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Solar Charging Time Calculator

Estimate how much sun you need to restore a chosen amount of battery energy. Enter how much battery energy you want to restore in Wh, or in Ah with your battery's nominal voltage, along with your panel rating and sun conditions.

Not sure what to enter? Read the step-by-step guide.

Your inputs

What gets replaced?

Compatible fields are replaced with values from your saved setup, including blanks. Capacity values and units are replaced together where supported. Other fields stay unchanged. You can undo the replacement until you manually change one of the replaced fields. Edit My Setup

Battery energy to restore
What does energy to restore mean?
Enter only the energy missing from the battery that you want to replace, not its full rated capacity unless you are charging from empty to full. This estimate assumes no loads are using the battery while it charges.

Switching units clears the value so it is not interpreted using the wrong unit.

Your solar setup
What does rated panel power mean?
Add together the rated watts shown on the labels of all panels in the array. This is nameplate power, not a promise of continuous output.
What are peak sun hours?
Peak sun hours express a day's solar energy as the equivalent number of hours at full rated sunlight. Use an estimate for your location, season, and panel orientation.

Peak sun hours are not the same as daylight hours.

What does solar charging efficiency mean?
The percentage of estimated solar energy from the panels that is ultimately stored in the battery after panel, wiring, controller, and charging losses. This measures energy stored in the battery, before it is later used to power devices.

Enter the percentage of panel energy stored after losses. The calculator does not choose a default.

This model assumes no battery use while charging. Results update in this browser; no location or weather data is fetched.

Solar charging estimate

How much sun you could need

Estimated sun needed to restore the battery energy you entered.

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Enter the battery energy to restore, panel watts, peak sun hours, and charging efficiency.

The “sunny days” result is an energy-equivalent estimate, not elapsed clock time or a promise that the battery will reach full charge. Weather, shade, changing charge rate, controller limits, battery condition, and loads running during charging can change the outcome.

Edit your inputs
See calculation details, assumptions and sources

If entering Ah, energy to restore (Wh) = Ah to restore × nominal battery V. Stored Wh/day = panel W × peak sun hours/day × overall charging efficiency (as a fraction). Sunny days = Wh to restore ÷ stored Wh/day. Equivalent full-sun hours = Wh to restore ÷ (panel W × efficiency).

For example, restoring 1,200 Wh with 400 W of panels, 4 peak sun hours per day, and 75% overall charging efficiency gives 1,200 Wh/day stored: 4 equivalent full-sun hours, or 1 idealized sunny day. These are example inputs, not recommended defaults.

Peak sun hours summarize daily solar energy, not a period of constant midday output. The efficiency must represent the whole path from panel nameplate energy to energy stored in the battery. Do not count the same loss twice if your sun-hour input already includes it. This simplified model assumes no concurrent battery loads and does not simulate a battery's changing charge rate near full.

References: NREL explanation of peak sun hours and NREL PVWatts manual on solar resource and losses. See the site methodology.