Practical Calculations for DIY Solar Systems: Covering Consumption, Storage, and Seasonal Variations
I. Basic Parameter Collection
Daily Electricity Consumption (Wh)
- Calculate the power (W) and daily usage time (h) of all devices:
Daily Consumption = Device Power × Usage Time
(Include inverter losses, e.g., if inverter efficiency is 90%, actual power required = Device Power / 0.9) - Example: A 60W device running 7 hours daily consumes 60W × 7h = 420Wh.
- Calculate the power (W) and daily usage time (h) of all devices:
Local Peak Sun Hours (h)
- Obtain from local meteorological data (typically 3-6 hours/day; lower in winter, higher in summer).
Backup Days for Rain/Clouds
- Set the number of days the system must operate independently (e.g., 3-5 days)25.
II. Battery Capacity Calculation
Total Reserve Power (Wh)
Total Reserve = Daily Consumption × Backup Days
Example: 420Wh/day × 3 days = 1260Wh.Battery Capacity (Ah)
- Convert based on system voltage (e.g., 12V/24V):
Capacity = Total Reserve / System Voltage / Depth of Discharge- Depth of Discharge (DoD): Lithium batteries use 0.97, lead-acid batteries use 0.7.
- Example: 12V system with lead-acid battery → 1260Wh / 12V / 0.7 ≈ 150Ah.
- Convert based on system voltage (e.g., 12V/24V):
III. Solar Panel Power Calculation
Daily Power Generation Requirement (Wh)
- Cover daily consumption and recharge battery losses:
Generation Requirement = Daily Consumption / Charging Efficiency
(Charging efficiency: 0.8-0.9).
- Cover daily consumption and recharge battery losses:
Solar Panel Power (W)
Panel Power = Generation Requirement / Peak Sun Hours / System Efficiency- System Efficiency: Accounts for wiring losses, dust, etc. (0.7-0.8).
- Example: 1260Wh required, 4h sun, 0.7 efficiency → 1260Wh / 4h / 0.7 ≈ 450W.
IV. System Optimization
Panel Series/Parallel Configuration
- Parallel Panels: Total power / Single panel power (e.g., 450W requires three 150W panels).
- Series Panels: System voltage / Single panel voltage (e.g., 24V system needs two 12V panels in series).
Adjustment Factors
- Increase power redundancy (+20%) for low winter sun; adjust panel tilt at high latitudes.
- Reserve battery capacity for overcharge/discharge protection (e.g., lead-acid batteries use 70-85% of rated capacity).
V. Example Validation
- Streetlight System (12V/60W, 7h/day, 3 backup days)
- Battery Capacity = (60W × 7h × 4 days) / 12V / 0.7 ≈ 200Ah.
- Solar Panel Power = (60W × 7h) / 4h sun / 0.7 ≈ 150W.
