How to Size Your Solar Battery Correctly

The Three-Step Sizing Method

Step 1: Calculate Your Evening Energy Usage

Your battery needs to cover energy use from sunset to sunrise when solar isn't generating. In Perth, this is typically 6pm to 6am (12 hours).

Step 2: Account for Depth of Discharge

Most batteries shouldn't be fully discharged. Modern LFP batteries allow 90-100% depth of discharge, but it's wise to plan for 80-90% to extend lifespan.

Formula: Required capacity = Evening usage ÷ 0.9
Example: 15.2 kWh ÷ 0.9 = 16.9 kWh battery needed

Step 3: Consider Your Backup Priorities

Do you want to power your entire home during a blackout, or just essentials? This affects sizing:

Common Perth Household Profiles

Small Household (1-2 people, no pool)

• Daily usage: 10-15 kWh
• Evening usage: 6-8 kWh
• Recommended battery: 8-10 kWh
• Examples: BYD HVM 8.3, Sungrow SBR096, Pylontech Force L2 10.65

Medium Household (3-4 people, moderate AC use)

• Daily usage: 20-30 kWh
• Evening usage: 12-15 kWh
• Recommended battery: 13.5-16 kWh
• Examples: Tesla Powerwall 3, Sungrow SBR128, Alpha ESS SMILE-B3

Large Household (4+ people, pool, heavy AC)

• Daily usage: 35-50+ kWh
• Evening usage: 18-25 kWh
• Recommended battery: 20-25+ kWh
• Examples: BYD HVM 22.1, Sungrow SBR192, Multiple Powerwall 3s

Perth-Specific Considerations

Summer Air Conditioning

Perth summers drive significant evening energy use. A 2.5 kW reverse-cycle AC running 4 hours per evening adds 10 kWh to your daily battery requirement. Consider:

• Sizing for peak summer demand (December-February)
• Pre-cooling your home during solar generation hours (3-6pm)
• Using battery power for evening cooling (6-10pm)

Pool Pumps

Run pool pumps during solar generation hours (10am-3pm) rather than from battery. A typical pool pump uses 1-2 kW for 6-8 hours daily - that's 6-16 kWh that shouldn't come from your battery.

DEBS Rebate Optimization

The WA DEBS rebate provides $130/kWh for the first 10 kWh. This creates a sweet spot:

• 10 kWh battery: Maximum $1,300 rebate
• 13.5 kWh battery: Still $1,300 rebate (only first 10 kWh)
• 20 kWh battery: Still $1,300 rebate

The rebate doesn't increase beyond 10 kWh, so size based on your needs, not the rebate cap.

Modular vs Fixed Capacity

Start Small, Expand Later (Modular)

Best for: Uncertain usage, budget constraints, future-proofing

• Pros: Lower upfront cost, can add capacity later, flexible
• Cons: Higher per-kWh cost, expansion not always possible years later
• Examples: BYD HVM, Pylontech Force L2, Sungrow SBR

Right-Size From Day One (Fixed)

Best for: Known usage patterns, long-term planning, premium features

• Pros: Better value per kWh, integrated design, simpler installation
• Cons: Higher upfront cost, can't expand if needs change
• Examples: Tesla Powerwall 3 (13.5 kWh), Enphase IQ 10 (10.08 kWh)

Common Sizing Mistakes

Mistake 1: Sizing for Total Daily Usage

Your battery doesn't need to cover 24-hour usage - solar handles daytime. Only size for evening/overnight consumption (typically 50-60% of daily usage).

Mistake 2: Ignoring Seasonal Variation

Perth's summer AC demand can double evening energy use. Size for summer peaks, not winter averages.

Mistake 3: Over-Sizing for Blackouts

Blackouts in Perth are rare (average 1-2 per year, lasting 1-3 hours). Don't oversize purely for backup unless you have medical equipment or work-from-home requirements.

Mistake 4: Under-Sizing for VPP Events

Synergy VPP events (up to 30/year) discharge your battery for grid support. If you're participating in VPP, add 20-30% capacity buffer to ensure you still have power for your home.

Use Our Calculator

Our battery calculator uses your actual usage data and Perth-specific factors to recommend the optimal battery size. It accounts for:

• Your daily and evening energy consumption
• Existing solar system size
• Backup priorities (essential vs whole-home)
• VPP participation requirements
• DEBS rebate optimization
• Budget constraints