Most Australian households land between 10kWh and 20kWh of storage, and the honest answer to “what size solar battery do I need” comes from two numbers on the last power bill: how many kilowatt-hours the home uses each day, and how much of that happens after dark. Everything else — brand, backup, three-phase, a future EV — sits on top of those two numbers. Clean Earth Solar sizes every system on real consumption data, and this guide walks through the same reasoning any homeowner can do before a site visit.
Start With Your Bill, Not With a Battery Size
Solar battery sizing begins with daily consumption in kilowatt-hours. Every retail electricity bill in NSW carries the figure, and it is the single most useful number in the whole decision. Those same figures also drive your solar battery payback period.
Find Your Average Daily kWh Use
- Look for “average daily usage” on the bill summary, usually printed next to a comparison bar for the same quarter last year.
- If the bill only shows the total for the period, divide the total kWh by the number of days billed.
- Compare a summer bill with a winter one. Hawkesbury homes with ducted air conditioning or a pool pump often swing 40% between seasons, and sizing to the mildest quarter guarantees disappointment in February.
A typical four-person Hawkesbury household lands around 20kWh to 25kWh a day. The average matters less than knowing your own.
Split Daytime Use From Overnight Use
A battery only earns its keep on power used when the panels are not producing, so the sizing target is the overnight and early-morning slice.
- Households where everyone leaves by 8am use roughly 60% to 70% of their power outside daylight hours.
- Homes occupied all day — shift workers, retirees, a home office — self-consume more solar directly and need less storage than their total usage suggests.
Interval data settles it properly. A solar monitoring system shows consumption half-hour by half-hour, which is why existing solar owners usually get sized more accurately than first-time buyers.
Usable Capacity Is Not the Same as the Number on the Box
A battery advertised at 10kWh will not always deliver 10kWh to the house. Two specifications decide what a household gets, and both belong in any quote comparison. If the kW and kWh figures on a quote are not making sense side by side, the difference between kW and kWh explains what each number actually measures.
- Usable capacity (kWh), which is what remains after the depth-of-discharge reserve the manufacturer holds back to protect cell life.
- Continuous power output (kW), which sets how many appliances can run at once — a 5kW output will not start a ducted air conditioner and an induction cooktop together, no matter how many kWh sit behind it.
Tesla Powerwall 3, Sungrow and Sigenergy systems publish both figures on their datasheets. Comparing nominal capacity on one quote against usable capacity on another is the most common way homeowners talk themselves into the wrong size. The Clean Energy Council maintains the approved battery product list worth checking any shortlisted model against.
What a 10kWh vs 20kWh Solar Battery Actually Covers
The practical difference is how many hours of normal living the home runs without the grid. A 10kWh solar battery carries an average household through the evening peak and overnight. A 20kWh solar battery starts covering a full day of cloudy weather, or an evening with the air conditioning running.
| Household Profile | Typical Daily Use | Indicative Storage Target | What It Realistically Covers |
|---|---|---|---|
| Couple, small home, most days out | 8–12kWh | Around 8–10kWh | Evening cooking, lighting, TV and overnight standby loads |
| Family of four, gas cooking and hot water | 18–22kWh | Around 10–13kWh | The full evening peak plus overnight, with morning use topped up by early sun |
| All-electric family home, ducted air conditioning | 25–35kWh | Around 15–20kWh | Evening peak with climate control running, and most of a poor solar day |
| Home with a pool, or a large all-electric home | 35–45kWh | 20kWh or more, often modular | Overnight plus pump cycles shifted off the grid |
| Home with an EV charged overnight | Add 8–15kWh per charge | 20kWh or more, or daytime charging instead | Household loads and a partial vehicle charge |
| Rural property chasing independence | Varies widely | Sized on worst-case winter week | Days of autonomy — see off-grid design |
These bands start the conversation; they are not a quote. Ranges shift with roof orientation, shading and tariff structure, which is what a site assessment for solar battery installation exists to pin down.
How Solar Array Size Changes the Battery You Need
A battery can only store the solar the household does not use at the time. Storage larger than the daily surplus means paying for capacity that sits half empty most of the year.
- A 6.6kW Hawkesbury array produces roughly 24kWh to 26kWh on a good day, and under half that in wet mid-winter weather.
- Subtract daytime consumption from daily generation, and the leftover surplus is the most a battery can absorb.
- Where the surplus is small, the better first move is often more panels rather than more storage — panels cost less per kilowatt-hour delivered.
