Solar Charge Time Calculator: Why Is It So Slow?
Solar Charge Time Calculator: Why Is It So Slow?
You bought a 100Ah battery and a 100W panel. You assume it will charge in 12 hours of daylight. In reality, it might take 3 days. Why? Because physics doesn’t care about marketing. Use this tool by ShockTrail to calculate the real-world charge time of your system.
The Charging Estimator
Enter your battery size and total solar wattage to see how many “Peak Sun Hours” you need.
Charge Time Calculator
Solar Logic by ShockTrail0.0 Hrs
The “5-Hour” Rule (Peak Sun Hours)
The sun is only strong enough to produce maximum power for a few hours around noon. This is called “Peak Sun Hours.”
- 8:00 AM: Sun is low. Your 100W panel might produce 20W.
- 12:00 PM: Sun is high. You get ~90W.
- 5:00 PM: Sun is low. Back to 20W.
To find the exact Peak Sun Hours for your location, use the PVWatts Calculator by the National Renewable Energy Laboratory (NREL).
System Efficiency Losses
Energy is lost at every step. According to Solar Reviews, real-world system efficiency is typically around 75-85%. Where does the power go?
| Loss Factor | Estimated Loss | Cause |
|---|---|---|
| Heat | -10% | Solar panels lose efficiency as they get hotter. |
| Wiring & Controller | -5% | Resistance in copper wires and MPPT conversion. |
| Dirt / Dust | -5% | A thin layer of dust blocks photons. Clean your panels! |
| Total Real Power | ~80% | A 100W panel gives you ~80W on average. |
Battery Chemistry Matters: Absorption Phase
Lead-Acid batteries (AGM/Gel) charge fast until they hit 80%, then they slow down drastically (Absorption Phase). It takes hours to top off that last 20%. Lithium (LiFePO4) accepts full current almost until 99%, making them much faster to charge.
Leading manufacturer Battle Born Batteries explains that Lithium’s low internal resistance allows for significantly reduced generator run times compared to lead-acid.
Frequently Asked Questions (FAQ)
How can I charge faster?
The only way is to add more solar panels (Watts). If your battery can accept 50 Amps of charge current, you can keep adding panels until you hit that limit.
Does MPPT charge faster than PWM?
Yes. In cloudy weather or early morning, an MPPT controller can harvest 30% more energy than a PWM controller, effectively reducing your charge time.
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