What Size Solar Battery Do I Need for My Home?
The right battery size depends on your objective. For hybrid backup — powering essential loads during a short outage — a household using 9,000 kWh/year might need about 5.1 kWh of battery storage. For off-grid autonomy over 2 days — powering everything without the grid — the same household might need about 40.6 kWh. These are very different use cases.
Key Factors
Battery sizing depends on what you are trying to achieve:
- Hybrid / everyday backup — sized to cover essential loads for a few hours during an outage. Depends on protected load share and backup duration.
- Off-grid autonomy — sized to cover total daily consumption for multiple days without sun. Depends on daily consumption and autonomy days.
- Usable battery fraction — the percentage of nominal capacity you can actually use (typically 80–95%).
- Battery-to-load efficiency — discharge losses, typically 85–95%.
Worked Examples
The table below shows two contrasting battery-sizing scenarios computed with the Home Solar battery engine:
Sizing mode — Hybrid backup
Sizing mode — Off-grid autonomy
Examples use the Home Solar battery sizing engine. No hidden margins or safety factors are applied.
How the Battery Size Is Calculated
For hybrid mode, the engine calculates: protectedDailyEnergy = averageDailyConsumption × (backupLoadShare ÷ 100); requiredLoadEnergy = protectedDailyEnergy × (backupDuration ÷ 24). For off-grid mode: requiredLoadEnergy = averageDailyConsumption × autonomyDays. Both then apply: requiredStoredEnergy = requiredLoadEnergy ÷ dischargeEfficiency; recommendedBattery = requiredStoredEnergy ÷ usableFraction, rounded up to the nearest 0.1 kWh. No hidden margins or safety factors — only upward 0.1 kWh rounding.
Why These Results Are So Different
The hybrid example covers only essential loads (~50% of consumption) for 8 hours — it needs about 5.1 kWh. The off-grid example covers ALL consumption for 2 full days — it needs about 40.6 kWh. The difference is roughly 8×, because off-grid means no grid fallback: the battery must power everything, day and night, for the entire autonomy period. This is why off-grid battery systems are dramatically more expensive than hybrid backup systems.
Limitations & Assumptions
- These are preliminary planning estimates — actual battery requirements depend on load profiles, seasonality, and temperatures.
- Self-consumption percentage, self-sufficiency percentage, and grid import/export are NOT calculated — these require detailed load-profile modeling.
- Exact backup runtime depends on which appliances are running during the outage, not just total consumption.
- Monthly production variation is NOT factored in — off-grid systems must account for worst-month solar production, not annual averages.
- Exact off-grid inverter size is not estimated here — off-grid inverter sizing requires load and surge data.
- Battery chemistry (LiFePO4, NMC, lead-acid) affects usable fraction, cycle life, and cost — this estimate does not recommend a specific chemistry.
- An installer's final design may differ based on your actual load profile, local regulations, and product availability.
Frequently Asked Questions
Related Tools
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Plan Your Solar + Battery System
The right battery size depends on your objective. For hybrid backup — powering essential loads during a short outage — a household using 9,000 kWh/year might need about 5.1 kWh of battery storage. For off-grid autonomy over 2 days — powering everything without the grid — the same household might need about 40.6 kWh. These are very different use cases.