What size solar battery do I need?

For backup of essential loads (refrigerator, lights, phone charging, ~500–1,000W), a 10–13 kWh battery like the Tesla Powerwall is usually sufficient for 8–16 hours. For whole-home backup or 24-hour coverage, 20–40 kWh is recommended. Battery sizing also depends on how much solar you have to recharge it.

How much does a home solar battery cost?

Home battery systems typically cost $8,000–$15,000 installed for a single 10–13 kWh unit, before incentives. The federal residential ITC (Section 25D) expired December 31, 2025 — batteries installed with solar after that date under homeowner ownership no longer qualify for the federal credit. Larger capacity systems (20–40 kWh) run $18,000–$35,000+. Prices have been declining roughly 10% annually.

What size solar battery do I need for whole-home backup?

Whole-home backup for a typical US home (900 kWh/month) requires 30–40 kWh of battery capacity to cover 24 hours without solar input. At $800–1,000/kWh installed, that's $24,000–40,000 for battery alone. Most homeowners choose partial-home backup (10–20 kWh, $8,000–20,000) covering critical loads like lights, refrigerator, outlets, and HVAC.

How many years does a solar battery last?

Most home batteries are warranted for 10 years at 70–80% of original capacity. Real-world lifespan is typically 10–15 years. Tesla Powerwall 3 and Enphase IQ are rated for 4,000+ cycles at 70% capacity. With one cycle per day, that's over 10 years of daily use. Battery chemistry (LFP vs NMC) affects both safety and cycle life.

Solar Battery Size Calculator

Determine the right battery capacity for your solar-plus-storage system. Enter your daily energy needs, solar production, and backup goals to find out how many kilowatt-hours of storage you need.

Daily Load (kWh)

Solar Production (kWh/d)

Backup Days (days)

Depth of Discharge (%)

About This Calculator

The Solar Battery Size Calculator helps you determine how much energy storage capacity your home needs to meet your backup and self-consumption goals. Battery sizing involves two distinct considerations: covering the daily shortfall when solar production falls short of usage, and providing backup power during grid outages. This calculator combines both by calculating your net daily energy gap, multiplying by your desired backup days, and then working backward through depth-of-discharge and efficiency factors to arrive at the raw battery capacity you should shop for.

Depth of discharge (DoD) is the percentage of a battery's total capacity that can be safely used. Most lithium iron phosphate (LFP) batteries like the Tesla Powerwall or Enphase IQ Battery are rated at 80–100% DoD, while older lithium-ion chemistries operate best at 80% or less. If your usable capacity need is 10 kWh but your battery only allows 80% DoD, you need a 12.5 kWh battery to get those 10 usable kWh. Battery round-trip efficiency (typically 90–97% for modern lithium batteries) adds another small overhead on top of that.

To determine your daily shortfall accurately, compare your actual daily electricity usage against your solar system's average daily production. Usage data comes from your utility bill (annual kWh divided by 365); solar production comes from your installer's simulation or the Solar Energy Production Calculator. Note that backup needs differ from self-consumption needs — for outage backup, you may only need to power critical loads (refrigerator, lights, medical equipment) rather than your whole home, which can dramatically reduce the required battery size.

Battery storage costs have dropped significantly but still run $800–$1,200 per usable kWh installed. While the federal residential tax credit (Section 25D) expired December 31, 2025, some states still offer incentives for battery storage. It is still worth evaluating whether adding storage at the time of solar installation makes financial sense. Adding batteries later typically costs 20–30% more than including them in the original installation.

Calculations based on NREL solar modeling data and industry-standard assumptions, built and maintained by the independent SolarToolsOnline research team.

Estimates only — not financial, tax, or legal advice. Verify important results with a licensed solar installer or financial professional before making decisions.