in short, a 20W solar panel will charge a 12v battery with 1.6 amp input which will take 12 hours to fully charge a 20A battery from 0-100% or 13 days (with 5 hours of sunlight per day) to charge 100Ah fully discharged battery
There are a few key points to remember before we do the calculation
- Convert the battery capacity from Amp-hour (Ah) to watt-hours (Wh)
- how many watts can a 20w solar panel per day
- The size of a 12v battery
Battery Capacity In Watt-hours (Ah – Wh)
Converting the battery capacity in watt-hours from amp-hours will give us a more clear idea because we measure the capacity of solar panels in watts and the battery capacity in amp-hours
a watt is a unit that describes the total number of power & an amp is a unit of current. by multiplying amps and volts you’ll get the total watts
Watts = Amps * volts
so in this case amps will be the total Amp-hours (Ah) of your battery and the number of volts will be 12v (12v battery)
For this, we’ll take a 100Ah lead-acid battery as an example with a 50% DOD (Depth of discharge) limit which means you can only draw 50% from the total capacity of the battery to keep the battery safe.
100 * 12 = 1200 Wh
1200 * 0.5 = 600 watts
which means you can draw 1200 watts from a 100Ah 12v lithium battery with 100% discharge and 600 watts from a lead-acid or AGM battery with a 50% DOD limit
there are different sizes of 12v batteries available starting from 5Ah to 200Ah. So now I’m gonna share a table with converting battery size from Ah to Wh according to its DOD limit
|12v battery size (Ah)||Equal to Wh (with 50% DOD limit)||Equal to Wh (with 80% DOD limit)||Equal to Wh (with 100% DOD limit)|
You got the idea! this is how you can calculate the amp-hours to watt-hours
we’ll go with 100Ah lead-acid battery with a 50% DOD limit for further calculation which means we need 600 watts of power from the solar panels to recharge the 50% discharged 12v 100Ah battery. but you can do the math according to your 12v battery size
How Much Energy Does a 20W Solar Panel Produce? (Wh)
After calculating the watt requirement for your battery now let’s calculate how many watt-hours your 20W solar panel will produce per day / per hour so we can determine how long will it to recharge the battery
First of all, let’s see how many hours of sunlight your geographic location receives on average per day. Visit this website, Enter your location in the search box. You’ll get different values but DNI (Direct Normal Irradiation) will give the average peak sun hours in your area
I live in Florida city and we receive on average 5 hours of peak sunlight (this value describes the peak sun hours for solar panels, not sun hours for whole a day) but on average we receive about 6-7 hours of sunlight but the problem is that your solar panels will not produce that much power during other day hours like early in the morning or at the evening time
we’ll go with 5 hours of peak sunlight per day for this calculation
Your 20W solar panel will produce 20 watts under standard testing conditions which are rare so on average you’ll get 15-18 watts of output per hour from your 20W 12V solar panel and 75-90 Watt-hours per day
How Fast will a 20W Solar Panel Charge a 12V Battery?
After calculating how many watts we need and how many watts a 20W solar panel can produce not let’s check how long will it take to charge a 12v battery
12v 100Ah battery with 50% DOD limit (600W)
600 / 90 = 6.6
This means you’ll need 7 12v 20W solar panels to fully charge 100Ah (50% discharged) battery in 5 hours or a single solar panel will take 7 days to recharge your battery
|12v Battery capacity (Ah)||Estimated charging time with 20W solar panel (50% DOD)||Estimated charging time with 20W solar panel (80% DOD)||Estimated charging time with 20W solar panel (100% DOD)|
|5Ah||1.6 hours||2.6 hours||3 hours|
|10Ah||3 hours||5.5 hours||6 hours|
|20Ah||6.5 hours||8 hours||2 days|
|50Ah||3.3 days||5.3 days||6.6 days|
|100Ah||6.6 days||10.6 days||13 days|
|150Ah||10 days||16 days||20 days|
|200Ah||13.5 days||21 days||27 days|
I have made this calculation on the base of 90wh per day from a 20W solar panel with 5 hours of peak sunlight hours. This is how you can calculate how long it will take to charge a 12v battery
Video – How to connect 20W solar panel with battery
Can you connect a solar panel directly to a 12V battery?
If you have a small size solar panel like 10 watts then you can connect your solar panel directly to the battery but for large solar panels then 10W you’ll need a charge controller which will adjust the voltage (18V) coming from the solar panel according to battery volts (12v).
a 12v solar panel will produce 18V under direct sunlight in the noontime but a 12v battery can accept 12-14v depending on the remaining percentage charge in the battery so you’ll need a charge controller which will regulate the voltage
Related post: How to use solar panels without batteries?
Will a 20W solar panel charge a car battery?
car batteries come in 12v so Yes, a 12v 20W solar panel will charge a 12v car battery but the charging time may take longer because you’ll get a maximum of 100wh of capacity from your 20W solar panel in a day. This means you can only add 100 watts of power to your car battery in a day
Will a 25 watt solar panel charge a 12 volt battery?
in short, Yes a 25W solar panel will charge a 12v battery. Which will add 100-125 watt-hours in a day.
Related Post: What can a 25W solar panel run?
Will a 30 watt solar panel charge 12V battery?
With a 30W solar panel, you can add up to 150 watts of power into your 12v battery in a day with 5 hours of good sunlight