Off-grid solar battery sizing is the one calculation where getting it wrong has real consequences — too small and you run out of power during a stretch of cloudy days; too large and you've spent $20,000–$30,000 more than necessary. The math isn't complicated, but most online guides skip the depth-of-discharge step that separates a working system from a dead battery bank. Here's how to do it correctly in 2026.
Disclaimer: Load and sizing calculations are based on standard industry methodology using depth-of-discharge limits from battery manufacturer specifications and NREL solar resource data. True off-grid system design should be reviewed by a licensed solar installer or engineer familiar with off-grid applications. Battery prices reflect 2026 market estimates. The federal Section 25D residential energy credit expired December 31, 2025 and does not apply to 2026 purchases. Get 3+ installer quotes before purchasing.
Key Takeaways
- The off-grid battery sizing formula is: daily load (kWh) × days of autonomy ÷ 0.5 (DOD limit) — a 30 kWh/day home needs a 180 kWh bank for 3-day autonomy
- Most "off-grid" residential installs are actually backup-capable grid-tied systems with 20–40 kWh storage — true off-grid requires 100–300 kWh banks
- Depth of discharge (DOD) is the most commonly ignored variable — exceeding 50% regularly on lead-acid, or 80% on lithium, cuts battery lifespan by 40–60%
- At 2026 lithium battery costs, a true off-grid battery bank for a typical U.S. home costs $50,000–$150,000 — most rural homes choose backup-capable grid-tied instead
True Off-Grid vs Backup-Capable Grid-Tied
Before running sizing calculations, it helps to be clear about what you're actually building. These are genuinely different systems.
True off-grid means no utility connection at all. The solar array and battery bank are the sole power source. For this to work reliably, the battery bank must hold enough energy to cover 3–5 days of full home load with no solar input — the "autonomy days" needed to ride out an extended cloudy period. In the Pacific Northwest, you might design for 5–7 days of autonomy. In Arizona, 2–3 days is typically sufficient.
Backup-capable grid-tied means you have utility service as the primary source and a solar + battery system that keeps your home running during outages. The battery bank is sized to cover 8–24 hours of critical loads — not 3–5 days of full home load. The cost difference is enormous: $15,000–$25,000 for a backup-capable grid-tied system vs $80,000–$200,000+ for true off-grid on a full-size home.
According to NREL's residential systems analysis, the vast majority of residential solar + battery installs in the U.S. are grid-tied with backup capability — not true off-grid. When homeowners say they want to "go off-grid," they usually mean they want backup protection from outages, which is a very different (and much cheaper) goal.
The Core Battery Sizing Formula
The fundamental off-grid battery sizing calculation has three inputs:
- Daily energy consumption (kWh/day)
- Days of autonomy (how many sunless days you want to ride out)
- Depth of discharge limit (how far you can safely drain the battery)
Formula:
Battery bank size (kWh) = Daily load × Days of autonomy ÷ DOD
The DOD (depth of discharge) limit is the percentage of a battery's rated capacity you can use before needing to recharge. Exceeding the limit repeatedly accelerates degradation:
- Lithium iron phosphate (LiFePO4): 80% DOD typical; some manufacturers rate to 90%
- Lead-acid (flooded): 50% DOD maximum for long cycle life
- Lithium-ion (NMC, as in Powerwall 3): 80–90% DOD
For conservative sizing, use 0.5 (50%) as the DOD denominator — this provides a margin and applies to lead-acid systems. For lithium systems, you can use 0.8, but the conservative 0.5 gives you a buffer.
Step-by-Step Sizing Example
Home profile: 2,000 sq ft, central U.S. location, daily consumption 30 kWh/day
Design target: 3 days of autonomy with 50% DOD limit
Calculation:
30 kWh/day × 3 days = 90 kWh total needed before DOD adjustment 90 kWh ÷ 0.5 = 180 kWh battery bank
At current 2026 pricing for lithium battery modules ($700–$1,000/kWh installed for commercial-grade LiFePO4 systems), a 180 kWh bank costs roughly $126,000–$180,000. That's why most residential "off-grid" systems are actually backup-capable grid-tied — the true off-grid cost is prohibitive for average home loads.
| Daily Load | Autonomy Days | DOD Limit | Required Battery Bank | Estimated Cost (LiFePO4, installed) |
|---|---|---|---|---|
| 15 kWh/day (small/efficient home) | 3 days | 50% | 90 kWh | $63,000–$90,000 |
| 25 kWh/day (average home, reduced load) | 3 days | 50% | 150 kWh | $105,000–$150,000 |
| 30 kWh/day (average full-size home) | 3 days | 50% | 180 kWh | $126,000–$180,000 |
| 15 kWh/day (small home, high-sun region) | 2 days | 80% (lithium) | 37.5 kWh | $26,000–$38,000 |
| 30 kWh/day (high-sun, shorter autonomy) | 2 days | 80% (lithium) | 75 kWh | $52,500–$75,000 |
The table illustrates why high-sun regions with shorter autonomy needs (Arizona, New Mexico, Nevada) can make true off-grid pencil at lower cost — fewer cloudy-day risk days means less autonomy buffer needed.
