Home energy planning

Home Battery Size Calculator

Estimate the home battery capacity needed for selected household loads and an outage duration. The result uses average household energy, an editable backup-scope estimate and transparent reserve and efficiency assumptions.

Estimate home battery capacity

Quick Backup Scenarios:

How much home battery storage do I need?

Start with household energy, choose the share of normal use you want to support and enter the backup duration. The calculator estimates stored-energy capacity in kWh and assumes the battery starts fully charged.

Essential loads versus whole-home backup

Critical, partial and whole-home scope values are planning estimates. They do not classify every appliance automatically. Choose Custom when you know a better share of your household energy.

Home battery size formula

The model converts monthly use to an average daily value, applies the selected scope and backup hours, then adjusts for inverter efficiency, one SOC window, battery health and design margin:

Recommended kWh = daily energy × scope × backup hours ÷ 24 ÷ (efficiency × usable SOC window × health) × (1 + margin)

Average-load and multi-day assumptions

Energy is assumed to be spread proportionally across the backup period. Multi-day estimates repeat the normalized average daily load. Actual outage loads can vary by time of day, appliance schedule and day-to-day conditions. Solar or generator recharge is not included.

Worked home battery sizing example

For 300 kWh/month, a 50% scope and 12 hours, average use is about 9.856 kWh/day and selected-scope backup energy is about 2.464 kWh. With 20% minimum SOC, 90% inverter efficiency, 100% health and a 10% margin, the result is approximately 3.77 kWh recommended capacity.

Limitations

This is a stored-energy estimate. It does not size inverter power, peak or surge demand, service panels, transfer equipment, wiring, breakers, installation compatibility or high-power whole-home loads. Large motors and heating equipment may require separate electrical engineering.

Home Backup Battery Sizing Reference Matrix

Recommended 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 DOD)
Daily Household Energy	12-Hour Outage (Critical 30%)	24-Hour Outage (Partial 50%)	24-Hour Outage (Whole Home 100%)	Typical Battery Equivalent
15 kWh / day (Energy-Efficient Home)	~3.2 kWh	~10.6 kWh	~21.2 kWh	1× 10 kWh – 13.5 kWh unit
30 kWh / day (US National Average)	~6.4 kWh	~21.2 kWh	~42.4 kWh	2× 13.5 kWh units (e.g. Powerwall)
45 kWh / day (Large Home + Central AC)	~9.5 kWh	~31.8 kWh	~63.5 kWh	3× 13.5 kWh units
60 kWh / day (All-Electric + EV + Heat Pump)	~12.7 kWh	~42.4 kWh	~84.7 kWh	4× 13.5 kWh units or commercial stack

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.

FormulaBattery_kWh = [(Daily_kWh × Scope × (Outage_Hours / 24)) / (Usable_SOC × Inverter_Eff × Health)] × (1 + Margin)

Variable Definitions

Daily_kWhAverage Household Consumption(kWh/day): Daily baseline electricity consumption (Monthly kWh ÷ 30.4375).
ScopeBackup Coverage Scope(fraction): Share of normal loads backed up (30% Critical Essentials, 50% Partial, 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%).
MarginDesign Buffer(fraction): Safety margin for degradation and inverter standby tare power (typically 10%–15%).

Engineering Notes & Standards

A typical residential home battery unit (e.g. Tesla Powerwall, Enphase 5P, FranklinWH) provides 5.0 to 13.5 kWh of nominal capacity.
Whole-home backup for 240V HVAC or heat pumps requires checking the inverter continuous kW and peak surge LRA ratings.

Frequently Asked Questions (FAQ)

How many kWh of battery storage do I need to power a house for a day?

The average US household uses ~28 to 30 kWh per day. For whole-home backup including heating/cooling, you typically need 25 to 30 kWh of battery storage. For critical essentials only (refrigerator, lighting, Wi-Fi, basic outlets), a 10 to 13.5 kWh battery provides 24 to 48 hours of backup.

Can a single 10 kWh or 13.5 kWh home battery run central air conditioning?

Energy-wise, central AC consumes 3 to 4 kWh per hour of continuous running, which would deplete a 10 kWh battery in under 3 hours. Peak starting surge (LRA) may also require a soft-starter or higher inverter peak kW capacity.

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

Critical loads backup routes essential 120V circuits (fridge, internet, medical devices, lighting) to a dedicated subpanel (~25%–35% of total household energy). Whole-home backup connects all circuits (100% of loads) including heavy 240V appliances (HVAC, EV charger, oven).

How long will a 13.5 kWh battery last during a power outage?

With conservative critical load management (drawing ~500W average), a 13.5 kWh battery with a 20% reserve window delivers ~10.8 kWh usable energy, lasting approximately 20 to 24 hours.

Related calculators

Use the Electricity Usage Calculator to build a more detailed appliance profile. For solar or off-grid autonomy, use the Solar Battery Bank Size Calculator.