Households adding storage to an existing system often upsize the array in the same visit, which is where a hybrid solar system earns its place. Panel efficiency matters here too: AIKO panels with built-in shading optimisation hold output better on the partly shaded blocks common around Richmond and Windsor, lifting the surplus available to charge a battery.
What Changes if You Have an EV or a Pool?
Both add a large but movable load, and movable loads change sizing more than they change capacity.
Electric Vehicles
A typical EV commute draws 8kWh to 15kWh a night — an entire small battery on its own. Charging from the sun during the day, rather than from the battery after dark, keeps storage sized for the house. A smart EV charger installation that follows surplus solar output does this automatically, and it is usually cheaper than buying another 10kWh of storage.
Pools and Hot Water
Pool pumps and heat-pump hot water shift to the middle of the day on a timer, drawing straight from the array. Moving those loads into daylight hours often shrinks the battery needed by several kilowatt-hours.
Do You Want Bill Savings, Backup, or Both?
This changes the size more than any other choice, because backup capacity sits in reserve rather than cycling nightly.
- Bill savings only: size to the overnight surplus and stop there, since capacity beyond it cycles rarely.
- Essential-circuit backup: fridge, lights, internet and a few power points need only a modest reserve held back.
- Whole-home backup: the battery must cover the peak simultaneous load, not just the daily total — and in a three-phase home, that means a system built for it, such as the Sungrow SBH range.
Blackouts are a real consideration on the Hawkesbury fringe, where storm season and long rural feeders make outages more than theoretical. Properties chasing full independence move into off-grid battery installation territory, which is sized on the worst week of the year rather than the average day.
Common Solar Battery Sizing Mistakes
Most sizing regret traces back to a short list of errors, all avoidable before the deposit.
- Sizing to total daily usage instead of overnight usage, which buys capacity the sun already covers.
- Ignoring continuous power output, then discovering the air conditioner trips the system during a blackout.
- Choosing a fixed unit when household load is about to change — a modular system leaves room to expand when an EV or a pool arrives.
- Treating an online solar battery size calculator as the final word. Calculators use averages; interval data and a roof assessment use the actual home.
Sizing is a design decision, so it deserves a designer rather than a sales script. Every Clean Earth Solar consultation runs with an SAA-accredited installer, not a salesperson.
Get the Size Right Before You Buy
Storage capacity is the one decision that is expensive to reverse. Clean Earth Solar has installed more than 1,000 systems across the Hawkesbury and greater Sydney since 2018, all with in-house SAA-accredited electricians and a lifetime workmanship warranty, and every design starts with the household’s own consumption data. Send through a recent bill and the team will model what a battery would realistically cover in your home — including whether a larger array or smarter EV charging would serve you better. Get a free quote and find out what size actually suits the way your home uses power.
Ready to size your storage properly? Talk to Clean Earth Solar on 1300 753 318.
Frequently Asked Questions
How much battery storage do I need for a family home?
Most Australian family homes are sized between 10kWh and 20kWh of usable storage, based on how much power the household uses after dark rather than its total daily consumption. A family using 20kWh a day with roughly 60% of that overnight generally lands around 12kWh. A site assessment with real interval data narrows it further.
Is a 10kWh solar battery big enough?
A 10kWh solar battery covers the evening peak and overnight loads for an average household with gas cooking and hot water. Homes running ducted air conditioning, a pool or an EV usually need more, or need those loads shifted into daylight hours instead.
Can I add a battery to solar panels I already have?
Yes. Existing solar can usually be retrofitted, either through a hybrid inverter upgrade or an AC-coupled unit alongside the current inverter. Clean Earth Solar checks the existing inverter and switchboard first, and the interval data from an established system makes sizing far more accurate.
Is a solar battery size calculator accurate?
Online calculators give a useful ballpark, but they run on state-level averages rather than a home’s own load profile, roof orientation or shading. Treat the result as a starting range and confirm it against the daily kWh figure on your bill.
What size solar battery do I need if I have an electric vehicle?
An EV adds roughly 8kWh to 15kWh per overnight charge, so covering the car from storage typically means 20kWh or more. Charging the vehicle from surplus solar during the day is usually the better answer, which keeps the battery sized for the house.
How much does a solar battery cost to install?
Battery pricing moves with capacity, brand, backup requirements and any switchboard work involved, so a figure quoted without seeing the property is guesswork. Contact Clean Earth Solar directly for an accurate quote — all pricing is confirmed before work begins, with no obligation to proceed.