Interactive Whole-Home Battery Storage Sizing Calculator (2026)

Home Battery Engineering & Sizing Principles

When planning residential battery backup, confusing storage capacity (kWh) with continuous power (kW) and surge capacity (kVA) is the most common reason backup systems fail during an outage. Here is how electrical engineers calculate fail-safe residential resilience:

1. Power (kW) vs. Energy (kWh) Explained

• Continuous Inverter Output (kW): How many appliances can run at the exact same instant. A 7,200W (7.2 kW) inverter can simultaneously power a 3,500W central air conditioner, a 1,500W well pump, an 800W refrigerator, and 500W of lighting/Wi-Fi ($3,500 + 1,500 + 800 + 500 = 6,300W \le 7,200W$).
• Usable Battery Capacity (kWh): How long those appliances can run before the battery is depleted. If your household draws a continuous average load of 1,500W (1.5 kW), a 6.1 kWh battery will sustain that load for approximately 4 hours ($6.1 \text{ kWh} \div 1.5 \text{ kW} \times 0.95 \text{ efficiency} \approx 3.86 \text{ hours}$). Scaling to 18 kWh extends that runtime to over 11 hours without solar input.

2. The 240V Split-Phase Imperative

Standard North American residential electrical panels receive 120V/240V single-phase split-phase power across two 120V hot legs ($L_1$ and $L_2$) and a neutral:

• 120V Appliances: Lights, refrigerator, microwave, televisions, Wi-Fi routers, and CPAP machines run on a single 120V leg.
• 240V Appliances: Central AC compressors, heat pumps, well water pumps, electric dryers, and Level 2 EV chargers connect across both $L_1$ and $L_2$ hot legs (240V).
• Engineering Takeaway: A 120V-only battery power station cannot power your well pump or central AC, regardless of how many watts it has. Systems like the EcoFlow DELTA Pro Ultra and Tesla Powerwall 3 feature native dual-inverter architectures that synthesize true 240V split-phase power straight into your breaker panel.

3. Motor Inrush Currents (Locked Rotor Amperage - LRA)

Electric motors found in central AC compressors, well pumps, and refrigerators require a brief burst of massive electrical current (up to 3x to 5x their running wattage) for 150 to 300 milliseconds to break mechanical inertia:

• The Soft-Starter Solution: If you have a 3-ton to 5-ton central AC with an LRA rating of 70A to 110A, installing a Micro-Air EasyStart or similar soft-starter reduces the starting surge by up to 65-70%. This allows a single 7.2 kW / 10.8 kW surge inverter (such as the DELTA Pro Ultra) to start your entire home’s central AC without tripping the breaker.

4. 30% Federal Clean Energy Tax Credit (Section 25D)

Under the Inflation Reduction Act, battery storage systems installed in residential homes with a capacity rating of 3 kWh or greater qualify for a 30% non-refundable federal tax credit.

• This applies whether or not the battery is paired with solar panels.
• Eligible expenses include the battery unit, expansion packs, smart transfer panels, and professional electrical installation labor.