A standard 100 watt solar panel with full sun exposure could provide complete daily charges for 35-50 Ah of lead acid battery capacity at 12V, or around 50 Ah at 24V.
For lithium ion batteries which require specialized charging, you may get ~50% of the rated battery capacity. So at 24V nominal, a 100 watt panel could charge a 100-200 Ah battery from 50% to full over longer periods.
What Factors Determine Battery Capacity
When determining how many batteries a solar panel can charge, you first need to consider the battery’s capacity and voltage. Common batteries used for solar energy storage include lead acid batteries, lithium ion batteries, and deep cycle batteries. The capacity of these batteries is measured in amp hours (Ah) or milliamp hours (mAh). Higher Ah ratings indicate larger battery capacity and require more power to fully recharge.
Deep cycle lead acid batteries generally range from 35-200 Ah in capacity for common solar usage. Lithium ion batteries often fall between 100-300 Ah. The voltage of most lead acid batteries is 12V or 24V, while lithium ion batteries usually operate at 3.6V or 3.7V per cell. Connecting batteries in parallel or series configurations can increase capacity and operating voltage. To estimate recharge requirements, you need to know the total battery capacity and voltage.
Sun Exposure and Solar Panel Output
Another key factor is the solar panel’s maximum power output in watts, as well as the average sun exposure it will receive. A standard 100 watt solar panel typically produces 4-5 amps of current in optimal conditions. Less sunny areas will lower energy production. Most solar panels have a power tolerance rating around +/- 5%, so actual performance may range from 95-105 watts.
Keeping efficiency losses to a minimum is also important. Using high quality cables, charge controllers, and inverters can help maximize usable solar power for battery charging needs. Monitoring system performance helps ensure maximum battery charging capacity.
Lead Acid Battery Charging Requirements
Given these variables, we can estimate how many lead acid batteries a 100 watt solar panel can charge. For example, a common 12V 35Ah sealed lead acid battery requires about 420 watt hours (12V * 35Ah) for a full charge. A 100 watt solar panel operating at 95% efficiency (around 95 watts) could fully charge a 35 Ah battery from 0% to 100% in about 4.5 hours with optimal sun exposure.
To estimate the maximum number of batteries our 100 watt panel can handle, we need to make some assumptions. Getting 6 peak sun hours per day is fairly reasonable for solar usage. At 95% efficiency, our panel could produce around 570 watt hours (95 watts * 6 h) of energy in a day. With our sample 12V 35Ah batteries requiring 420 watt hours each, our 100 watt solar panel could theoretically charge one battery per day.
Larger batteries or longer run times would require additional panels. Two 150 Ah batteries at 12V for example (3600 watt hours total), would likely require around 300 watts of solar capacity for a full daily charge. An off-grid solar powered cabin may use a 24V battery bank with 500-1000 Ah capacity, requiring 1500-3000+ watt hours per day. This would need a solar array with 500 watts or more, along with added battery capacity to store excess energy.
Lithium Ion Battery Considerations
Lithium ion batteries offer advantages over lead acid in terms of size, weight, efficiency, and cycle life. But they also require special solar charge controllers to avoid damage from overcharging. Their higher voltage output per cell means wiring batteries in series instead of parallel to achieve desired system voltage.
For a lithium ion battery with a 24V nominal voltage and 100 Ah capacity, the basic calculation would be:
- Watt hours = Voltage x Amp hours
= 24V x 100Ah = 2,400 watt hours
So our 100 watt panel could charge a 24V 100 Ah lithium battery from 0% to 100% in around 24 hours assuming full sun. But only partial daily charges are recommended for lithium ion, so a charge controller is required. Lithium also cannot discharge 100% without shortening lifetime. 50% is optimal.
So while technically a 100 watt panel could fully charge a 24V 100Ah lithium battery overnight, actual usable capacity from one charge would likely be 1,200 watt hours or less. This still translates to sizable battery storage potential from a standard 100 watt panel.
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”.