Home energy planning

Home Battery Size Calculator

Estimate the total home battery storage capacity in kilowatt-hours (kWh) needed to protect your household during a blackout, from critical emergency circuits to whole-home backup.

Estimate home battery capacity

⚡ 1-Click Autofill: Top 5 Outage Scenarios

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Home backup estimate

Recommended home battery capacity

3.76 kWh

Technical References & Model Basis:

🛡️NFPA 855 🛡️UL 9540 🛡️IEEE 2030.5 🛡️NEC Art. 706

📦 Illustrative Battery Module Equivalents

Illustrative capacity equivalents only. Actual battery quantity depends on manufacturer specifications, usable capacity, inverter compatibility, continuous/peak power requirements, system voltage, code requirements and installation design.

1×13.5 kWh Units
(Residential ESS unit)

1×5.12 kWh Modules
(48V Lithium Module)

4×12V 100Ah Batteries
(1.2 kWh Drop-in Unit)

🔋 Home Power Flow & Outage Sim

Outage Autonomy12.0 Hours

Power ProtectionPartial Home (50%)

System Storage3.8 kWh Pack

Energy Loss Waterfall

Backup Window Energy2,464 Wh

Direct energy needed for 12h backup

Battery-Side Energy Demand2,738 Wh

Energy required from battery after inverter losses

Recommended Battery Capacity3,765 Wh

Total nominal capacity with reserve and design margin

Average daily household use: 9.86 kWh/day
Selected scope daily energy: 4.93 kWh/day · 50% estimate
Backup-window energy: 2.46 kWh
Minimum nominal capacity: 3.42 kWh
Recommended capacity: 3.76 kWh

Capacity by backup scope

Recommended capacity using the selected backup duration
Scope	Recommended capacity
Critical loads — 30% estimate	2.26 kWh
Partial home — 50% estimate — selected	3.76 kWh
Whole home — 100%	7.53 kWh

Assumptions used

Household energy basis: Average daily use
Backup scope: 50% planning estimate
Backup duration: 12 hours
Starting SOC: 100%
Minimum SOC: 20%
Usable SOC window: 80%
Inverter efficiency: 90%
Battery health / available capacity: 100%
Design margin: 10%
Chemistry: LiFePO4 / LFP planning preset

Energy Capacity vs. Power Output: This result estimates battery energy capacity (kWh) only. It does not determine inverter kW, motor-starting surge, HVAC LRA requirements, transfer equipment, service capacity or wiring. For whole-home backup, heavy loads such as HVAC, pumps, compressors, and EV chargers require separate continuous and surge-power evaluations.

Use this result elsewhere

Build a more accurate appliance profile Size an off-grid or solar battery bank

💡Quick Answer & Key Rule of Thumb

To power critical home essentials (refrigerator, lighting, router, electronics: ~30% planning estimate of normal consumption) for 12 to 24 hours during an outage, a typical home needs approximately 6.9 kWh to 13.8 kWh of nominal battery storage (under 80% usable SOC, 90% inverter efficiency, and 10% margin). Whole-home backup (100% load) for 24 hours typically requires 40 kWh to 50+ kWh depending on baseline use. Note that battery energy (kWh) sizing is separate from inverter continuous and surge power (kW) requirements.

\sum Governing Formula & Sizing Principle 📋 Copy Formula Battery kWh = [(Daily kWh \times Scope \times (Outage Hours / 24)) / (Usable SOC \times Inverter Eff \times Health)] \times (1 + Margin) Benchmark: 300 kWh/mo home (~9.86 kWh/day), 50% partial scope for 12h: [(9.86 \times 0.50 \times (12 \div 24)) \div (0.80 \times 0.90 \times 1.00)] \times 1.10 = 3.76 kWh

🏛️Governing Standard: Technical Reference Basis: NEC Article 706 & IEEE 2030.5

🧭 Connected Home Energy Planning Pathways

Integrate your battery storage sizing with appliance load audits and utility bill analysis:

⚡ Itemize Appliance Running & Surge Loads

Audit specific watts and starting surge (LRA) to verify your inverter and battery continuous kW ratings.

Electricity Usage Calculator →

📊 Benchmark Daily kWh Baseline

Compare your baseline consumption against U.S. EIA national averages (~29–30 kWh/day) and seasonal demand profiles.

Daily kWh Usage Guide →

💵 Project Electric Bill & TOU Savings

Model utility cost savings from rate arbitrage, peak shaving, and solar self-consumption during peak tariff periods.

Energy Bill Calculator →

How to Size a Home Battery Backup System

Enter Monthly Electricity Usage (kWh): Check your utility bill for average monthly consumption (US EIA residential average is ~880–900 kWh/mo).
Select Backup Scope: Choose Critical Essentials (~30% planning estimate), Partial Home (~50%), or Whole-Home (100%).
Set Outage Duration Target: Select how many continuous hours of blackout protection you require without grid power.
Review Battery Unit Recommendations: View required nominal kWh capacity and illustrative module equivalents (e.g. 13.5 kWh units).

