Solar Battery Size Calculator

Determine how much battery storage you need for your solar system. Enter your daily energy usage, desired backup duration, depth of discharge, and round-trip efficiency to calculate the optimal battery capacity and how many units you need.

Daily Usage (kWh)

Desired Backup Duration (hours)

Depth of Discharge (%)

Round-Trip Efficiency (%)

How to Use

1 Calculate your daily kWh usage from your monthly electricity bill (divide by 30).

2 Decide how many hours of backup power you need during an outage.

3 Set depth of discharge based on your battery chemistry (80% for lithium-ion).

4 Keep round-trip efficiency at 90% for lithium-ion or adjust for your battery type.

5 Review the recommended capacity and number of battery units needed.

Formula

Energy For Backup daily_kwh_usage × (backup_hours / 24)

Required Kwh energy_for_backup / (depth_of_discharge / 100) / (round_trip_efficiency / 100)

Recommended Capacity required_kwh × 1.2

Tesla Powerwalls ceil(recommended_capacity / 13.5)

Cost Estimate recommended_capacity × $800

Battery Backup for an Average Home

A home using 30 kWh/day wanting 12 hours of backup with 80% DoD and 90% efficiency: Energy for backup = 30 × (12/24) = 15 kWh, Required capacity = 15 / 0.80 / 0.90 = 20.83 kWh, Recommended = 20.83 × 1.2 = 25 kWh, Powerwalls = ceil(25/13.5) = 2 units, Cost ≈ $20,000.

Why It Matters

Proper battery sizing ensures you have enough stored energy to power essential appliances during outages without overspending. Battery storage also maximizes your solar investment by storing excess daytime production for use at night or during peak rate periods.

Who Uses This Calculator?

• Homeowners and renters estimating electricity costs, appliance usage, solar savings, or EV charging expenses.
• Solar shoppers, EV owners, and households comparing energy upgrades before requesting quotes or changing usage habits.
• Anyone who wants a quick estimate of home energy costs using clear assumptions.

Frequently Asked Questions

What is depth of discharge (DoD)?

Depth of discharge is the percentage of a battery's total capacity that has been used. For example, a 10 kWh battery discharged to 80% DoD has used 8 kWh. Regularly discharging beyond the recommended DoD shortens battery lifespan.

What is round-trip efficiency?

Round-trip efficiency measures how much energy you get out of a battery compared to what you put in. A 90% round-trip efficiency means for every 10 kWh stored, you retrieve 9 kWh. The remaining 10% is lost as heat during charging/discharging.

How long does a Tesla Powerwall last?

A Tesla Powerwall is warrantied for 10 years with unlimited cycles for daily use. It retains about 70-80% capacity after 10 years. The Powerwall 3 has 13.5 kWh usable capacity and can output 11.5 kW continuous power.

Can I add more batteries later?

Yes, most modern battery systems are modular and can be expanded. However, it's usually more cost-effective to size your system correctly from the start. Mixing old and new batteries may reduce overall efficiency.

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This calculator provides estimates for informational purposes only. Verify results with a qualified professional for important decisions.