Sizing a Solar Battery for a Sydney Household: Achieving 100% Coverage

For Sydney households aiming to maximize solar energy use and minimize grid reliance, selecting the right solar battery size is critical. A well-sized battery, paired with a hybrid inverter, can achieve up to 100% coverage of household electricity needs, leveraging excess photovoltaic (PV) energy and accounting for seasonal variations and weather challenges like rainy days. This blog post explores the science behind battery sizing, analyzes Sydney-specific electricity consumption and solar export data, and evaluates three models for achieving 100% coverage: using existing excess PV, upgrading the PV system, and designing for near off-grid capability. We’ll also address the impact of rainy days and the role of hybrid inverters in providing backup power.

Understanding Sydney Household Electricity Consumption

Sydney’s electricity consumption varies by household size, lifestyle, and season. According to data from the Australian Energy Regulator and solar industry sources, the average daily electricity usage in Sydney is:

• 2-person household: 14.35 kWh/day, ~$523.97/quarter
• 3-person household: 17.43 kWh/day, ~$618.45/quarter
• 4-person household: 20.03 kWh/day, ~$698.26/quarter
• 5+ person household: 24.68 kWh/day, ~$840.89/quarter

For this analysis, we’ll use a 4-person household consuming 20 kWh/day as a baseline, reflecting a typical Sydney family with moderate energy use (e.g., air conditioning, appliances, lighting). Electricity costs average 33.62¢/kWh, with a daily supply charge of 91.78¢. Seasonal variations impact consumption, with higher usage in summer (November–March) due to cooling and lower usage in winter (June–August).

Solar Production and Export in Sydney

Sydney’s solar potential is strong, with an average of 5.1 peak sun hours (PSH) per day annually, though this drops to ~3.5 PSH in winter and rises to ~6.5 PSH in summer. A typical 6.6 kW PV system generates:

• Annual average: ~26 kWh/day (6.6 kW × 5.1 PSH × 0.77 efficiency factor, accounting for inverter and system losses).
• Summer: ~30 kWh/day (6.6 kW × 6.5 PSH × 0.77).
• Winter: ~18 kWh/day (6.6 kW × 3.5 PSH × 0.77).

Without a battery, a 6.6 kW system covers ~39% of a 20 kWh/day household’s energy needs, with excess energy exported to the grid (6–8 kWh/day in summer, less in winter) at a low feed-in tariff (8–13¢/kWh in NSW). A battery can store this excess, increasing self-consumption to 80–100%.

Impact of Rainy Days

Sydney averages 120 rainy days per year, with ~8–12 days per month, particularly in autumn (March–May) and winter (June–August). On overcast days, solar output can drop to 10–20% of normal (e.g., 2–5 kWh/day for a 6.6 kW system). Batteries must provide autonomy for 2–3 consecutive rainy days to ensure reliability.

Summary Table: Battery Sizing Models for Sydney

Science of Battery Sizing and Operation

Battery sizing depends on energy storage capacity (kWh), depth of discharge (DoD), round-trip efficiency, and integration with a hybrid inverter. The science involves electrochemical and electrical principles:

• Lithium-Ion Chemistry (LiFePO4): Most solar batteries, like the Fox ESS EQ4800 (4.66 kWh/module), use LiFePO4 cells for their high cycle life (6,000–10,000 cycles at 90% DoD) and thermal stability. Lithium ions move between the cathode (lithium iron phosphate) and anode (graphite) through an electrolyte, storing/releasing energy. LiFePO4’s robust structure minimizes degradation from repeated cycling, unlike NMC chemistries.
• Battery Management System (BMS): The BMS monitors voltage, current, temperature, SoC, and SoH, using algorithms to balance cells, prevent overcharging (avoiding electrolyte decomposition), and limit deep discharges (preventing lithium plating). It optimizes charging to maintain 15°C–25°C, reducing SEI layer growth.
• Round-Trip Efficiency: LiFePO4 batteries achieve 90–95% efficiency, meaning 5–10% of energy is lost as heat during charge/discharge cycles. High-efficiency inverters (e.g., 97% for Fox ESS H1(G2)) minimize additional losses.
• Depth of Discharge (DoD): Limiting DoD to 60–80% (e.g., 20%–80% SoC) extends cycle life by 30–50%, as shallow cycles reduce electrode stress and SEI growth.
• C-Rate: Charging/discharging at low rates (e.g., 0.5C, or 2.33 kW for a 4.66 kWh battery) minimizes heat and mechanical stress, preserving cell integrity.

Three Models for 100% Coverage

We evaluate three battery sizing models for a 4-person Sydney household (20 kWh/day), aiming for 100% coverage (minimal grid reliance) while addressing rainy days and inverter requirements. Each model assumes a hybrid inverter with backup capability (e.g., Fox ESS H1(G2), 5–10 kW, 97% efficiency).

The smartest battery size for a Sydney household depends on your goals:

• For most, a 5–10 kWh battery with a 6.6 kW solar system maximises savings and provides basic backup.

