Solar charging time calculator
Estimate energy balance—not outdoor clock time, electrical compatibility, or a weather forecast.
Enter assumptions and calculate.
Set the charging target
Enter nominal battery capacity, the assumed capacity factor, and starting and target charge percentages. The calculator estimates only the energy between those charge levels. If the target is already reached, no additional charging time is needed.
Separate panel output from station limits
Panel watts are the array's nameplate rating. The field-output factor represents your assumed fraction of that rating under the scenario. The model then caps power at the station's solar watt limit and applies charging efficiency. Use the manufacturer's voltage, current, polarity, and connector requirements to check the physical setup separately.
Concurrent load is power taken from the battery side during charging. Include the applicable conversion and inverter overhead in this input; entering only appliance watts would understate battery demand. Leave it at zero only when modeling charging without a concurrent load.
Read the time boundary correctly
Equivalent steady-input hours = energy still to store ÷ net charging watts. When additional charge is needed but load equals or exceeds stored input power, there is no net charging time to report. Outdoor power varies and charging can taper, so this is not an elapsed-time prediction.
Worked example
A 1,000 Wh battery at 100% assumed health charging from empty to full, with 400 W of panels, a 75% field-output factor, a 200 W station input limit, 90% charging efficiency, and no concurrent load needs 5.56 equivalent steady-input hours. The station limit caps input before charging efficiency is applied. This does not predict elapsed outdoor time.
Calculated with the same formulas as the interactive tool. Inputs are illustrative and results are rounded for display.
What does the optional daily balance mean?
Peak-sun-hours represent daily solar energy as equivalent hours at peak irradiance, not the length of daylight. Enter a location- and season-appropriate assumption from a source you trust. The daily model multiplies stored input power by peak-sun-hours and subtracts 24 hours of the concurrent load.
A positive daily balance does not prove that the battery can carry the load overnight or through poor weather. For an intermittent load, this constant-load daily scenario may not fit. Read the solar interpretation guide and size the battery-only requirement in the main calculator.