What Size Solar Panel to Charge 120ah Battery?


Before we explore the answer to this question, we need to know do you have removed the solar battery? When i can ensure 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 discharging any kind of solar battery 100%, adn this situation cannot be seen at most times. While it’s true that some lithium-type batteries can be discharged more than 95%, most lead-acid types are discharged between 20% to 80%, depending on their design.

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

In general, a 120Ah deep-cycle lead-acid battery would require a 200 watt solar panel to recharge from 50% Depth of Discharge (DOD) if we assume 4 peak-sun-hours per day. It would take one day to fully recharge assuming clear skies.

How to Size Solar Panels Power Using Irradiance?

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

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 under a day, so estimate 5 to 6 hours, 2 to 3 hours either side of mid-day.

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

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

There are always losses in solar circuits, so it’s always prudent to add 50% to any sizing you come up with. Also, there might not be clear skies – every passing cloud reduces the solar panel energy output.

Now, please see the table below gives a good idea of the energy variation found between geographical locations and  shows how the value of peak-sun-hours can make quite a difference when sizing solar panels:


Glasgow, UK

Chicago, Il

Lancaster, Ca





Solar panel rating in watts needed to fully charge each battery type in one day - full capacity  120Ah

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

155 watts solar panel rating required

75 watts solar panel rating required

36 watts solar panel rating required

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

390 watts solar panel rating rerquired

180 watts solar panel rating required

90 watts solar panel rating required