SolarMathKit

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Battery charging

Solar battery charging time calculator from solar panels

Use this solar battery charging time calculator with the battery state you have now and the charge level you need. The result separates bulk-equivalent solar time from the editable finishing-stage allowance.

Estimate solar charging time

Editable starting point. Check the battery maker's charging limits.
Charging assumptionsEditable planning values

Your planning estimate

Local calculation

Enter your values, then calculate. Results will show the formula inputs and rounding used.

Formula used

solar days = [battery V × Ah × (target SoC − start SoC) ÷ (panel W × controller efficiency × battery charge efficiency) + finishing hours] ÷ peak sun hours

The energy portion converts the missing battery capacity to effective panel time. A separate, visible finishing allowance represents taper or absorption behavior instead of hiding it inside a single efficiency factor.

See the full assumptions and rounding policy.

Maintenance and verification

Maintained by: SolarMathKit Formula version: 1.3.0 Last reviewed:

The calculator calls the shared, tested TypeScript calculation modules documented in the methodology. Automated tests cover published examples, invalid inputs, rounding, and result invariants.

SolarMathKit maintains this implementation. The review is an internal technical check, not an independent engineering certification.

Worked example: 12V 100Ah LiFePO₄ with 200W solar

Charge from 20% to 100% using 95% controller efficiency, 98% charge efficiency, five peak sun hours per day, and a 0.5-hour finishing allowance.

  1. Energy to restore: 12V × 100Ah × (1.00 − 0.20) = 960Wh.
  2. Effective charging power: 200W × 0.95 × 0.98 = 186.2W.
  3. Bulk-equivalent sun: 960Wh ÷ 186.2W ≈ 5.16 hours.
  4. Add 0.5 finishing hours, then divide 5.66 hours by 5 peak sun hours/day.
Result: The planning estimate is about 1.13 solar days, before cloud, shade, active loads, temperature limits, and controller clipping.

Common mistakes to avoid

  • Treating daylight hours as peak sun hours.
  • Assuming panel nameplate watts reach the battery continuously.
  • Ignoring the battery's maximum charge current or low-temperature limit.
  • Using the same finishing-stage allowance for every chemistry and charger.

Questions people ask

How long will a 100W panel take to charge a 100Ah battery?

It depends on voltage, starting state of charge, controller and battery losses, peak sun hours, and charge-stage behavior. Enter those values instead of assuming the battery is empty and the panel produces 100W continuously.

Why is lead-acid finishing time longer?

Lead-acid charging commonly includes an absorption stage where current tapers. Use the battery and charger documentation to replace the editable planning allowance.

Does the estimate include loads running while charging?

No. Subtract simultaneous load energy from available solar production or use a measured net charging rate.

Can I use this for a portable power station?

Use its documented usable capacity, solar input limit, and conversion behavior. Proprietary charge curves can make the generic estimate less accurate.

Before you build: Check voltage, current, temperature, protection, cable sizing and installation rules with the equipment manuals and a qualified professional.