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Solar charging time calculator

Solar charging time is the watt hours you need divided by the watts that actually reach the battery. Enter your station, panels and sun below and see the hours of good sun needed, plus how much charge one day adds.

By The WattFort TeamUpdated Runs in your browser, nothing is sent anywhere

Your station, panels and sun

Panel presets are common portable sizes. Change any figure to match your panels and your station spec sheet.

Solar panels on a house roof beside palm trees

How the calculator works

  • Watt hours needed = capacity × (target charge minus current charge) ÷ 100.
  • Effective charging watts = panel watts × efficiency × the sunlight condition factor. Panel watts are first capped at the station solar input limit, if you enter one. Full sun uses a factor of 1, partly cloudy about 0.57 and overcast about 0.17, applied on top of your efficiency figure.
  • Hours to charge = watt hours needed ÷ effective charging watts. Those are hours of sun at that condition, not clock hours on a dull day.
  • Charge gained per day = effective charging watts × peak sun hours a day, shown capped at the amount you still need.

The assumptions are stated so you can change them. A 75 percent efficiency factor in full sun is a planning figure for heat, angle, dust, cables and the charge controller. Panels that are flat, shaded or behind glass can do far worse, so lower the efficiency or pick partly cloudy. Charging also slows near full on many stations, so allow extra time for the last part of the charge. For sizing the station itself, start with our power station runtime calculator, then return here for the recharge plan.

Frequently asked questions

How do I calculate solar charging time for a power station?

Work out the watt hours you need, which is capacity multiplied by the charge you want to add, then divide by the watts that actually reach the battery. With 400 W of panels at 75 percent efficiency, 300 W reaches the battery, so taking a 1,000 Wh station from 20 percent to full means 800 Wh divided by 300 W. That is about 2.7 hours of good sun.

Why do panels rarely give their rated watts?

Panel ratings are measured in laboratory conditions. Heat, sun angle, dust, cable losses and the charge controller all take a share. This calculator uses 75 percent as the default planning factor for good sun, and you can lower it for fixed, flat or shaded panels.

What are peak sun hours?

Peak sun hours are not daylight hours. One peak sun hour is one hour of sun at the full strength used to rate panels. A bright day might give 4 to 6 peak sun hours, and a short winter day far fewer. The US DOE Energy Saver pages and the PVWatts calculator from the National Laboratory of the Rockies (formerly NREL) can help you estimate them for your location.

Does a bigger panel always charge faster?

Only up to the maximum solar input of your power station. If the station accepts 400 W of solar and your panels could give 600 W, the extra mainly helps in weak light. Check the maximum input watts and the allowed voltage on the maker spec sheet before you buy panels.

Can I charge and run appliances at the same time?

Many stations allow pass through use, but the loads take power that would otherwise charge the battery, so charging slows. For the clearest estimate, work out charging with no loads first, then expect longer times while a fridge or router is running from the station.

Sources

Figures are typical values for planning, not a substitute for your appliance labels or an electrician's load calculation.

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