Solar Panel Charge Time Calculator
Estimate how long it takes to charge your battery bank using solar power.
Estimated Charge Time
How to Calculate Solar Charge Time
Calculating the time it takes for a solar panel to charge a battery is critical for designing off-grid systems, van life setups, or emergency backups. While the math is straightforward, real-world variables like weather and system efficiency play a major role in the actual results.
The Fundamental Formula
To find the charge time, we first convert the battery capacity into Watt-hours (Wh) and then divide it by the effective output of the solar panels.
Step 1: Calculate Battery Watt-Hours
Watt-Hours (Wh) = Amp-Hours (Ah) × Voltage (V)
Step 2: Calculate Effective Solar Output
Effective Watts = Panel Rating (W) × Efficiency Factor (%)
Step 3: Calculate Time
Time (Hours) = Total Watt-Hours / Effective Watts
Why Efficiency Matters
A 100-watt solar panel rarely produces exactly 100 watts. Factors that reduce output include:
- Angle of Incidence: Panels produce the most when the sun is directly perpendicular.
- Temperature: Solar panels are actually less efficient as they get hotter.
- Controller Loss: PWM controllers are less efficient (approx 70%) than MPPT controllers (approx 95%).
- Wiring Resistance: Voltage drop across long cables reduces energy transfer.
Real-World Example
Imagine you have a 100Ah 12V Lead Acid battery and a 200W solar panel. You are using a standard system with roughly 75% efficiency.
| Component | Calculation | Result |
|---|---|---|
| Battery Capacity | 100Ah × 12V | 1,200 Wh |
| Effective Panel Power | 200W × 0.75 | 150 Watts |
| Charge Time | 1,200 / 150 | 8 Hours |
Pro Tip: Peak Sun Hours
Remember that "hours" in solar terms usually refers to Peak Sun Hours. If your calculation says 8 hours, but your location only receives 4 peak sun hours per day, it will take two full days to charge that battery from empty to full.