What size solar panel to charge a 120 Ah battery?


Before we explore the answer to this question, we need to know if you have removed the solar battery. When I can make sure that the number of solar batteries are removed or discharged, now I can definitely know the answer of what size solar panel to charge a solar panel.

In fact, I cannot ensure that any type of solar battery is 100% discharged, and this situation cannot be seen in most cases. While it is true that some lithium-type batteries can be more than 95% discharged, most lead-acid batteries are between 20% and 80% discharged, depending on their design.

Also, in this post, let's pay attention to lead-acid type batteries, after all, this type of battery is the most common type used. Even so, there are still two distinct variations of the lead acid cell structure.

In general, a 120 Ah deep cycle lead acid battery would require a 200 watt solar panel to recharge from 50% depth of discharge (DOD) assuming 4 peak hours of sunshine per day. It would take a day to fully recharge assuming clear skies.

How to dimension the power of solar panels using irradiation?

In general, the average value of 400 Wh/day is good to use as an example, to illustrate the process. The following calculation gives the number of 100-watt solar panels needed for each of the batteries described above:

The car battery: (120Ah x 20%) x 12 volts = 24 x 12 = 288 watt-hours.

A 100W solar panel would easily charge this battery in less than a day, so plan on 5-6 hours, 2-3 hours at noon.

Leisure-style deep cycle battery: (120 Ah x 50%) = 60 x 12 volts = 720 watt-hours.

This solar battery will need 2 x 100 watt solar panels to recharge in one day (2 x 100 watt solar = 800 Wh/day on average).

There are always losses in solar circuits, so it's always wise to add 50% to any size you can think of. Also, there may not be clear skies - each passing cloud reduces the power output of the solar panel.

Now, check out the table below which gives you a good idea of the variation in energy found between geographic locations and shows how the value of peak sun hours can make a big difference when sizing solar panels:

Solar panel rating of 155 watts required


Glasgow, UK

Chicago, Il

Lancaster, Ca





Solar panel rating in watts needed to fully charge each type of battery in one day: 120 Ah total capacity

Car battery - 24Ah discharged (20%) - 288 Wh required

Solar panel rating of 155 watts required

Solar panel rating of 75 watts required

Solar panel rating of 36 watts required

Deep cycle: 60 Ah discharged (50%) - 720 Wh required

Solar panel rating of 390 watts required

Solar panel rating of 180 watts is required

Solar panel rating of 90 watts is required