A 200-watt solar panel in full sun can charge a typical 100 amp-hour 12-volt battery from empty to full in an average of 6-8 hours or less, with higher capacity batteries taking longer.
Efficiency losses mean real-world charge times usually exceed the theoretical minimum.
Solar Panel Power Rating
The most basic factor is the power rating of the solar panel itself. Power is measured in watts, so a 200-watt solar panel has a peak power output of 200 watts under ideal sunlight conditions. This number provides a starting point for calculations, but real-world conditions usually result in lower actual output.
The intensity and quality of the available sunlight impacts solar output. Factors like cloud cover, pollution, shade, and the sun’s position in the sky all play a role. In general, a 200-watt panel in full, direct noonday sun will produce closer to its rated output than one in early morning shade. Tracking panels that follow the sun can help improve intensity.
The capacity and voltage of the battery being charged also impact charge times. A 100 amp-hour 12-volt battery requires 1,200 watt-hours to go from empty to a full charge. So even in ideal conditions, a 200-watt panel would take around 6 hours to fully charge such a battery. Larger or smaller batteries will vary the charge time.
Solar panels produce direct current (DC), while batteries run on DC but need controlled charging voltages. A charge controller regulates the voltage and current from the solar panels to prevent over or under-charging the batteries. It has its own efficiency losses, resulting in charge times around 5-10% longer. Controllers also usually turn off charging when batteries are full.
Resistance losses in the wires connecting components will result in some wasted power. Thicker solar wires minimize this effect, improving charging efficiency. But running wires over long distances inevitably increases resistance and charge times. Keeping wire runs as short as possible helps maximize power transfer.
Extreme cold or hot ambient air temperatures can reduce solar efficiency by up to 20%. Colder weather tends to have the bigger impact. Solar panels and batteries also run less efficiently at temperature extremes. Location and climate are a factor for any solar power system.
No battery charges at 100% efficiency from solar panels, thanks to system losses from resistance, heat, controllers, and other factors. 80% is considered very efficient. So a solar panel rated at 200 watts may effectively deliver around 160 watts after losses, extending charge times. High quality equipment improves efficiency.
Battery State & Chemistry
A battery’s existing state of charge impacts how readily it accepts a charge. A fully depleted battery will initially soak up current faster than one that is already half charged. Battery chemistry also plays a role, with lithium solar batteries now rivaling traditional lead-acid ones in charge rate capabilities. But for a given battery, state of charge is key.
Eng. Matthew Joseph Nandirio is the Founder of walkingsolar.
After graduating from the University of Houston in 2002, matt started working as a Solar Electrical Engineer for several multi-national solar energy companies.
He has a wide range of experiences including solar system requirement analysis, planning, maintaining, debugging and even solar device development through research.
He now shares his 20 years of expertise through his articles on the walkingsolar website.
Further, he is also the author of two books on Solar Technology, “Solar Power for Villages” and “DIY Solar System for Dummies”.