What size battery do I need? is the wrong question. The better one is: What size battery makes financial sense for my home?
Because you could install a 20kWh battery and theoretically power your entire home for a full day without the sun. But you’d spend $25,000–$30,000 and wait 15+ years to break even. That’s not smart; it’s over-engineering.
Here’s how to actually size a battery that’ll save you money without burning through capital you don’t need to spend.
The Math: Understanding Your Night-Time Load
Your battery’s job is simple: store solar energy from the day and power your home when the sun’s down. So the first question isn’t “how big?” It’s “how much power do you actually use at night?”
Step 1: Find Your Night-Time Usage
Look at your last quarter’s electricity bill. You’ll see a breakdown like:
- Daytime usage: 8am–5pm (when solar’s working)
- Night-time usage: 5pm–8am (when you need grid power or battery)
- Total: Your quarterly consumption
Let’s work with a real Melbourne example:
Sarah’s household:
- Quarter bill: $380
- Estimated quarterly usage: 2,900 kWh
- Roughly 970 kWh per month
- Night-time usage: ~30% of daily consumption = ~290 kWh per month
Divided across ~14 hours of night (5pm to 7am), that’s about 20 kWh per night.
But here’s the critical bit: peak night-time power demand, not total consumption. You don’t use power evenly. You use it in peaks:
- Evening peak (5pm–10pm): Aircon, cooking, hot water, lighting, TV
- Baseload (10pm–7am): Fridge, some HVAC, standby loads
- Morning peak (7am–8am): Showers, breakfast, washing machine
Sarah’s peaks average 8–10 kW simultaneous demand in the evening, but her overall night energy is 20 kWh.
A battery needs to handle *both*: peak instantaneous power AND total energy storage.
Step 2: Understand Usable Capacity vs. Rated Capacity
A “10kWh battery” doesn’t give you 10 kWh of usable power every night. It’s the *total* capacity, but:
- Most batteries only discharge to 80–90% (below that, lifespan degrades)
- You want to preserve some charge for unexpected demand spikes
- Degradation over time means you get slightly less over the battery’s life
- Real usable capacity: roughly 80% of rated capacity.
A 10kWh battery realistically gives you 8 kWh usable per night, every night, for 15 years.
Common Battery Sizes for Melbourne Homes
5kWh Battery
- Best for: Light users, single-person homes, apartment dwellers with solar
- Night energy: ~4 kWh usable
- Cost: $10,300–$12,800 installed (after rebates)
- Who it suits: Pre-solar bill under $150/quarter, minimal evening load
- Payback: 10–14 years
10kWh Battery (Most Common)
- Best for: Families, homes with 2–4 people, regular evening usage
- Night energy: ~8 kWh usable
- Cost: $12,800–$16,500 installed (after rebates)
- Who it suits: Pre-solar bill $250–$450/quarter, moderate to high evening load
- Payback: 8–12 years
15kWh Battery
- Best for: Large families, homes running pools/spas, homes with high baseload
- Night energy: ~12 kWh usable
- Cost: $17,500–$22,000 installed (after rebates)
- Who it suits: Pre-solar bill $450+/quarter, or multiple air-con units
- Payback: 10–15 years
20kWh+ Battery (Rare for Residential)
- Best for: Serious off-grid aspirations, homes with backup resilience requirement, commercial setups
- Cost: $22,000–$30,000+
- Payback: 15+ years (or never, financially)
- Real talk: Most Melbourne residential homes never need this
Real Melbourne Examples
Example 1: Sarah in Balwyn (4-bedroom, family of 4)
- Pre-solar bill: $420/quarter
- Estimated night usage: 18–22 kWh
- Evening load: Aircon (summer), heating (winter), kids’ devices, cooking
- Recommendation: 10kWh battery
- Why: Captures most evening demand, payback is realistic (9–10 years), fills the “peak shift” need
- Usable capacity: 8 kWh covers typical nights; peak demand (AC startup) handled by battery power limit (5–6 kW continuous)
Example 2: Mark in Chadstone (2-bedroom, couple, both work)
- Pre-solar bill: $280/quarter
- Night usage: ~8–10 kWh
