Clinic Solar System That Fails at Night
A rural clinic has four 250 W solar panels, a 12 V 200 Ah battery, and a 1 kVA inverter. The daily loads are a vaccine refrigerator drawing 80 W continuously, six 10 W LED lamps for five hours, a 20 W radio for four hours, and phone charging at 60 W for three hours. The inverter efficiency is 85%, the battery should not be discharged below 50%, panel derating is 75%, and the site receives an average of 5 kWh/m²/day. On sunny days the clinic still loses power at night, and the refrigerator sometimes fails to restart in the morning. The installer says the panels are more than enough because 4 × 250 W is 1 kW. Staff want to add another battery of the same type in parallel.
1. Calculate daily AC energy demand, DC energy required from the battery, and usable battery capacity; compare them.
Read the worked answer
AC daily energy:
- Refrigerator: 80 W × 24 h = 1,920 Wh = 1.92 kWh
- Lamps: 6 × 10 W × 5 h = 300 Wh = 0.30 kWh
- Radio: 20 W × 4 h = 80 Wh = 0.08 kWh
- Phones: 60 W × 3 h = 180 Wh = 0.18 kWh Total AC = 2.48 kWh/day.
DC energy from battery/inverter: 2.48 / 0.85 = 2.92 kWh/day.
Usable battery capacity: 12 V × 200 Ah = 2,400 Wh = 2.4 kWh nominal. At 50% depth of discharge, usable = 1.2 kWh.
Comparison: Usable battery (1.2 kWh) is less than half of one day's DC demand (2.92 kWh). The battery cannot carry the clinic through the night, let alone a cloudy day. The PV array, on average, would produce 1 kWp × 5 kWh/m²/day × 0.75 = 3.75 kWh/day, which is just above daily demand on a good day, but there is almost no reserve and no autonomy.
2. Diagnose why the clinic loses power at night and why the refrigerator struggles to restart.
Read the worked answer
The clinic loses power at night because the load runs from the battery after sunset, and the battery's usable energy is only 1.2 kWh. The refrigerator alone uses 1.92 kWh AC per day, or about 2.26 kWh DC after inverter losses; if it runs significantly at night, it exhausts the battery. The PV array may recharge the battery during the day, but it cannot store enough for a full night plus a cloudy day.
The refrigerator restart problem has a second cause: at 12 V, a 1 kVA inverter can draw about 83 A continuously at full load, and much more during compressor start-up. That causes voltage drop, overload trips, and battery voltage sag. A 12 V system is poorly matched to a 1 kVA inverter and a motor load. The battery may also be ageing, so its real capacity is below 200 Ah. Adding an identical parallel battery would double nominal capacity but would not fix the 12 V voltage-drop and surge problem.
3. Recommend a sizing and wiring upgrade; say whether adding an identical parallel battery is sufficient.
Read the worked answer
Recommended upgrade:
- Move to a 48 V DC bus with an MPPT charge controller and a 48 V inverter, or use a 24 V system only if the load is kept small.
- Battery: at least 5 kWh usable, preferably 8–10 kWh, sized for two to three days of autonomy and 50% depth of discharge. Use a battery bank designed for cyclic duty, not a single 12 V unit.
- PV: at least 2 kWp to recharge the larger battery and cover cloudy days; keep the existing 1 kWp as an expandable sub-array if the controller allows.
- Inverter: choose a unit with surge capacity at least three to five times the refrigerator's running load and a low-voltage disconnect that protects the battery.
Is another identical parallel battery enough? No. It doubles the nominal Ah but keeps the 12 V bus, the same high currents, and the same surge problem. It may also create imbalance if the batteries are not identical in age and condition. The clinic should first do a load and autonomy calculation, then replace the battery bank and reconfigure the voltage. Until then, run the refrigerator during daylight and switch off non-essential loads at night.