Solar charging time calculator
Enter the battery or power station capacity, how full it is now, your panel watts and how much sun you get. You'll see how many hours of full sun the charge takes, how many days that means, and how other panel sizes compare.
Other panel sizes, same battery and sun
| Panels (rated) | Realistic power | Peak sun hours | Days |
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How the calculation works
Charging power (W) = panel rated watts × real-world output, capped at the solar input limit
Charging time (peak sun hours) = energy needed ÷ charging power
Days = charging time ÷ peak sun hours per day
A panel's rated watts are measured in lab conditions: full sun of 1,000 W per square meter at a cell temperature of 25 °C. Outdoors, heat, dust, angle, wiring and the charge controller all take a share, so the calculator scales the rating down. Around 75% is a fair planning figure for fixed panels pointed at the sun. Portable panels lying flat on the ground often do worse, so the default here is 65%.
The result comes out in peak sun hours, the equivalent number of hours at full lab-strength sun. A day with 4.5 peak sun hours might have 12 hours of daylight, but most of it is weak morning and evening light. Our peak sun hours guide explains how to look up the number for your location and month.
If your capacity is in amp-hours, switch the unit to Ah and enter the battery voltage. The calculator multiplies the two to get watt-hours (100 Ah × 12.8 V = 1,280 Wh). The Ah to Wh converter does the same conversion on its own.
Worked examples
A 1,024 Wh power station with a 200 W portable panel
The power station is at 20% and you want it full. It needs 1,024 × 0.8 ≈ 819 Wh. A 200 W panel laid flat at 65% gives about 200 × 0.65 = 130 W. 819 ÷ 130 ≈ 6.3 peak sun hours. At 4.5 peak sun hours a day, that's about 1.4 days of sun. Doubling to 400 W of panels halves it to about 3.2 peak sun hours, which fits in one good day.
When the solar input limit decides
A 2,048 Wh unit charging from 10% to 100% needs 2,048 × 0.9 ≈ 1,843 Wh. With 800 W of fixed panels at 75%, the panels could deliver about 600 W, but suppose the unit's solar input is limited to 500 W. The charging power is then 500 W, and the time is 1,843 ÷ 500 ≈ 3.7 peak sun hours, under one day at 4.5. Adding a ninth or tenth panel would change nothing. Moving the panels you have into better sun would matter more.
Capping an average figure like this is a slightly pessimistic shortcut. In reality, the panels only hit the limit around midday. In the morning and late afternoon their output is below the limit and nothing is lost, so the real charge often finishes a little sooner.
What makes real charging slower or faster
- The last few percent. Lithium batteries usually charge at a steady rate until they're nearly full, then the charger tapers the current to top them off. The last stretch to 100% can take longer than the calculator shows. If you only need 80% or 90%, set that as the target.
- Using power while charging. Anything plugged in during the day uses solar output first. As a rough fix, subtract its average watts from the realistic charging power, or add its daytime watt-hours to the energy needed.
- Heat. Panels produce less as they get hotter, so a cool, bright spring day can beat a hot summer afternoon.
- Angle and shade. Tilting panels toward the sun and moving them a couple of times a day helps a lot. Shade on even part of a panel can cut its output far more than the shaded area suggests.
- Battery charging losses. Lithium batteries lose a few percent as heat while charging. Lead-acid batteries lose more, and they spend a long time in the slow topping-off stage.
Frequently asked questions
How long does a 100 W solar panel take to charge a 100 Ah battery?
For a 12 V lithium battery (100 Ah × 12.8 V = 1,280 Wh) going from 20% to 100%, it needs about 1,024 Wh. A 100 W panel at 75% gives about 75 W, so that's 1,024 ÷ 75 ≈ 13.7 peak sun hours, or about 3 days at 4.5 peak sun hours a day. A 200 W panel takes about half as long. Set the unit to Ah above to try your own battery.
How many solar panels do I need to charge a power station in one day?
Divide the energy needed by your peak sun hours and the real-world output. For 1,000 Wh with 4.5 peak sun hours at 75%: 1,000 ÷ (4.5 × 0.75) ≈ 296 W, so about 300 W of panels, as long as the power station accepts that much solar input. The solar panel size calculator works this out from your daily use.
Why is my power station charging slower than the panel rating?
That's normal. A 200 W panel almost never delivers 200 W outside the lab. Readings of 60–80% of the rating in good sun are common, and much less in haze, heat, shade or at a low angle. If you're far below that in clear midday sun, check the angle, shade, cable connections and whether the panel voltage is within the unit's input range.
Can I charge in cloudy weather?
Yes, but slowly. Heavy overcast can cut output to a small fraction of the rating. Set the peak sun hours to the cloudy preset to see what a run of gray days does to your charging time.