• For those seeking full backup or off-grid resilience, 30–40 kWh of storage and a much larger PV array are required.

• Always size for winter and rainy periods, and ensure your hybrid inverter matches your backup needs.

Model 1: Using Excess PV Available

• Concept: Leverage excess PV from an existing 6.6 kW system to charge a battery, covering 100% of daily needs (20 kWh).
• Battery Size Calculation:
  • Daily need: 20 kWh.
  • Excess PV: In summer, a 6.6 kW system generates ~30 kWh/day, with ~10 kWh exported after covering daytime loads (10–12 kWh). In winter, ~18 kWh/day, with ~3–5 kWh excess.
  • Battery size: To cover 20 kWh/day, a battery must store enough to supplement low winter production and rainy days (2–3 days autonomy). Assuming 90% DoD and 95% efficiency:
    • Daily usable capacity: 20 kWh ÷ 0.95 ÷ 0.9 ≈ 23.4 kWh.
    • For 2 rainy days (40 kWh total, assuming 5 kWh/day PV production): 40 kWh ÷ 0.95 ÷ 0.9 ≈ 46.8 kWh.
  • Recommended battery: 3–4 Fox ESS EQ4800 modules (3 × 4.66 kWh = 14 kWh usable at 90% DoD, or 4 × 4.66 kWh = 18.64 kWh usable). For full coverage, a 5-module system (23.3 kWh, ~21 kWh usable) is ideal.
• Hybrid Inverter: A 5 kW hybrid inverter (e.g., Fox ESS H1(G2)) suffices for a 6.6 kW PV system, supporting backup for critical loads (e.g., 3–5 kW). A bypass switch is recommended to maintain grid power if the inverter fails.
• Performance:
  • Summer: Excess PV (10 kWh) charges the battery, covering evening/overnight loads. 100% coverage is achievable.
  • Winter: Limited excess (3–5 kWh) requires grid charging during off-peak periods (e.g., 10 PM–7 AM, ~25¢/kWh) to reach 20 kWh.
  • Rainy days: A 23.3 kWh battery provides ~2 days autonomy (20 kWh/day), assuming minimal PV input (5 kWh/day). Grid charging supplements during extended overcast periods (e.g., 8 rainy days, as seen in some winters).
• Science: The BMS maintains 20%–80% SoC to extend cycle life to ~9,000 cycles, reducing SEI growth. Low C-rate charging (0.5C, ~2.33 kW) minimizes heat. The inverter’s MPPT optimizes solar harvest, ensuring efficient battery charging.
• Pros: Cost-effective, uses existing PV system, achieves high self-consumption (80–90%).
• Cons: Winter and rainy-day coverage may require grid charging, limiting full energy independence.
• What to Look For: Battery with scalable modules (e.g., Fox ESS EQ4800), BMS with TOU tariff integration, and inverter with high efficiency and backup capability.

Model 2: Upgrading PV System

• Concept: Upgrade the PV system to increase excess energy, enabling 100% coverage with a smaller battery, ideal for households with roof space.
• Battery Size Calculation:
  • Upgraded PV: A 10 kW PV system generates ~39 kWh/day annually (10 kW × 5.1 PSH × 0.77), ~47 kWh/day in summer, and ~27 kWh/day in winter.
  • Excess PV: After covering 12 kWh/day daytime loads, ~27 kWh (summer) and ~15 kWh (winter) are available for battery charging.
  • Battery size: For 20 kWh/day, a smaller battery suffices due to increased excess PV:
    • Daily usable capacity: 20 kWh ÷ 0.95 ÷ 0.9 ≈ 23.4 kWh.
    • For 2 rainy days (40 kWh total, 5 kWh/day PV): ~46.8 kWh, but higher winter PV reduces grid reliance.
    • Recommended: 3 EQ4800 modules (14 kWh, ~12.6 kWh usable) for daily needs, or 4 modules (18.64 kWh, ~16.8 kWh usable) for rainy days.
• Hybrid Inverter: An 8–10 kW inverter (e.g., Sungrow SH-RS series) handles the larger PV array, with dual MPPTs and smart PID recovery to minimize panel degradation. Backup capability supports critical loads.
• Performance:
  • Summer: Excess PV (~27 kWh) easily charges a 14–18.64 kWh battery, achieving 100% coverage.
  • Winter: ~15 kWh excess PV covers most needs, with minimal grid charging on rainy days.
  • Rainy days: A 14–18.64 kWh battery provides 1–2 days autonomy, with grid charging for extended overcast periods.
• Science: Increased PV output reduces battery cycling frequency, lowering electrochemical stress. The BMS optimizes charging to 80% SoC, and the inverter’s smart algorithms prioritize solar over grid power, maximizing efficiency.
• Pros: Higher PV output reduces battery size and cost, achieves near-100% coverage year-round.
• Cons: Requires roof space and higher upfront cost (~$8,000–$10,000 for 10 kW PV).
• What to Look For: High-efficiency panels (e.g., monocrystalline, >20% efficiency), inverter with oversizing capability (e.g., 1.3x PV capacity), and battery with robust BMS.

