Why Sizing a Solar Battery Correctly Matters More Than Anything Else
A solar battery system that is undersized will leave you without power. One that is oversized costs significantly more than necessary. Getting the sizing right — based on real data, not rules of thumb — is the single most important step in any solar project specification.
Step 1: Define Your Daily Energy Requirement
List every load in the system. For each load, multiply power draw (watts) by hours of use per day.
Example — small commercial solar system (resort in the Philippines):
- Lighting (LED, 20 fixtures × 10W × 8 hours): 1,600Wh/day
- Air conditioning (1,500W × 6 hours): 9,000Wh/day
- Refrigeration (200W × 24 hours): 4,800Wh/day
- Wi-Fi and security (100W × 24 hours): 2,400Wh/day
- Total: 17,800Wh/day ≈ 18kWh/day
This is the minimum energy the battery must supply during periods without solar generation.
Step 2: Determine Required Days of Autonomy
Autonomy = number of cloudy days the battery must bridge without solar input.
| Application | Recommended Autonomy | Typical Scenario |
|---|---|---|
| Grid-tied with backup | 1 day | Grid fails, generator starts |
| Off-grid with generator backup | 2–3 days | Multi-day cloudy period |
| Remote off-grid (no generator) | 3–5 days | Remote telecom, monitoring station |
| Critical infrastructure | 5–7 days | Hospital, data center |
For most commercial solar projects, 2 days autonomy is the practical minimum.
Step 3: Size the Battery Bank for Depth of Discharge Limit
Batteries should never be regularly discharged below their recommended depth of discharge (DoD) limit. Operating beyond DoD dramatically reduces cycle life.
| Battery Type | Recommended Max DoD | Design DoD for Daily Cycling |
|---|---|---|
| Flooded lead-acid | 50% | 50% |
| AGM VRLA | 50–60% | 50% |
| OPzV tubular gel | 60–80% | 50–60% |
| LiFePO4 | 80% | 80% |
Battery bank size formula:
Required bank (kWh) = Daily usage (kWh) × Autonomy (days) ÷ Max DoD
Example: 18kWh/day, 2 days autonomy, OPzV gel at 60% DoD
= 18 × 2 ÷ 0.60 = 60kWh battery bank
Step 4: Convert kWh to Battery Units
OPzV tubular gel cells (2V)
For a 48V system: 48V = 24 cells × 2V
To get 60kWh at 48V:
→ 60kWh ÷ 48V = 1,250Ah required
→ Recommended: 24 × 2V 1,500Ah OPzV cells
Lead-acid blocs (12V × 4 = 48V)
For a 48V system: 4 × 12V blocs in series
To get 60kWh at 48V:
→ 60kWh ÷ 48V = 1,250Ah required
→ Recommended: 4 × 12V 1,250Ah lead-acid blocs (or 4 × 12V 1,000Ah + 8 × 2V cells for a larger bank)
Step 5: Solar Panel Sizing
The solar array must be large enough to recharge the battery each day AND supply the daily load simultaneously.
Recharge requirement:
Panel array (W) = Battery bank (kWh) × 1.2 (charging losses) ÷ Peak sun hours × Days to recharge target
For 18kWh/day load in the Philippines (average 4.5 peak sun hours):
- Array needed for daily load: 18kWh ÷ 4.5h = 4,000W
- Array needed to recharge 60kWh bank in 1 day: 60kWh × 1.2 ÷ 4.5h = 16,000W
Minimum recommended array: 16kWp (to fully recharge battery while powering loads on a cloudy day)
Solar Battery Sizing Examples
Residential off-grid (Philippines, family of 4)
Loads: 10kWh/day
Autonomy: 2 days
Battery: 48V LiFePO4 at 80% DoD
→ 10 × 2 ÷ 0.80 = 25kWh bank → 48V 400Ah LiFePO4 system
Array: 5kW (to recharge in 1 day with loads)
Commercial solar storage (Kenya, safari lodge)
Loads: 30kWh/day
Autonomy: 3 days
Battery: 48V OPzV gel at 60% DoD
→ 30 × 3 ÷ 0.60 = 150kWh bank → 48V OPzV system with 24 × 2V 1,500Ah cells
Array: 15kW
Telecom tower (Nigeria, off-grid mast)
Loads: 8kWh/day (typical LTE tower)
Autonomy: 5 days (remote location)
Battery: 48V OPzV gel at 60% DoD
→ 8 × 5 ÷ 0.60 = 66.7kWh → 48V 1,000Ah OPzV system
Array: 4kW with 48-hour recharge target
Key Sizing Mistakes to Avoid
Mistake 1: Not accounting for inverter efficiency
Battery kWh ÷ inverter efficiency = usable AC kWh. A 90% efficient inverter means 10% of your battery capacity is lost before it reaches your loads. Size battery and inverter together.
Mistake 2: Ignoring temperature derating
Battery capacity falls at low temperatures. A lead-acid battery bank rated at 25°C delivers only 70–80% of rated capacity at 0°C. For outdoor installations in cold climates, increase battery bank size accordingly.
Mistake 3: Oversizing for future loads you never add
Adding planned capacity during system design is prudent — but do not double the battery size “just in case.” Size for the loads you actually have, and add a 20% contingency instead.
Mistake 4: Ignoring the charge controller’s current limit
A 100A MPPT charge controller can only accept a limited solar array size regardless of battery capacity. Array watts ÷ battery voltage = maximum charge current. Do not exceed the controller’s current rating.
CHISEN Battery Solar Storage Solutions
CHISEN Battery supplies battery banks for solar installations from residential to utility scale:
- OPzV tubular gel series: 2V 100–3,000Ah — the standard for commercial and utility solar storage
- AGM VRLA battery banks: Pre-assembled 24V, 48V, and 96V packs for commercial buildings
- LiFePO4 energy storage systems: 48V residential and custom rack systems for C&I projects
- Containerized energy storage: Complete 100kWh–2MWh container solutions available
- Technical support: Free battery sizing service — send your daily load profile and location for a sizing recommendation
- Certifications: CE, IEC 62619, UN38.3, UKAS, TUV Rheinland (select models)
Send your project specifications for a free battery sizing and quotation:
📧 jack@chisen.cn | WhatsApp: +86 131 6622 6999 | www.chisen.cn
