Depending on the location of the installation, it can be more convenient to use solar instead of extending a power cable and a battery charger to the battery, for example, in the case of an outdoor LED light.
If so then how many watts of solar should you use and what time will it take to charge the battery?
To find these and other answers, read on. In this post, you’ll find how many watts of solar you can use for a 40Ah battery, expected charging time, and other related information so that you have the right expectations from the get-go.
How Many Watts of Solar to Use
Depending on how many hours of sunshine are available at the installation location, you’ll probably need at least 300Wh of solar panels to fully charge a 40 Ah battery assuming the battery is only discharged to only 50% of its capacity.
If your location receives an average of 4 hours of sunshine a day then you can use a single 75 – 100 watt solar panel.
Using a larger capacity solar panel will be overkill unless you have plans to grow the size of the solar bank later.
The calculation for Watt Hours of Solar
You can use the formula
Solar panel watt hours = (Battery capacity(Ah) x depth of discharge of the battery (%) x battery voltage)/ efficiency of the charging system.
For efficiency, I worked with 80%, i.e. you’ll need to supply more energy to the battery to replenish the used-up capacity because of the losses in converting electrical to chemical battery energy.
Using actual values in the formula, solar panel watts = (40Ah x 50% x 12V)/ 0.8 =300 watt hours.
How Many Watts of Solar
To estimate how many watts of solar panels to use, divide the number of watt-hours of solar by the average hours of sunshine received at your location.
For example, with 300 watt hours and assuming your location receives 4 hours of sunshine on average then, you should use 300 watt hours/ 4 = 75 watt or 100 watt solar panel to keep the battery charged.
How Many Solar Panels
There are several possible combinations you can use to attain a solar panel capacity in the range of 75-100 watts.
You can use a single 75 or 100 watt solar panel, 2 x 40 watt solar panels in parallel, or 2 x 50 watt solar panels in parallel. These are just a few of the possible configurations that you may be able to use.
How Long It Takes to Charge the Battery
It can take at least 4 hours (estimate) to fully charge the battery. This assumes it has been discharged to only 50% of its capacity.
Actual charge time will depend on if there is cloud cover, and whether the solar panels are clear of any debris or obstructions that can restrict how much sunlight the solar panels receive.
How Many Hours the Battery Lasts
A fully charged 40Ah AGM battery can power a 60 watt appliance for an estimated 4 hours before its capacity is reduced to 50% (recommended for maximum lifespan).
A lithium battery, on the other hand, has more usable capacity (80%) and therefore you can expect the same 60W load to run for much longer, in this case, 6.4 hours (estimated) before it is discharged to 80% of its capacity recommended).
How long the battery lasts depends on the power consumption of the connected appliance (watts), the level of charge of the battery, and the usable capacity of the battery.
Aging or damaged batteries are not able to hold as much charge and therefore expect the appliances to last for a shorter duration when connected to such batteries.
Closing Thoughts
Plan for at least a 75 – 100 watt solar pane to keep your 40Ah AGM fully charged each day assuming a location that receives 4 hours of sunshine a day.
As a guide, a 60 watt appliance will run for about 4 hours before 50% of its capacity is used up.
Actual runtime depends on the power consumption of the connected appliance, the level of charge of the battery, its usable capacity, and its condition.
Recommended Posts
