Can a 100 watt solar panel charge a 100Ah battery?

Yes, but slowly. At 75% system efficiency a 100 watt panel delivers about 75 Wh per peak sun hour, half of what a 200 watt panel does. Refilling 600 Wh would take about 8 peak sun hours, so plan on more than one sunny day.

Do I need a charge controller?

Yes. A charge controller sits between the panel and the battery and regulates the charge so the battery is not overcharged. Simple controllers switch off at a set voltage, while PWM and MPPT controllers adjust the charging rate as the battery fills.

Does a solar panel charge a battery on a cloudy day?

It still produces some power, but much less. Peak sun hours measure how much usable sunlight a day delivers, and cloud, shade and short winter days all lower that number. Fewer peak sun hours means more days to charge.

Solar Battery Charging Time | Panel Watts, Sun Hours and Losses

To work out how long a solar panel takes to charge a battery, divide the watt-hours the battery needs by the watt-hours the panel delivers per peak sun hour. A 100 Ah, 12 volt battery going from 50% to full needs 600 Wh. A 200 watt panel at 75% system efficiency delivers 150 Wh per peak sun hour. So the charge takes 600 / 150 = 4 peak sun hours, or 0.8 days at 5 peak sun hours a day.

Try your own panel and battery in the solar charging time calculator. Here is each step.

The solar charging formula

Watt-hours needed = Battery Ah x Volts x (End % - Start %)
Wh per sun hour   = Panel watts x System efficiency
Peak sun hours    = Watt-hours needed / Wh per sun hour
Days              = Peak sun hours / Sun hours per day

For the example:

Watt-hours needed: 100 x 12 x 0.50 = 600 Wh.
Panel output per sun hour: 200 x 0.75 = 150 Wh.
Peak sun hours: 600 / 150 = 4.
Days at 5 sun hours a day: 4 / 5 = 0.8.

Charging the same battery from empty means 1,200 Wh, or 8 peak sun hours. That is 1.6 days at 5 sun hours a day.

Why watt-hours, not amp-hours

Solar panels are rated in watts, and batteries in amp-hours. Converting the battery to watt-hours puts both in the same unit. Multiply amp-hours by the battery voltage: 100 Ah x 12 V = 1,200 Wh. Half of that is the 600 Wh used above.

Peak sun hours

A panel’s watt rating is what it produces in strong, direct sunlight. Real days start and end with weak light. Peak sun hours squeeze a day’s sunlight into the number of hours at full strength that would deliver the same energy.

That is why the formula uses peak sun hours instead of daylight hours. A long summer day might have many hours of light but fewer peak sun hours. Your number depends on where you live, the season, the weather and which way the panel faces. The example uses 5 hours a day.

System losses: why 75%

A 200 watt panel rarely puts 200 watts into a battery. Some power is lost to heat, wiring, the charge controller, dust and panel angle, and the battery itself loses some during charging. The example rolls all of that into one 75% system efficiency, which turns 200 watts into 150 Wh per peak sun hour. If you have measured your own system, use your figure.

Battery chemistry matters here too. Lead acid is about 90% coulombic efficient, while lithium-ion is above 99%, according to Battery University. The guide to lithium vs lead-acid charging covers the differences.

PWM vs MPPT charge controllers

A charge controller sits between the panel and the battery. According to Wikipedia’s charge controller article, simple controllers cut off charging at a set high voltage and turn it back on when the battery voltage drops. PWM (pulse-width modulation) and MPPT (maximum power point tracking) controllers are more advanced. They adjust the charging rate to the battery’s level so it charges closer to full.

An MPPT charge controller also tracks the panel’s peak operating point as sunlight, shading and temperature change, as Wikipedia’s maximum power point tracking article explains. It can let a higher voltage array charge a lower voltage battery. Wikipedia’s example is a 150 volt array charging a 24 or 48 volt battery. A higher array voltage means lower current, which can cut wiring costs on long runs.

How much extra energy an MPPT controller gains over PWM depends on the panel, battery and conditions. Check the controller maker’s specs rather than relying on a single headline figure.

The slow end of the charge

Solar has the same tail as any other charging. A lead-acid battery fills its last 30% slowly during the topping stage, which Battery University says takes another 7 to 10 hours on a wall charger. If the sun goes down before that stage finishes, the battery may not reach full. Battery University also notes that lithium-ion does not need a full saturation charge, which is an advantage for solar setups that can’t always fill the battery.

How many panels do you need?

Flip the formula around. Decide how many watt-hours you need per day and divide by your sun hours and system efficiency. One 200 watt panel at 75% gives 150 Wh per peak sun hour, or 750 Wh in a 5 sun hour day. That is more than enough to refill the 600 Wh in the example. The FAQ on how many solar panels you need for a 100Ah battery covers this, and the battery runtime calculator shows how long the stored energy will last.

How Long to Charge a Battery With a Solar Panel