Off-Grid Medical Clinic in Rural Nepal

Scenario

A solar-powered off-grid medical clinic in Solukhumbu District, Nepal (elevation ~2,800 m), serves 120 patients monthly and operates refrigerated vaccine storage, LED lighting, diagnostic equipment, and a laptop-based EMR system. Grid connection is impossible; diesel backup is prohibited per local environmental policy. Key constraints: extreme diurnal temperature swings (−5°C to 25°C), limited roof space for PV, and strict reliability requirements—no power interruption for cold-chain storage.

Given Data

• Daily energy consumption: 4,200 Wh (measured via 7-day load audit, including 800 Wh for vaccine fridge cycling)
• Days of autonomy: 5 (monsoon season with 5+ consecutive cloudy days)
• Temperature derating factor: 0.75 (based on battery manufacturer’s LiFePO₄ spec sheet at −5°C ambient)
• Depth of discharge (DoD): 0.8 (selected to balance cycle life and capacity; batteries rated for 3,000 cycles @ 80% DoD)
• System voltage: 48 V (chosen to reduce current, minimize voltage drop over 15 m cable run from battery shed to clinic)
• Inverter efficiency: 0.92 (high-efficiency pure-sine-wave inverter, tested under partial load)

Calculation

The Battery Bank Sizing Calculator uses the formula:

Battery Capacity (Ah) = (Daily Energy Consumption × Days of Autonomy) / (System Voltage × Depth of Discharge × Inverter Efficiency × Temperature Derating Factor)

Substituting values:

• Numerator = 4,200 Wh × 5 days = 21,000 Wh·days
• Denominator = 48 V × 0.8 × 0.92 × 0.75 = 48 × 0.552 = 26.496 V·dimensionless
• Capacity = 21,000 / 26.496 ≈ 792.6 Ah

Rounded up to nearest standard battery configuration: 800 Ah @ 48 V.

Result and Decision

An 800 Ah, 48 V LiFePO₄ battery bank was specified—comprising four 100 Ah × 48 V modules wired in parallel (each module = 4 × 12 V cells in series). This met autonomy, cold-temperature performance, and footprint constraints. The design included integrated battery management system (BMS) with low-temp charge inhibition and heated enclosure (passive + resistive trace) to maintain >5°C cell temperature.

Lesson

Temperature derating isn’t just a safety margin—it’s a performance gatekeeper in high-altitude off-grid systems; always validate the factor against battery-specific low-temp discharge curves—not generic tables.