Battery system sizing
Battery Size Calculator
Estimate the battery capacity (Ah and kWh) needed to power your electrical appliances for a desired backup runtime, factoring in inverter conversion losses, DOD reserve limits, and planning margin.
Estimate required battery size
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How to Size a Battery Bank for Power Outages
- Enter Continuous Power Load (Watts): Enter the total wattage of all devices running simultaneously or add individual appliances.
- Set Target Backup Duration (Hours): Specify how many hours or days the battery must sustain the load without grid power.
- Choose Battery Chemistry (LiFePO4 vs Lead-Acid): LiFePO4 allows 80%–90% usable DOD, while Lead-Acid/AGM is limited to 50%.
- Select System Voltage (12V / 24V / 48V): Review required Amp-hour (Ah) capacity across voltage configurations.
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Battery Storage Sizing & Backup Load Hierarchy
Multi-source charging, chemical storage management, and pure sine wave inverted backup delivery.
🔌InputGrid / Solar InputPrimary Energy Input
⚡ProtectionSmart BMS ChargerMulti-Stage CC/CV Charging
🔋ReservesBattery Bank12V / 24V / 48V Storage
🔄InverterPure Sine InverterDC to 120V/240V AC (88–92%)
💡ProtectedCritical SubpanelRefrigeration, Medical, Wi-Fi
Engineering Principle: Continuous AC backup runtime is determined by usable Depth of Discharge (DOD) and inverter conversion efficiency.
Common Emergency Backup Battery Sizing Scenarios
Estimated battery capacities needed for common residential power outage durations (assuming 12V LiFePO4 with 90% usable DoD, 88% AC inverter efficiency, and 0% planning margin):
| Backup Scope & Average Load | 4-Hour Outage | 8-Hour Outage | 12-Hour Outage | 24-Hour Outage |
|---|---|---|---|---|
| Essential Communications (Wi-Fi + Phone + LED Lights: 50W) | ~253 Wh (21 Ah) | ~505 Wh (42 Ah) | ~758 Wh (63 Ah) | ~1.52 kWh (126 Ah) |
| CPAP Machine + Phone Charging (70W avg) | ~354 Wh (29 Ah) | ~707 Wh (59 Ah) | ~1.06 kWh (88 Ah) | ~2.12 kWh (177 Ah) |
| Refrigerator + Wi-Fi + Lights (180W avg) | ~909 Wh (76 Ah) | ~1.82 kWh (152 Ah) | ~2.73 kWh (227 Ah) | ~5.45 kWh (455 Ah) |
| Home Office / Remote Work (Laptop + 2 Monitors + Starlink: 220W) | ~1.11 kWh (93 Ah) | ~2.22 kWh (185 Ah) | ~3.33 kWh (278 Ah) | ~6.67 kWh (556 Ah) |
| Critical Household Circuit (Fridge + Sump Pump + Internet: 400W) | ~2.02 kWh (168 Ah) | ~4.04 kWh (337 Ah) | ~6.06 kWh (505 Ah) | ~12.12 kWh (1,010 Ah) |
Calculation Formulas
Estimates nominal stored-energy capacity (Wh and Ah) required to sustain a continuous or cycling electrical load for a desired backup duration for planning purposes.
Variable Definitions
Load_WattsContinuous Electrical Load(W)- Average real-time power draw (Watts × Duty Cycle).
Runtime_HoursTarget Autonomy Duration(hours)- Desired continuous operating hours without recharging.
Planning_MarginPlanning Margin(fraction)- User-selected capacity buffer for extra sizing headroom (e.g. 10%–20%).
Usable_SOCUsable DOD Window(fraction)- Fraction of nominal energy available above minimum reserve (e.g. 80% for LiFePO4, 50% for Lead-Acid).
Conversion_EfficiencyConversion Efficiency (η)(fraction)- Inverter efficiency for AC loads (88%–93%) or DC conversion efficiency.
Battery_HealthState of Health (SOH)(fraction)- Available capacity factor relative to original factory rating (default 100%).
Calculation Notes
- Amp-Hour equivalent at selected voltage V: Ah = Nominal_Battery_Wh / V.
- Planning margin provides an additional capacity buffer selected by the user. Temperature derating, cable resistance losses, and high-rate discharge physics are not individually modeled by this formula.
Technical References & Model Basis
The sizing methodology and energy reserve calculations in this tool reflect industry standards and engineering references:
IEEE Std 485 & IEC 62619
Recommended practices for battery sizing, depth-of-discharge margins, and safety reserve limits in stationary lithium-ion and lead-acid battery installations.
UL 1973 & NFPA 70 / NEC 706
Safety standards and National Electrical Code requirements governing energy storage system (ESS) integration, inverter pairing, and disconnect protection.
Frequently Asked Questions (FAQ)
How do I calculate what size battery I need?
Multiply your appliance load (in Watts) by your target runtime (in hours) to find required load energy (Wh). Divide by your conversion efficiency (e.g., 90% for an AC inverter) and usable Depth of Discharge (e.g., 80% for LiFePO4, 50% for Lead-Acid) to find minimum nominal capacity. Then apply a planning margin (e.g., 10%) for extra sizing headroom. Finally, divide by system voltage (12V, 24V, or 48V) to determine required Amp-hours (Ah).
What size battery do I need for a 500W load?
To run a 500W AC load for 4 hours (2,000Wh of load energy), you need a recommended battery bank of approximately 3.06 kWh (assuming 90% inverter efficiency, 80% usable depth-of-discharge, 100% SOH, and a 10% planning margin: 2,000 ÷ (0.80 × 0.90) × 1.10 = 3,055.56 Wh). At 12V this is ~255 Ah; at 24V this is ~127 Ah; at 48V this is ~64 Ah.
What size battery do I need to run a refrigerator during a 24-hour power outage?
A residential refrigerator with an illustrative average load of 50W (e.g. 150W compressor cycling at a 33% duty cycle) consumes approximately 1,200Wh over 24 hours. Assuming 80% usable Depth of Discharge, 90% inverter efficiency, and a 10% planning margin, the recommended battery size is approximately 1.83 kWh (1,200 ÷ (0.80 × 0.90) × 1.10 = 1,833 Wh), or ~153 Ah at 12V LiFePO4. If the refrigerator averages 80W (1,920Wh/day), required capacity is approximately 2.93 kWh (~244 Ah at 12V).
Why does system voltage affect required battery Ah but not kWh?
Watt-hours (Wh or kWh) measure total energy storage capacity, which remains constant regardless of wiring voltage. Amp-hours (Ah) measure electrical charge at a specific voltage (Ah = Wh ÷ Volts). A 2,400 Wh battery bank equals 200 Ah at 12V, 100 Ah at 24V, or 50 Ah at 48V.
How much extra capacity should I add for battery aging?
A planning margin can provide additional nominal capacity headroom, but actual capacity loss over time depends on battery chemistry, operating temperature, cycling frequency, depth of discharge, maintenance, and manufacturer specifications. For general planning, many system designers select a 10% to 20% margin.