enough stored energy.

Should I size a battery for my daily electricity consumption?

Total daily consumption is useful, but it doesn't tell the whole story. Electricity used while your solar panels are generating may already be supplied directly by solar. Evening and overnight consumption, along with excess daytime solar, is usually more informative for battery sizing.

How do I know how much electricity I use at night?

Smart-meter interval data can provide a detailed breakdown of when your electricity is being consumed. Solar monitoring systems may also provide useful consumption and generation information where suitable monitoring equipment is installed.

Do I need a bigger battery if I want blackout backup?

Potentially, but capacity is only part of backup design. You also need to consider which circuits will be backed up, the power those appliances require, the battery and inverter's output capability and how long you expect the system to operate.

Get a battery sized around your home

The best battery isn't necessarily the biggest one.

A well-designed system should balance the solar energy you have available, the electricity you actually use, your backup expectations and the way your home is likely to change in the future.

If you're considering battery storage in Albury, Wodonga or the surrounding area, Cross Border Power can assess your energy usage and property before recommending a suitable system.

Request a quote or site inspection to discuss a battery system designed around your actual energy needs.