What Size Solar Battery Do You Need?
Three numbers decide it. The smallest one wins.
Here’s the short answer, then how to check it.
Find Your House
Find the row that sounds like your household. That’s your starting figure — the rest of the page is how to check it.
| Your household | Battery | Solar to fill it |
|---|---|---|
| Couple or small family, gas cooking, no pool or EV 10–14 kWh a day | 8–10 kWh | 6.6kW is enough |
| 3–4 bedrooms, mixed gas and electric 15–20 kWh a day · most common | 10–13.5 kWh | 10kW, or 6.6kW plus panels |
| Larger home, ducted air con or a pool 21–28 kWh a day | 13.5–16 kWh | 10kW minimum |
| All-electric, or an EV charging overnight 29–40 kWh a day | 20–27 kWh | 13kW |
| Large all-electric home, EV and pool 40 kWh+ a day | 27 kWh+ | 13kW+, often three-phase |
Don’t know your daily use? It’s on your bill as a daily average — or the battery size calculator will ask you.
One warning about that table. It assumes your solar can actually charge the battery. On a lot of local roofs it can’t, and that’s the most common sizing mistake we see.
The Three Limits
Your battery size isn’t one calculation. It’s three, and whichever gives the smallest number is your answer. Most quotes only do the first one.
1. Demand
How much power you use after the sun goes down. Not your daily total — solar already covers your daytime. This sets what you’d ideally want.
2. Supply
What your solar can spare in winter, after the house has taken its share. This sets what you can actually fill. It’s the limit nobody checks.
3. Value
Where the rebate rate falls away. Since May 2026 it drops sharply past 14 kWh. This sets where extra capacity stops being good value.
Limit 1 — What You Use After Dark
The mistake nearly everyone makes is sizing against their whole daily consumption. You don’t need to store power you’re using at midday — the panels are making it in real time. The battery only carries you from late afternoon to sunrise.
For a typical household, around 60% of consumption falls outside solar hours: the evening peak when everyone’s home, cooking and heating, plus the overnight baseload of the fridge, standby loads and hot water.
So: daily average kWh off your bill, take 60%, that’s your target usable capacity.
Usable, not nominal. A battery advertised at 15 kWh might deliver 13.5 kWh, because manufacturers hold back a reserve to protect cell life. Usable is the number that matters, and it’s also what the rebate is calculated on.
Don’t size for your worst night. A battery that covers you on 90% of evenings is a good battery. Sizing to never touch the grid in the darkest week of July means buying capacity that idles for most of the year.
Limit 2 — What Your Solar Can Spare
This is the check almost nobody does, and it decides whether a battery earns its keep or sits half-empty from May to August.
Your battery charges on surplus — what’s left after the house has taken what it needs during daylight. In midsummer there’s plenty. In midwinter there often isn’t.
| Solar | Winter day | Spare to charge | Battery ceiling |
|---|---|---|---|
| 6.6 kW | ~17 kWh | ~10 kWh | 10 kWh |
| 10 kW | ~25 kWh | ~18 kWh | 16 kWh |
| 13 kW | ~33 kWh | ~26 kWh | 20 kWh+ |
North-facing, unshaded, around Penrith. Shaded Mountains roofs generate less again in winter.
If you’ve a 6.6kW system and someone quotes a 13.5 kWh battery, ask them how it fills in July. Mostly it won’t. Check your output →
Limit 3 — Where the Rebate Rate Falls Away
This changed on 1 May 2026 and most battery advice online hasn’t caught up.
When the federal Cheaper Home Batteries Program launched in July 2025, every kWh earned the same rate. It no longer does — the rate now tapers by size, so the first slice of your battery is worth far more than the last.
| Usable capacity | Share of the rate |
|---|---|
| First 0–14 kWh | 100% |
| 14–28 kWh | 60% |
| 28–50 kWh | 15% |
| Above 50 kWh | Nil |
The first 14 kWh is where nearly all the value sits — and it happens to line up with the size most local households actually need. Past that, each extra kWh earns noticeably less back.
The rate also steps down over time. It fell in May 2026, falls again in January 2027, then roughly every six months to 2030. Your installation date sets your rate, not your quote date.
Rebate calculator → · Eligibility → · NSW incentives →
A Real One: Springwood, 22 kWh a Day
Four bedrooms on a shaded block. 6.6kW installed in 2019. Reverse-cycle heating through winter, gas cooking, two teenagers. They’d been quoted a 13.5 kWh battery. Here’s the three limits, run properly.
| Limit 1 Demand | 22 kWh a day, about 60% of it after dark — roughly 13 kWh to carry overnight. On demand alone, the 13.5 kWh quote looks right. | 13 kWh |
|---|---|---|
| Limit 2 Supply | Here’s where it falls apart. That 6.6kW array makes about 17 kWh on a clear midwinter day — but it’s shaded, so call it 14. The house uses around 7 during daylight. Roughly 7 kWh spare to charge with. | 7–8 kWh |
| Limit 3 Value | Anything up to 14 kWh earns the full rebate rate, so this limit isn’t binding. No constraint from here. | 14 kWh |
The smallest number wins. Supply is the binding limit at 7–8 kWh.
That 13.5 kWh battery would have spent every winter around half full — the season they most wanted it. Not because it’s a bad battery, but because the roof couldn’t fill it.
So they had two honest options. Fit a 10 kWh battery now and accept it’s sized to the roof. Or add panels at the same time, lift the array, and then the 13.5 kWh makes sense — with the panels and the battery going in on one job, one lot of paperwork, one rebate claim.
They added panels. If nobody had run limit 2, they’d have bought a battery that underperformed for the life of the system and never known why.
Already Been Quoted? Audit It.
Most people reading this already have a number from somebody. Here’s how to tell whether it was worked out or guessed — including if the quote came from us.
Signs it was worked out
- They quoted usable capacity and said so unprompted
- They asked for a full year of bills, or pulled your interval data
- They modelled winter, not just an annual average
- They checked whether your existing solar can fill it
- They showed you what one size smaller would look like
- They named which circuits would stay live in a blackout
- They explained where you land on the rebate bands
Signs it was guessed
- The size came off a package list before anyone saw your bill
- Nominal capacity quoted, or they wouldn’t say which it was
- No mention of whether your array can charge it in winter
- Pushed past 14 kWh with no mention of the rebate taper
- “Go off grid” framing on a grid-connected house
- Urgency about a rebate deadline that doesn’t exist
- Backup promised without naming the circuits or the changeover gear
What's Different Up Here
Sizing advice written for Sydney’s coastal suburbs doesn’t quite transfer up the line.
Winter squeezes from both ends. Households up the Mountains use more power in winter, mostly on heating — and that’s exactly when solar output is lowest and canopy shading is worst. The gap between what you need and what you can charge is widest in July. Size for that month, not March.
The cold affects the battery itself. Lithium loses usable capacity and charges more slowly at low temperatures, and the upper Mountains gets genuinely cold. Where it’s mounted — garage versus an exposed external wall — makes a real difference. Our guide to keeping batteries working in the cold is the most-read page on our site, for good reason.
Outages are routine, not rare. Long feeders, storms and bushfire season. If backup matters, that shapes the sizing and the circuit selection more than anything else here. See blackout protection.
Bushfire zones affect where it can go. BAL requirements influence mounting location. Better raised at quote stage than on install day.
500+ batteries installed across Penrith, Emu Plains, Springwood, Katoomba and Lithgow.
