Start With Your Electricity Bill, Not a Salesperson
The right system size is determined by how much electricity your household uses, not by what a salesperson tells you sounds impressive. Your electricity bill tells you everything you need to know. Look for the section that shows your daily average consumption in kWh. Most NSW households fall between 15–30 kWh per day.
If your daily usage isn't on your bill, divide your quarterly consumption (listed in kWh) by 90. That's your average daily usage.
Quick check: A quarterly bill showing 2,700 kWh ÷ 90 days = 30 kWh/day average usage. That's a high-consumption household that would benefit significantly from both solar and battery storage.
Choosing the Right Solar System Size
Solar panels generate electricity during daylight hours. The goal is to generate roughly what you consume during the day, with enough surplus to either export to the grid or charge a battery for overnight use.
The basic rule of thumb
A 6.6kW solar system generates approximately 25–28 kWh per day in Sydney under average conditions. A 10kW system generates approximately 38–42 kWh per day. These figures vary based on roof pitch, orientation, and shading.
| Daily Usage | Recommended Solar Size | Typical Household |
|---|---|---|
| 10–15 kWh/day | 5–6.6 kW | 1–2 people, small home |
| 15–25 kWh/day | 6.6–10 kW | Family of 3–4, medium home |
| 25–35 kWh/day | 10–13 kW | Large family, pool, EV |
| 35 kWh+ /day | 13–20 kW+ | Very large home, business |
North-facing vs east/west-facing roofs
A north-facing roof in Sydney will generate roughly 15–20% more electricity than the same panels facing east or west. If your best roof space faces east or west, you may want to size up by 10–15% to compensate. South-facing panels are generally not recommended in NSW as they can generate 30–40% less than north-facing equivalents.
Don't undersize to save money upfront
One of the most common mistakes is installing a 5kW system to keep costs down when a 10kW system would have paid back 60% faster due to greater bill reduction. The difference in cost between a 6.6kW and a 10kW system is often only $1,500–$2,500, while the difference in daily generation is roughly 15 kWh. At Sydney electricity rates of around 30–35 cents per kWh, that's an extra $1,600–$1,900 per year in savings — meaning the larger system pays back the price difference in under 18 months.
NSW-specific tip: Federal STC rebates are proportional to system size. A larger system attracts proportionally more STCs, so the after-rebate price difference between a 6.6kW and 10kW system is often smaller than you'd expect.
Choosing the Right Battery Size
Battery sizing is about one thing: how much electricity do you use after the sun goes down?
Most households use roughly 40–60% of their daily electricity after sunset — cooking dinner, running the dishwasher, charging devices, watching TV, running the aircon in summer evenings. For a 20 kWh/day household, that's approximately 8–12 kWh of overnight consumption.
The sizing formula
As a starting point: battery size = overnight consumption × 1.2 (the extra 20% accounts for battery efficiency losses and depth of discharge limits).
| Daily Usage | Est. Overnight Consumption | Recommended Battery |
|---|---|---|
| 15 kWh/day | ~6–8 kWh | 10 kWh battery |
| 20 kWh/day | ~8–12 kWh | 10–15 kWh battery |
| 30 kWh/day | ~12–18 kWh | 15–25 kWh battery |
| 40 kWh/day | ~16–24 kWh | 25–53 kWh battery |
What about our large 50–53kWh battery systems?
Our flagship Solis Alpha 53.2kWh and Growatt 50kWh systems are designed for high-consumption households (35 kWh+/day), homes with electric vehicles, or households wanting 2+ days of backup power independent of the grid. They're also the sweet spot for the Federal Cheaper Home Batteries Program rebate before 1 May 2026 — the rebate at ~$300/kWh usable means these systems attract $10,000–$15,000+ in rebates at current rates.
Can I add more battery capacity later?
Yes — most modern hybrid inverters (including those we install) are designed to accept additional battery modules. However, it's generally more cost-effective to size correctly upfront because the labour cost of installation is the same whether you add one module or three. If you think your usage might grow (EV in the next 2 years, growing family), size up now.
Real example: A Castle Hill family of 4 with a pool and 30 kWh/day usage chose a 10kW solar + 53.2kWh Solis Alpha battery. Their quarterly electricity bill went from $820 to $38. The system paid back in under 4 years.
Does My Roof Affect System Size?
Absolutely. Before finalising a system size, check these factors:
- Available roof space: A standard 400W panel takes up approximately 1.7m². A 6.6kW system (16 panels) needs roughly 28m² of usable roof space.
- Shading: Even partial shading from a chimney, tree or neighbouring building can reduce a system's output by 20–40%. A good installer will assess shading using satellite imagery and site inspection.
- Roof pitch and orientation: Ideally 15–30° pitch, north-facing. Flat roofs can use tilt frames to optimise angle.
- Structural integrity: Older tile roofs (30+ years) may need assessment before panel installation.
Single Phase vs Three Phase — Does It Matter?
Most NSW homes are single phase. A single phase inverter and battery work perfectly in a single phase home. They also work in three-phase homes — the battery and inverter operate on one phase, and net metering at the meter level accounts for solar generation and exports across all phases. You don't need a three-phase inverter to get full benefit from solar and battery in a three-phase home.
Practical Steps to Get Sized Correctly
- Gather 12 months of bills — seasonal variation matters. Summer aircon use can double consumption. You want an annual average, not a single quarter.
- Note your peak usage times — if you're home during the day, a smaller battery may suffice. If everyone leaves at 7am and returns at 6pm, you need a battery that can store most of your solar generation for the evening.
- Consider future changes — planning to buy an EV? Add a heat pump? These add 15–40 kWh/week to consumption. Size for where you'll be in 3 years, not where you are today.
- Get a detailed assessment — a reputable installer will use your actual usage data (not guesswork) to model the exact system performance before recommending anything.
Get a Free System Sizing Assessment
Our team will analyse your actual electricity usage data and recommend the exact system size that maximises your return — without overselling. No obligation, no pressure.
Book Free Assessment →Common Questions
Not always. If you export most of your solar to the grid at low feed-in tariff rates (currently 5–10 cents/kWh in NSW), you're not getting full value. A battery lets you use more of what you generate at full retail value (30–35 cents/kWh). The right balance is a system sized to cover your daytime usage with enough surplus to fill your battery — not generate three times what you consume.
Prioritise the best-performing roof space. A smaller, optimally-placed system often outperforms a larger poorly-oriented one. High-efficiency panels (400W+) generate more per square metre, giving you more generation in a smaller footprint. Ask your installer about panel efficiency ratings.
Adding a battery later is possible but usually costs more in total than installing both at once due to additional labour and potential inverter upgrade costs. More importantly, the Federal Cheaper Home Batteries Program rebate (~$300/kWh) applies now — delaying means risking missing the current rebate rate. If budget allows, do both together.
Online calculators give a rough ballpark — usually accurate within 20–30%. For an accurate sizing recommendation, you need an installer to analyse your actual bill data, assess your roof, and model your specific usage patterns. A free on-site or virtual assessment from a reputable company costs you nothing and gives you a much more reliable recommendation.