Solar Array Sizing for Off-Grid
The battery bank is only half the equation. You also need enough solar panels to recharge the battery within the available sun hours — otherwise even a large bank eventually depletes.
Solar array sizing formula:
Required array (kW) = Daily load (kWh) ÷ peak sun hours × recharge factor (1.25–1.5)
The recharge factor accounts for system inefficiencies (wiring losses, inverter losses, temperature derating).
| Location | Peak Sun Hours | Daily Load | Array Needed (1.25x factor) |
|---|---|---|---|
| Phoenix, AZ | 6.5 hrs | 30 kWh | ~5.8 kW |
| Dallas, TX | 5.2 hrs | 30 kWh | ~7.2 kW |
| Atlanta, GA | 4.7 hrs | 30 kWh | ~8.0 kW |
| Seattle, WA | 3.2 hrs | 30 kWh | ~11.7 kW |
| Portland, ME | 3.8 hrs | 30 kWh | ~9.9 kW |
Seattle's low sun hours mean an off-grid home needs nearly twice the solar capacity of an equivalent Phoenix home — and requires a larger battery bank to survive longer cloudy stretches. These are the markets where off-grid rarely makes financial sense without significant load reduction.
Use NREL's PVWatts to find peak sun hours for your specific location and tilt angle. The Solar ROI Calculator can help estimate production for grid-tied systems, though it's designed for grid-tied rather than full off-grid sizing.
Reducing Load: The First Step Before Sizing
The most cost-effective off-grid design starts with load reduction, not battery bank expansion. Every kWh you eliminate from daily consumption reduces the battery bank requirement by 2× (due to the DOD formula) and the solar array by a proportional amount.
Common load reductions for off-grid homes:
- Replace electric resistance water heater with heat pump water heater: Reduces water heating load by 60–70% (from ~4 kWh/day to ~1.2 kWh/day)
- Switch to mini-split heat pumps: Reduces HVAC energy 40–50% vs resistance electric heat
- LED lighting: Already standard, but ensures ~80% less lighting load than incandescent
- Energy-efficient refrigerator: Modern ENERGY STAR models use 300–450 kWh/year vs 800–1,200 kWh/year for older units
- Eliminate electric dryer: Propane or air-drying removes 2–4 kWh/day
Reducing load from 30 kWh/day to 18 kWh/day (aggressive but achievable for an efficient 2,000 sq ft home) cuts the required 3-day battery bank from 180 kWh to 108 kWh — saving $50,000–$70,000 in battery cost.
Check your Panel Capacity Checker results to understand your current loads and where the largest draws are before designing an off-grid system.
What 20–40 kWh Systems Actually Are
Most residential battery installs in 2026 fall in the 13.5–27 kWh range (1–2 Powerwalls) or 10–20 kWh (2–4 Enphase modules). These aren't true off-grid systems — they're backup-capable grid-tied systems that provide 1–2 days of essential load coverage, not 3–5 days of full-home off-grid operation.
That's not a flaw — it's the right design for most homeowners. Even in outage-prone areas, most grid restoration happens within 24–48 hours. A system providing 24–36 hours of essential backup covers 95%+ of outage events. True off-grid sizing ($50,000–$200,000) is appropriate for remote properties without grid access — not for suburban homes trying to reduce utility dependence.
If you're looking for backup power rather than true off-grid capability, the home battery guide covers the full range of mainstream systems at realistic costs.
Bottom Line
True off-grid solar battery sizing follows a simple formula — daily load × autonomy days ÷ DOD limit — but the result is a battery bank that costs $50,000–$200,000 for a full-size U.S. home. That math works for remote properties without grid access; it rarely makes financial sense for homes with grid service. Most homeowners who want "off-grid" actually want backup capability, which a 13.5–27 kWh battery achieves at a fraction of the cost. Size your system honestly against your real goal.
Related Guides
- Home Battery Storage Cost in 2026 — Full cost breakdown for mainstream home battery systems and installation variables.
- Is Solar Worth It in 2026? — Solar ROI analysis including system sizing and payback for grid-tied installs.
- How Many Powerwalls Do I Need in 2026? — Practical guidance on backup system sizing for grid-tied homeowners.
- Net Metering Guide 2026 — How net metering rules affect the value of grid-tied vs battery-storage solar.