- Evening load: Low—home after 6:30pm, mostly entertainment, bedroom heating/cooling
Recommendation: 5kWh battery
- Why: You’re not home most of the day; evening load is modest; 5kWh covers most nights
- Cost-benefit: $4,000 cheaper than 10kWh; payback is still reasonable (11–13 years)
Example 3: Linda in Werribee (3-bedroom, remote worker, pool)
- Pre-solar bill: $520/quarter
- Night usage: 25–30 kWh (pool equipment, AC all day, high baseload)
- Evening load: Very high—AC runs evenings, pool pump 6 hours/day
Recommendation: 15kWh battery
- Why: Your night load is significant; baseload is real; 10kWh would deplete too fast
- Reality: Even at 15kWh, you’re likely exporting surplus solar in summer and drawing grid power some winter evenings
- Payback: 12–14 years (longer, but the load justifies it)
How to Actually Figure Out Your Size
The Quick Estimate
- Find your last quarter’s bill (total kWh)
- Divide by 90 days to get daily consumption
- Assume 30–35% of that is night-time (5pm–7am)
- That’s your daily night-time kWh
Result: This number is your baseline. If you need 15 kWh at night, a 15kWh battery is undersized (remember: only 80% usable, plus degradation).
The Better Way: Let a Professional Assess
We analyze:
- Your actual quarterly usage pattern
- When you’re home and using power
- Seasonal swings (aircon in summer, heating in winter)
- Peak instantaneous demand (when your AC compressor kicks in, how much power does everything running simultaneously need?)
- Your existing solar system size (bigger solar = more to store)
This takes an hour, but it’s the difference between a battery that saves you real money and one that sits half-charged most nights.
Melbourne Seasonal Reality
Your night-time load changes by season:
- Summer (Dec–Feb): High evening AC demand (8–12 kW peak). You’ll use the battery hardest.
- Winter (Jun–Aug): Lower evening load, but less sun means less generation. You’ll rely on battery + grid.
- Spring/Autumn: Moderate, balanced loads.
A battery sized for summer’s peak might seem oversized in winter. A battery sized for winter might struggle in summer’s evening rush. The real size is a compromise—and it’s why “one-size-fits-all” advice doesn’t work.
The Financial Reality Check
Here’s the honest bit: bigger batteries have longer paybacks.
Adding 5kWh to a battery costs roughly $3,500–$4,500 (the battery itself, plus some integration). But it might only save you $150–$200 per year in additional energy costs, because you’re already capturing the evening peak with a smaller size.
That $4,000 extra outlay takes 20+ years to pay back. It’s usually not worth it.
The sweet spot for most Melbourne homes is 10kWh—big enough to capture full evening demand, small enough that payback is realistic.
Common Sizing Mistakes
Mistake 1: “I’ll oversize so I have backup power for outages.”
Reality: A battery sized for financial payback isn’t really an outage-backup system. If outages are your driver, say that upfront—you might choose a smaller battery *with* backup switch for resilience, rather than a larger system for storage alone.
Mistake 2: “I want to go off-grid.”
Reality: Melbourne’s cloudy winters mean an off-grid home would need a 30–50 kWh battery system and a backup generator. Cost: $40,000+. Not realistic. Grid is your safety net.
Mistake 3: “Bigger is always better.”
Reality: No. Oversizing costs you money upfront, extends payback into the irrelevant timeframe, and leaves capacity unused most nights. You’re paying for theoretical benefit, not real savings.
Your Next Step
The only way to truly know what size you need is to get your home assessed. We’ll look at your bill, your home’s usage pattern, your existing solar, your evening loads, and your financial timeline.
Then we’ll recommend a size that makes sense—not the biggest, not the smallest, but right for your home and wallet.
Book a free battery assessment. We’ll show you real sizing for your home and honest payback timelines. No surprises, no overselling.
