Solar Storage Sizing Guide
Sizing solar battery storage is the most consequential design decision in a solar energy system. Get it right, and your system delivers reliable power for a decade or more. Get it wrong, and you face chronic underperformance, accelerated battery degradation, or unnecessary cost. This guide covers the complete engineering process — from consumption analysis through battery chemistry selection.
The Sizing Framework
Solar storage sizing follows a deterministic engineering process. Four variables drive the result: daily consumption, autonomy days, depth of discharge, and system efficiency. Understanding how each variable affects the outcome lets you make informed trade-offs between cost, resilience, and performance.
Daily Consumption
The total watt-hours your household or facility uses per day. This is the foundation of all sizing calculations. Accurate load audits prevent both oversizing (wasted cost) and undersizing (power shortages).
Autonomy Days
How many days the battery must power loads without solar input. Ranges from 1 day (sunny climate, generator backup) to 5+ days (cloudy region, critical loads, no generator).
Depth of Discharge
The percentage of total capacity you can safely use. LFP allows 80–90%; lead-acid is limited to 50%. Lower DoD means larger banks for the same usable energy.
System Efficiency
Accounts for inverter losses, wiring resistance, and BMS overhead. Typically 0.90–0.95 for modern equipment. Higher efficiency means smaller required capacity.
Core Sizing Formulas
These formulas are universal — they apply to any battery chemistry, system voltage, or installation type. The variables change; the method does not.
Step 1: Consumption Analysis
Begin by cataloging every electrical load the solar system must support. For each device, record the power draw in watts and the estimated daily runtime in hours. The product gives watt-hours per day. Sum all devices to get total daily consumption.
Common household consumption ranges from 5–30 kWh/day depending on size, climate, and electrification level. Homes with electric heating, air conditioning, and EV charging consume 20–40 kWh/day. Efficient small homes with heat pumps and LED lighting may use 5–10 kWh/day.
| Home Type | Typical Consumption | Key Loads |
|---|---|---|
| Small efficient home | 5–8 kWh/day | LED lights, efficient fridge, laptop, router |
| Average 3-bedroom home | 15–25 kWh/day | Standard appliances, AC/heat pump, washer |
| Large all-electric home | 25–40 kWh/day | Electric heat, EV charging, dryer, hot tub |
| Off-grid cabin | 3–8 kWh/day | Minimal loads, efficient appliances only |
Step 2: Battery Storage Reference Table
The table below maps daily consumption levels to required rated battery storage for common autonomy scenarios. All values assume LFP chemistry at 85% DoD and 92% system efficiency.
| Daily Consumption | 1 Day Autonomy | 2 Days Autonomy | 3 Days Autonomy | With 25% Margin |
|---|---|---|---|---|
| 5 kWh/day | 6.4 kWh | 12.8 kWh | 19.2 kWh | 24.0 kWh |
| 10 kWh/day | 12.8 kWh | 25.6 kWh | 38.4 kWh | 48.0 kWh |
| 15 kWh/day | 19.2 kWh | 38.4 kWh | 57.6 kWh | 72.0 kWh |
| 20 kWh/day | 25.6 kWh | 51.1 kWh | 76.7 kWh | 95.9 kWh |
| 30 kWh/day | 38.4 kWh | 76.7 kWh | 115.1 kWh | 143.9 kWh |
The "With 25% Margin" column accounts for battery aging, temperature derating, and load growth over the system's lifetime. This is the recommended column for real-world sizing decisions.
Step 3: Battery Chemistry Selection
Battery chemistry determines the usable capacity, cycle life, safety profile, and cost per kWh. For solar storage, LFP (LiFePO4) has become the dominant choice due to its combination of deep discharge capability, long cycle life, and thermal stability.
| Chemistry | DoD | Cycle Life | Cost/kWh | Safety |
|---|---|---|---|---|
| LFP (LiFePO4) | 85–90% | 3,000–5,000 | $250–400 | Excellent — no thermal runaway |
| NMC (Lithium) | 80–90% | 2,000–3,000 | $300–500 | Good — requires thermal management |
| Lead-Acid (AGM) | 50% | 500–1,000 | $150–250 | Good — mature technology |
| Sodium-Ion | 80–90% | 2,000–4,000 | $150–300 | Excellent — emerging technology |
When comparing cost, look at lifetime cost per kWh delivered, not just upfront price. A $350/kWh LFP battery delivering 4,000 cycles at 85% DoD costs $0.10 per kWh delivered. A $200/kWh lead-acid battery delivering 800 cycles at 50% DoD costs $0.50 per kWh delivered — five times more expensive over its lifetime.
