How Much Battery Storage Do I Need?
Battery storage capacity is measured in kilowatt-hours (kWh) — the total energy your batteries can store and deliver. Sizing this correctly ensures your system can power your loads through the night and survive cloudy days without running dry. This guide explains how to calculate the exact storage capacity your installation requires.
Understanding Usable vs. Rated Capacity
Every battery has a rated (nameplate) capacity and a usable capacity. The usable portion is determined by the depth of discharge (DoD) limit — the percentage of total capacity you can safely draw before recharging. Exceeding this limit damages cells and shortens cycle life.
For example, a 48V 200Ah LFP battery has a rated capacity of 9,600 Wh (9.6 kWh). At 85% DoD, its usable capacity is 8,160 Wh (8.16 kWh). When sizing storage, always work from usable capacity, not rated capacity.
| Chemistry | Typical DoD | Usable per Rated kWh |
|---|---|---|
| LFP (LiFePO4) | 80–90% | 0.80–0.90 kWh usable per 1 kWh rated |
| NMC (Lithium) | 80–90% | 0.80–0.90 kWh usable per 1 kWh rated |
| Lead-Acid (AGM/Gel) | 50% | 0.50 kWh usable per 1 kWh rated |
Storage Sizing Formula
System efficiency (typically 0.90–0.95) accounts for inverter losses, wiring losses, and BMS overhead. DoD is expressed as a decimal (e.g., 0.85 for 85%).
Step-by-Step Sizing Method
The sizing process follows three straightforward steps. Start with your daily consumption in watt-hours, multiply by autonomy days to get total required usable energy, then divide by DoD and efficiency to find the rated capacity you need to purchase.
Step 1
Audit your daily consumption. List every load, multiply power by runtime, and sum the total watt-hours per day.
Step 2
Multiply by autonomy days. Two days is standard; three or more for critical loads or cloudy regions.
Step 3
Divide by DoD and efficiency. This gives you the rated kWh to purchase. Round up to the nearest available battery size.
The result is your minimum rated storage capacity. Adding a 20–30% margin for load growth, temperature derating, and aging is standard practice.
Storage Requirements by Daily Consumption
The table below shows required rated battery storage (in kWh) for common daily consumption levels. Values assume 85% DoD (LFP), 92% system efficiency, and the indicated number of autonomy days.
| Daily Consumption | 1 Day Autonomy | 2 Days Autonomy | 3 Days Autonomy |
|---|---|---|---|
| 5 kWh/day | 6.4 kWh rated | 12.8 kWh rated | 19.2 kWh rated |
| 10 kWh/day | 12.8 kWh rated | 25.6 kWh rated | 38.4 kWh rated |
| 15 kWh/day | 19.2 kWh rated | 38.4 kWh rated | 57.6 kWh rated |
| 20 kWh/day | 25.6 kWh rated | 51.1 kWh rated | 76.7 kWh rated |
| 30 kWh/day | 38.4 kWh rated | 76.7 kWh rated | 115.1 kWh rated |
For context, a typical Tesla Powerwall stores 13.5 kWh. A home consuming 10 kWh/day with 2 days autonomy needs roughly two Powerwalls (27 kWh rated) to meet the requirement.
Worked Example
Given:
- Daily consumption: 10,000 Wh (10 kWh)
- Desired autonomy: 2 days
- Battery chemistry: LFP at 85% DoD
- System efficiency: 92%
Step 1: Calculate required usable storage:
Step 2: Convert to rated capacity:
Step 3: Convert to kWh:
You need approximately 25.6 kWh of rated LFP battery storage. In practice, this could be three 48V 100Ah modules (15.36 kWh rated) plus one 48V 200Ah module (10.24 kWh rated), or any combination of LFP modules totaling at least 25.6 kWh nameplate. Adding a 20% margin for load growth brings the target to roughly 30.7 kWh.
Common Battery Sizes for Solar Storage
The table below maps typical daily consumption levels to recommended battery configurations using standard 48V LFP modules.
| Consumption | Storage Needed (2 days) | Recommended Config |
|---|---|---|
| 5 kWh/day | ~12.8 kWh | 3 × 48V 100Ah (15.4 kWh) |
| 10 kWh/day | ~25.6 kWh | 5 × 48V 100Ah (25.6 kWh) |
| 15 kWh/day | ~38.4 kWh | 8 × 48V 100Ah (41.0 kWh) |
| 20 kWh/day | ~51.1 kWh | 10 × 48V 100Ah (51.2 kWh) |
| 30 kWh/day | ~76.7 kWh | 15 × 48V 100Ah (76.8 kWh) |
When to Oversize Storage
Seasonal Buffer
Winter produces 30–50% less solar energy than summer. If your system must maintain autonomy year-round, size storage for the worst-case season. A 30–40% oversize beyond summer requirements covers the winter gap.
Load Growth Margin
Household consumption tends to increase over time — new appliances, EV charging, home office equipment. Adding a 20–30% margin now avoids the cost of retrofitting the battery bank later.
Battery Aging
LFP batteries retain 80% capacity after 3,000–5,000 cycles. Sizing to 100% of your need on day one means you will be undersized by year five. A 15–20% oversize compensates for capacity fade over the system's lifetime.
Critical Loads
Medical equipment, refrigeration for medications, or communication systems cannot tolerate interruption. For critical loads, size for 3–5 days of autonomy regardless of climate, and consider a backup generator.
Try It
Use the Solar Battery Sizing Calculator to determine the exact storage capacity for your specific consumption and autonomy needs.
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Read Guide →Frequently Asked Questions
How much battery storage do I need for my home?
A typical home consuming 10–15 kWh/day needs 25–50 kWh of usable battery storage for 2 days of autonomy at 85% DoD. The exact number depends on your consumption, desired backup duration, and battery chemistry. Use the formula: Required Capacity = Daily Consumption × Autonomy Days / (DoD × Efficiency).
Is 10 kWh of battery storage enough?
10 kWh of usable storage provides roughly 1 day of backup for a home consuming 10 kWh/day, or half a day for a 20 kWh/day household. For most homes, 10 kWh is a starting point — adequate for overnight bridging but insufficient for multi-day outages without solar recharge.
Should I size for peak demand or average consumption?
Size for average daily consumption multiplied by your autonomy days. Peak demand is handled by the inverter's power rating, not the battery's energy capacity. The battery stores watt-hours (energy); the inverter delivers watts (power). These are separate specifications.
How does depth of discharge affect storage size?
Lower DoD means you need more total capacity to access the same usable energy. At 50% DoD (lead-acid), a 20 kWh bank only gives you 10 kWh usable. At 85% DoD (LFP), the same 20 kWh bank gives you 17 kWh usable. LFP chemistry allows smaller banks for the same usable energy.