Designing an off-grid solar system requires more precision than grid-tied. Get it wrong and your batteries die early, your loads go dark, or you massively overspend. Here is the engineering process used by licensed electrical engineers — adapted for Philippine conditions.
Step 1: Calculate Your Daily Load (Wh/day)
List every appliance, its wattage, and daily hours of use:
Appliance | Watts | Hours/day | Wh/day |
|---|---|---|---|
LED lights (8 pcs) | 80W | 5h | 400 |
Refrigerator | 150W | 24h (30% duty) | 1,080 |
Electric fan (3 pcs) | 150W | 10h | 1,500 |
TV | 100W | 5h | 500 |
Phone charging | 30W | 3h | 90 |
Total | 3,570 Wh/day |
Step 2: Size the Battery Bank
Formula: Battery Ah = (Load Wh × Autonomy Days) ÷ (System Voltage × DoD × Battery Efficiency)
For 2 days autonomy, 48V system, 80% DoD LiFePO4, 95% efficiency:
= (3,570 × 2) ÷ (48 × 0.80 × 0.95) = 7,140 ÷ 36.5 = 196 Ah @ 48V
Use two 200Ah 48V LiFePO4 batteries in parallel = 400Ah (comfortable margin).
Step 3: Size the Solar Array
Philippine average peak sun hours: 4.5 PSH/day (conservative). Apply system losses (0.70 derate factor):
Array Wp = Load Wh ÷ (PSH × derate) = 3,570 ÷ (4.5 × 0.70) = 1,133 Wp
Round up to 1,200 Wp — use three 400W panels or four 330W panels.
Step 4: Size the Charge Controller and Inverter
MPPT Charge Controller: Array Isc × 1.25 = size required. For 1,200W at 48V: 25A minimum — use 40A MPPT
Inverter: Peak simultaneous load × 1.25. If peak load = 800W: use 1,000W inverter minimum
Find verified off-grid installers who follow this sizing method at solarenergyph.shop.
Engr. Jason Morales — Founder, SolarEnergyPH