Whole-House Battery Backup
Whole-house battery backup means powering every circuit in your home — including HVAC, appliances, lighting, and electronics — entirely from stored battery energy during a grid outage. This guide covers the engineering requirements for sizing a system large enough to sustain full-home consumption, along with the inverter and panel considerations that differ from smaller critical-loads systems.
Whole-House vs. Critical-Loads Backup
The fundamental difference between whole-house and critical-loads backup is scope. Critical-loads systems back up 5–15 kWh of essential devices. Whole-house systems back up 25–60+ kWh of everything. This difference in scope cascades through every component: larger battery, higher-capacity inverter, more complex panel integration, and significantly higher cost.
The decision between the two approaches is primarily economic. Whole-house backup requires 3–5× more battery capacity, which is the dominant cost in the system. However, it eliminates the need to triage loads or change behavior during an outage. Everything works as if the grid were still online, which is the ideal outcome for homeowners who prioritize seamless resilience.
| Factor | Critical-Loads | Whole-House |
|---|---|---|
| Battery capacity | 5 – 15 kWh | 25 – 60+ kWh |
| Inverter size | 3 – 5 kW | 10 – 15+ kW |
| Panel work | Subpanel or critical loads panel | Full panel integration or smart panel |
| Typical cost | $5,000 – $15,000 | $15,000 – $40,000+ |
| Behavior change | Must manage loads during outage | No changes — everything runs normally |
Step 1: Calculate Total Household Consumption
Whole-house backup requires knowing your total daily energy consumption, not just critical loads. The most accurate method is to pull your highest-usage month from your utility bill and divide by the number of days. This gives you the peak daily consumption your battery system must sustain.
For homes with solar panels, your net consumption from the grid may understate your actual usage. During an outage with no solar input, you need to cover the full gross consumption. Check your solar inverter's monitoring app for total production and consumption data to get the true picture.
Worked Example: Determine household consumption from utility data.
- Highest summer month bill: 900 kWh
- Billing period: 31 days
Calculation:
This home consumes approximately 29 kWh per day at peak. For whole-house backup, size the battery to sustain this level for the desired number of hours.
Worked Example: 20 kWh/Day House
The following worked example sizes a complete whole-house backup system for a home consuming 20 kWh per day with an overnight outage scenario.
Given:
- Daily consumption: 20 kWh/day
- Target backup duration: 24 hours
- Battery chemistry: LFP (85% DoD)
- Inverter efficiency: 92%
- System voltage: 48V
- Peak simultaneous load: 8 kW
Step 1: Calculate total energy needed for 24 hours:
Step 2: Account for DoD and inverter efficiency:
Step 3: Convert to amp-hours at 48V:
Step 4: Inverter sizing for peak load:
Step 5: Practical battery configuration:
Option B: 5 × 48V 100Ah LFP batteries in parallel = 25.6 kWh
Recommended: Option A — two modular units provide redundancy and simpler wiring
Total system: 26.8 kWh battery bank + 10 kW hybrid inverter + automatic transfer switch + monitoring.
Whole-House Sizing Formulas
For whole-house backup, the inverter must handle peak load — not just average load. If your HVAC, oven, and water heater all cycle on simultaneously, the inverter must supply that surge. A 10 kW inverter with a 48V battery bank draws approximately 208A at peak, requiring appropriately sized DC cabling and fusing.
Electrical Panel Integration
Whole-house backup requires integrating the battery system with your main electrical panel. There are two approaches: a critical loads subpanel (which limits backed-up circuits) and full panel integration (which backs up everything). For true whole-house backup, you need the full panel approach, often implemented with a smart electrical panel that can shed non-essential loads during extended outages.
Standard Integration
The battery connects through an automatic transfer switch to the main panel. All circuits are backed up. Simple and straightforward, but the battery must be sized for the entire home's load. Best for homes with moderate total consumption.
Smart Panel Integration
A smart electrical panel (like Span or Lumin) allows individual circuit control via app. During an outage, non-essential circuits are automatically shed to extend battery runtime. Enables a smaller battery while maintaining critical load coverage.
Scaling by Home Size
The following table provides approximate whole-house battery sizing ranges by home size. These assume standard electric appliances, central HVAC, and 2–4 occupants. Actual requirements vary significantly based on climate, insulation, and occupant behavior.
| Home Size | Daily kWh | Battery (24h) | Inverter |
|---|---|---|---|
| Small (under 1,500 sq ft) | 12 – 20 kWh | 15 – 26 kWh | 5 – 8 kW |
| Medium (1,500 – 2,500 sq ft) | 20 – 35 kWh | 26 – 45 kWh | 8 – 12 kW |
| Large (2,500 – 3,500 sq ft) | 35 – 50 kWh | 45 – 65 kWh | 12 – 15 kW |
| Very Large (3,500+ sq ft) | 50 – 70+ kWh | 65 – 90+ kWh | 15 – 20+ kW |
Try It
Use the Home Backup Calculator to size a whole-house battery system based on your actual household consumption and desired backup duration.
Open Home Backup CalculatorRelated Tool
Use the Battery Sizing Calculator to determine the exact battery bank configuration for your selected voltage, chemistry, and capacity.
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Frequently Asked Questions
How big of a battery do I need for whole-house backup?
A typical home consuming 20–30 kWh per day requires 25–40 kWh of usable battery capacity for 24-hour whole-house backup. At 85% DoD and 92% inverter efficiency, a 30 kWh daily load requires approximately 39 kWh of total battery capacity. Larger homes with HVAC loads may need 50+ kWh.
Is whole-house battery backup worth the cost?
Whole-house backup is significantly more expensive than critical-loads-only backup — typically 3–5× the battery capacity. However, it provides complete peace of mind and seamless operation with no lifestyle changes during an outage. The value depends on your outage frequency, local energy costs, and whether you pair the system with solar for bill offset.
Can I back up my entire house with just batteries?
Yes, but the battery bank must be large enough to handle peak demand and sustained consumption. A house running HVAC, water heating, cooking, and lighting simultaneously may draw 5–10 kW peak and consume 25–40+ kWh per day. This requires a substantial battery bank, a high-capacity inverter (10–15 kW), and proper electrical panel integration.
How does whole-house backup differ from critical-loads backup?
Critical-loads backup powers a subset of circuits (fridge, lights, medical devices) using a smaller, less expensive battery (5–15 kWh). Whole-house backup powers every circuit including HVAC and appliances, requiring 3–5× more battery capacity. Whole-house systems use a critical loads panel or smart electrical panel to manage the full home.