Solar System Designer Tools
Solar Charge Time Calculator
Find out exactly how many hours of sunlight it takes your solar array to fully recharge your battery bank — accounting for real-world efficiency losses, charge controller overhead, and battery chemistry.
How Charge Time Is Calculated
The basic formula for solar charge time is straightforward, but real-world performance depends on several loss factors:
Charge Time (hours) = Energy to Restore (Wh) ÷ Effective Solar Power (W)
Effective solar power accounts for:
- Wiring losses: Typically 3–5% lost in cables and connections
- Charge controller efficiency: MPPT controllers are 93–97% efficient; PWM controllers 70–80%
- Battery acceptance rate: Batteries charge more slowly as they approach full capacity
- Temperature derating: Hot panels produce less power than rated (derating 0.4–0.5% per °C above 25°C)
MPPT
93–97% efficient — best for large systems and high-voltage arrays
PWM
70–80% efficient — suitable for small 12V systems only
Wiring Loss
3–5% typical — minimize with correct wire gauge
Charge Time Reference Table
Approximate charge times for common system sizes using MPPT controller (95% efficiency), 5% wiring loss, and 5 peak sun hours:
| Battery Bank (Wh) | Solar Array (W) | Approx. Charge Time |
|---|
| 1,200 Wh (100Ah 12V) | 200W | ~6.7 hours |
| 2,400 Wh (100Ah 24V) | 400W | ~6.7 hours |
| 4,800 Wh (100Ah 48V) | 600W | ~9.0 hours |
| 10,000 Wh | 2,000W | ~5.6 hours |
| 20,000 Wh | 5,000W | ~4.5 hours |
These are for fully depleted batteries charging from empty. Most systems only partially recharge each day.
Frequently Asked Questions
How many solar panels do I need to charge a 200Ah battery in one day?
A 200Ah 12V battery holds 2,400 Wh. With 5 peak sun hours and 90% system efficiency, you need roughly 2,400 ÷ (5 × 0.90) = 533W of panels. That's typically 2–3 × 200W panels or 1–2 × 400W panels.
What is a "peak sun hour" and why does it matter for charge time?
A peak sun hour is one hour of solar irradiance at 1,000 W/m² — the standard test condition for panel ratings. Most US locations receive 3.5–6 peak sun hours per day. Your total daily energy harvest equals panel wattage × peak sun hours. Use our
Peak Sun Hours Calculator to find your location's value.
Does battery state of charge affect charging speed?
Yes. Charging slows significantly in the final 20% (absorption phase). A 100Ah LiFePO4 battery might reach 80% in 2 hours but take another 1–2 hours to complete the last 20%. The calculator accounts for this with a charge acceptance factor.
Should I use MPPT or PWM charge controller?
MPPT (Maximum Power Point Tracking) is almost always the better choice. It's 15–30% more efficient than PWM, especially in cold weather and with higher-voltage panel strings. PWM only makes sense for very small 12V systems under 200W where cost is the primary concern.