Home Backup Battery Sizing Reference Matrix

Recommended nominal residential battery capacity (kWh) based on daily electricity consumption and desired blackout outage duration:

Recommended nominal battery kWh (Critical 30% vs Whole-Home 100% scope, 80% usable SOC, 90% inverter efficiency, 100% health, 10% margin)
Daily Household Energy	12-Hour Outage (Critical 30%)	24-Hour Outage (Partial 50%)	24-Hour Outage (Whole Home 100%)	Illustrative Battery Equivalent
15 kWh / day (Energy-Efficient Home)	~3.44 kWh	~11.46 kWh	~22.92 kWh	1× 13.5 kWh unit (or 1× 5.12 kWh module for critical)
30 kWh / day (US EIA National Average)	~6.88 kWh	~22.92 kWh	~45.83 kWh	1× 13.5 kWh unit (critical) / 4× 13.5 kWh units (whole)
45 kWh / day (Large Home + Central AC)	~10.31 kWh	~34.38 kWh	~68.75 kWh	1× 13.5 kWh unit (critical) / 6× 13.5 kWh units (whole)
60 kWh / day (All-Electric + EV + Heat Pump)	~13.75 kWh	~45.83 kWh	~91.67 kWh	2× 13.5 kWh units (critical) / 7× 13.5 kWh units (whole)

*Note: The 30 kWh/day baseline reflects the U.S. EIA national average for residential utility customers (~880–900 kWh/month). Values are calculated using the canonical formula with 80% usable SOC, 90% inverter efficiency, 100% health, and 10% general planning margin. Module counts are illustrative capacity approximations only and do not replace professional electrical engineering or inverter surge/power sizing.*

📊 Open Empirical Benchmark Data & Battery Storage Research

Need empirical discharge curves and Peukert derating coefficients for residential lithium iron phosphate (LiFePO4) storage systems? Explore our open Residential BESS Peukert Derating Benchmark Dataset (PL-DS-BESS-05), the Residential BESS Degradation & Thermal Loss Benchmark (PL-DS-BESS-06), and companion whitepaper on High-Discharge C-Rate Capacity Derating in Residential BESS (PL-TR-2026-BESS01).

Home Battery Backup Capacity Formulas

Calculates residential energy storage required to maintain home electrical circuits during power grid outages based on backup scope percentage and outage duration.

BatterykWh=[(DailykWh × Scope × (OutageHours / 24)) / (UsableSOC × InverterEff × Health)] × (1 + Margin)

Variable Definitions

Daily_kWhAverage Household Consumption(kWh/day): Daily baseline electricity consumption (Monthly kWh ÷ 30.4375).
ScopeBackup Coverage Scope(fraction): Illustrative planning share of normal loads backed up (30% Critical Essentials, 50% Partial Home, 100% Whole Home).
Outage_HoursTarget Autonomy Duration(hours): Continuous hours of grid blackout protection.
Usable_SOCUsable DOD Window(fraction): Fraction of battery energy above reserve cutoff (typically 80%–90%).
Inverter_EffHybrid Inverter Efficiency(fraction): DC-to-AC conversion efficiency (typically 88%–93%).
HealthBattery State of Health(fraction): Available capacity fraction of battery (typically 80%–100%).
MarginPlanning Margin(fraction): General planning margin applied to calculated battery capacity. It is not a separate degradation or standby-power model.

Calculation Notes

A standard residential home battery unit provides 5.0 to 13.5 kWh of nominal capacity.
Energy sizing (kWh) must be paired with power sizing (kW): whole-home backup for 240V HVAC, heat pumps, or compressors requires checking inverter continuous kW and peak motor starting (LRA) ratings.

Frequently Asked Questions (FAQ)

How many kWh of battery storage do I need to run a house during a power outage?

Required battery capacity depends on average daily energy consumption, backed-up load fraction, outage duration, usable SOC window, inverter efficiency, battery health, and planning margin. For example, for an average 30 kWh/day home, critical loads (~30% illustrative estimate) for 12 hours require ~6.88 kWh of nominal battery capacity, while whole-home backup (100%) for 24 hours requires ~45.83 kWh (assuming 80% usable SOC, 90% inverter efficiency, 100% health, and a 10% design margin). Actual circuit-level loads can differ substantially from average daily energy percentages.

How many 13.5 kWh battery units do I need for my home?

The required number of modular 13.5 kWh battery units is calculated by dividing total required battery storage by unit capacity: Required Units = ceil(Required_kWh / 13.5). For a 30 kWh/day home requiring 6.88 kWh for 12 hours of critical backup, 1 unit is sufficient. For 24 hours of whole-home backup requiring 45.83 kWh, 4 units (54 kWh nominal) are needed. In addition to energy storage (kWh), you must separately verify that total inverter continuous output (kW) and motor surge capacity (LRA) can start and run heavy 240V loads such as central AC, heat pumps, or well pumps.

What is the difference between critical load backup and whole-home backup?

Critical load backup (~30% planning estimate) powers a dedicated critical loads sub-panel containing only essential circuits (refrigeration, lighting, networking, medical devices). Whole-home backup (100%) connects to your main electrical panel to back up all household branch circuits. Whole-home backup requires significantly higher energy storage (kWh) and high continuous/surge inverter power (kW) to handle concurrent large appliance starts.

Can home batteries recharge from rooftop solar during an outage?

Yes, island-capable solar battery systems with a microgrid interconnect device (MID) or automatic transfer switch can form a local AC grid during utility blackouts. This allows rooftop solar panels to simultaneously power home circuits and replenish the battery bank during daylight hours.