Model 3: Near Off-Grid with Maximum Autonomy

• Concept: Design a system for near off-grid operation, covering 100% of needs during the lowest production period (winter, rainy days), minimizing grid reliance.
• Battery Size Calculation:
  • Lowest period: Winter (3.5 PSH) with 3 consecutive rainy days, PV output ~2–5 kWh/day. Daily need: 20 kWh/day.
  • Battery size: For 3 days autonomy (60 kWh total, assuming 5 kWh/day PV):
    • Usable capacity: (60 kWh – 15 kWh PV) ÷ 0.95 ÷ 0.9 ≈ 52.6 kWh.
    • Recommended: 12 EQ480 Alison modules (12 × 4.66 kWh = 55.92 kWh, ~50.3 kWh usable).
  • PV size: A 13 kW PV system generates ~50 kWh/day annually, ~61 kWh/day in summer, and ~35 kWh/day in winter, ensuring sufficient excess for charging.
• Hybrid Inverter: A 10–12 kW inverter (e.g., Goodwe ES series) with four MPPTs and V2L compatibility for EV backup. A bypass switch and backup box ensure reliability during inverter failure.
• Performance:
  • Summer: Excess PV (~40 kWh) fully charges the battery, achieving 100% coverage.
  • Winter: ~20–25 kWh excess PV covers daily needs, with the large battery handling rainy days.
  • Rainy days: 55.92 kWh battery provides ~3 days autonomy, covering 8+ overcast days with minimal grid use.
• Science: Large battery capacity reduces DoD per cycle (e.g., 40% DoD for 20 kWh), extending cycle life to ~10,000 cycles. The BMS and inverter coordinate to prioritize solar charging, minimizing grid draw. V2L integration allows EV batteries (e.g., 70 kWh) as additional backup, reducing generator reliance.
• Pros: Near off-grid independence, robust rainy-day coverage, future-proof for EV charging.
• Cons: High cost ($25,000–$45,000 for PV and battery) and significant roof space (50 m²).
• What to Look For: Large-capacity, modular battery (e.g., Fox ESS Energy Cube), inverter with high power output and VPP compatibility, and professional load analysis for sizing.

Addressing Rainy Days and Inverter Backup

• Rainy Days: Sydney’s 120 rainy days/year reduce PV output, requiring batteries sized for 2–3 days autonomy. Model 1 (23.3 kWh) handles 2 days, Model 2 (14–18.64 kWh) relies on grid charging for extended periods, and Model 3 (55.92 kWh) covers 3+ days, ideal for near off-grid setups.
• Hybrid Inverter Backup: All models use hybrid inverters with backup capability (<20ms switching) for critical loads (e.g., fridge, lights). Model 3’s larger inverter supports whole-house backup. A bypass switch is critical to maintain grid power during inverter failure, especially for Models 1 and 2.
• Off-Grid Consideration: Model 3 approaches off-grid capability but retains grid connection for reliability. True off-grid systems require larger batteries (e.g., 70 kWh) and backup generators, increasing costs significantly.

What to Look for in Equipment

When selecting a battery and hybrid inverter for Sydney households:

• Battery:
  • Chemistry: LiFePO4 for 6,000–10,000 cycles and safety (e.g., Fox ESS EQ4800).
  • BMS: Real-time monitoring, cell balancing, TOU tariff integration, and low C-rate support.
  • Scalability: Modular design for future expansion.
  • Warranty: 10+ years with >70% capacity retention.
• Hybrid Inverter:
  • Efficiency: >95% conversion, >99% MPPT efficiency.
  • Compatibility: Matches battery voltage (e.g., 96–512V for EQ4800) and communication protocols (CAN/RS485).
  • Backup: Fast switching (<20ms) and bypass switch for reliability.
  • Software: User-friendly monitoring (e.g., Fox Cloud 2.0) with tariff optimization and local control (e.g., Modbus).
• System Design:
  • Use load calculators (e.g., SunSPOT) to analyze consumption and excess PV.
  • Ensure professional installation by licensed electricians with Certificate of Compliance (CCEW).

Conclusion

Sizing a solar battery for a Sydney household requires balancing daily consumption (20 kWh for a 4-person household), excess PV availability, and rainy-day autonomy. Model 1 leverages existing 6.6 kW PV systems with a 23.3 kWh battery for 80–90% coverage, supplemented by grid charging. Model 2 upgrades to a 10 kW PV system with a 14–18.64 kWh battery, achieving near-100% coverage with lower costs. Model 3 uses a 13 kW PV system and 55.92 kWh battery for near off-grid independence, covering 3+ rainy days. The science of LiFePO4 chemistry, BMS optimization, and high-efficiency inverters ensures longevity and performance. By selecting modular batteries, high-efficiency inverters, and smart software, Sydney households can achieve 100% coverage, reduce bills, and enhance energy independence. Again it’s best to talk to a Ssolar designer so please ring solarwind technology and will walk you through it.