Worked Example: Complete Sizing
Given:
- Home type: Average 3-bedroom, all-electric
- Daily consumption: 15,000 Wh (15 kWh)
- Autonomy: 2 days
- Battery: LFP at 85% DoD
- System efficiency: 92%
- System voltage: 48V
- Peak sun hours: 5
Step 1: Required usable capacity:
Step 2: Required rated capacity:
Step 3: Add 25% margin:
Step 4: Minimum solar array:
This home needs a 48 kWh rated LFP battery bank and a 4.2 kW solar array. In practice, this could be nine 48V 100Ah LFP modules (46.1 kWh) or a combination of rack-mounted 48V batteries. The 4.2 kW array recharges the daily 15 kWh consumption in approximately 4.1 peak sun hours, leaving margin for system losses and cloudy days.
Recommended Battery Configurations
| Daily Consumption | Storage (2 days + 25%) | Recommended Battery Config | Solar Array |
|---|---|---|---|
| 5 kWh/day | ~16 kWh | 3 × 48V 100Ah LFP | 1.7 kW |
| 10 kWh/day | ~32 kWh | 6 × 48V 100Ah LFP | 3.4 kW |
| 15 kWh/day | ~48 kWh | 9 × 48V 100Ah LFP | 4.2 kW |
| 20 kWh/day | ~64 kWh | 12 × 48V 100Ah LFP | 5.6 kW |
| 30 kWh/day | ~96 kWh | 18 × 48V 100Ah LFP | 8.4 kW |
Design Best Practices
Size for Winter
Solar production drops 30–50% in winter. If your system must maintain autonomy year-round, size the array for winter peak sun hours and the battery for the worst-case month. A system sized only for summer will fail in January.
Use 48V Architecture
For systems over 10 kWh, 48V is the industry standard. It reduces copper losses by 75% compared to 12V, simplifies wiring, and is compatible with all modern hybrid inverters and MPPT charge controllers.
Match Battery Age
If expanding an existing bank, use batteries of the same chemistry, capacity, and ideally the same manufacturing batch. Mixing old and new cells creates imbalance and reduces overall bank performance and lifespan.
Monitor Performance
Install battery monitoring that tracks SOC, voltage, current, and temperature. Data logging reveals capacity fade trends, identifies cell imbalance early, and confirms that your sizing assumptions hold in practice.
Try It
Use the Solar Battery Sizing Calculator to compute the exact storage capacity and array size for your installation.
Open Solar Battery Sizing CalculatorCompare Chemistries
Use the Battery Sizing Calculator to compare LFP, NMC, and lead-acid configurations side by side.
Open Battery Sizing CalculatorRelated Articles
How Much Battery Storage Do I Need?
Calculate total storage capacity from daily consumption and autonomy days.
Read Guide →LiFePO4 for Solar Storage
Why LFP chemistry is the optimal choice for stationary solar battery systems.
Read Guide →Frequently Asked Questions
What is the best way to size solar battery storage?
Start with a complete load audit to determine daily consumption in watt-hours. Multiply by your desired autonomy days (typically 2–3). Divide by depth of discharge and system efficiency to get rated capacity. This four-step method accounts for all variables and gives you the minimum bank size to purchase.
How do I choose between LFP and NMC batteries for solar?
LFP (LiFePO4) is preferred for stationary solar storage due to its longer cycle life (3,000–5,000 vs 2,000–3,000), better thermal stability, and lower cost per kWh. NMC offers higher energy density but carries greater thermal management requirements and higher cost. For solar, LFP is the standard choice.
Should I oversize my solar storage?
Yes, by 20–30% beyond minimum calculations. This accounts for battery aging (capacity fades 15–20% over 10 years), seasonal variation, load growth, and temperature derating. Oversizing now is cheaper than retrofitting later, and it reduces battery stress by operating at lower average depth of discharge.
How does system voltage affect storage sizing?
System voltage does not change the required energy capacity (kWh), but it affects the number of modules and wiring complexity. A 48V system requires fewer parallel strings than 12V or 24V, reducing copper losses and simplifying installation. For systems over 10 kWh, 48V is strongly recommended.