What size solar panel to charge 120ah battery?

Updated on November 7, 2022

What Size Solar Panel For 120Ah Battery?

I would ask first – How much has been taken out of the battery? Once I know how much has been taken out, or discharged, then I’ll know how much to put back and size a solar panel.

I don’t remember ever discharging any kind of battery 100%, that would be rare. 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.

I’ll focus on lead-acid type batteries for this post, as they are by far 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.

Difference between regular car battery and deep-cycle

Lead-acid batteries can be classified as deep-cycle, also known as ‘leisure’ or ‘marine’ batteries, and car batteries.

The internal structure is designed so that they deliver current in different ways. Deep-cycle batteries can supply low-to-medium, steady currents for some hours, while car batteries can supply hundreds of amps for less than a minute.

They also differ in the amount they they can be discharged. Deep-cycle batteries can be discharged regularly up to 50% and now and again as far as 80% of their capacity without damage.

Car batteries shouldn’t be discharged much more than 15% – let’s say 20% maximum.

Why is the difference between deep-cycle and auto batteries important?

120Ah car batteries and deep-cycle lead acid batteries have different capacities

For a 120Ah car battery, we’ll assume a discharge of 20%:

120Ah x 20% = 24Ah

For a 120Ah deep-cycle lead-acid battery, then the recharge required would be:

120Ah x 50% = 60Ah

So we could possibly need more than twice as much charging energy for the deep-cycle battery than for the car battery.

Our calculations will include both types of batteries, and it’s best to assume the maximum discharge of 20% and 50%.

Battery DOD vs SOC – What is Battery SOC and DOD?

SOC stands for State of Charge and DOD stands for Depth of Discharge. Without splitting hairs, they are just two ways of saying the same thing – how much has been taken out of the battery i.e. how much needs to be replaced?

State of Charge tells us how many amp-hours is left in the battery, while Depth of Discharge tells us how much has been taken out. We’re interested in DOD, because this is the capacity we’re going to replace using solar.

How to estimate remaining battery capacity

A rule-of-thumb way to check a lead-acid remaining amp-hour capacity is by measuring the terminal volts with a multi-meter.

The voltage reading should be taken when the battery has neither been charging or discharging for around 6 hours.

Lead-acid battery depth of discharge chart

Using the table below, match your terminal voltage to the percentage column to read off your battery’s remaining capacity:

 State Of Charge % (12 V Pb) Battery terminal voltage 100 12.73 90 12.62 80 12.50 70 12.37 60 12.24 50 12.10 40 11.96 30 11.81 20 11.66 10 11.51
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How many watt hours in a battery – converting Ah to Wh  (amp-hours to watt-hours)

Most everybody knows that battery capacity is measured in amp-hours (Ah). Many people think that it’s a simple calculation, that you can either get 10 amps out of a 100Ah battery for 10 hours, or 100 amps for 1 hour.

Unfortunately, this isn’t the case. How much you can draw from a battery i.e. the rate of current draw, depends on several factors.

It’s much more useful to work out how much energy in watt-hours is left in the battery. Solar panel output over time is measured in watt-hours, so it makes complete sense.

Convert amp-hours to watt-hours to calculate battery recharge energy required – How many watt hours in a battery?

Assuming the maximum recommended DOD for the two types of batteries discussed, then the energy needed to recharge them fully would be:

Auto battery: (120Ah x 20%) x 12 volts = 24 x 12 = 288 watt-hours
Deep-cycle battery: (120Ah x 50%) = 60 x 12 volts = 720 watts-hours

Next step is to find out how much of the sun’s energy is available in your location.

Daily solar irradiance data by location – solar data

The biggest factor affecting how much power a solar panel puts out is irradiance, or the amount of sun’s energy available in your location.

It’s measured in kilowatt-hours per square meter per day (kWh/m2/day) but is also known as peak-sun-hours. This values changes according to your location.

For example, in Nevada it’s much greater than in Scotland, UK – about 3 times greater! This means that the same solar panel will generate 3 times more electricity in Nevada than Scotland.

The best way to determine the peak-sun-hours in your area is to go to the site Global Solar Atlas and find the historical data for your location – see the image below:

Historical data shows irradiance peak sun hours for any location

Once you know the number of peak-sun-hours for your city, multiply it by 100 and that will give you the energy in watt-hours that a 100 watt solar panel will produce where you live..

An average value is about 400 watt-hours/day, although it can be more or less.

How to size solar panels power using irradiance

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 100 watt 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 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.

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:

 Location Glasgow, UK Chicago, Il Lancaster, Ca Peak-sun-hours/day 1.86 4.03 8.07 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
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This is a good method IMO, as it matches like with like i.e. the energy need by the battery measured in kWh needs to be more than equalled by the kWh put out by the solar panels.

Measured values of current have little relation to solar panel output as it will vary naturally throughout the day and also if any shading occurs..

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The 4 hours centered around mid-day generate the most electrical power, while production in the mornings and evenings is very low.

Peak-sun-hours represents an average of the total sun’s energy throughout a day with clear skies and is a good assessment of the watt-hours available.

Do I need to use a solar charge controller?

A solar charge controller should always be used for 12 volt battery charging

The above calculations aren’t very accurate, as they assume full output from the solar panels and no losses. As a general rule, solar chargers should always use a solar charge controller and these always have losses.

There are two choices for solar controllers and there basic operation is different.

MPPT vs PWM – which is best and why?

The Pulse Width Modulation (PWM) solar charge controller checks the battery terminal voltage and adjusts its output voltage to a little above the battery voltage.

This design doesn’t take into account the Maximum Power Point of the solar panels – this is when the panel voltage is at the right level to generate the most charging current.

Every electrical device has an internal resistance. For a solar panel it’s known as the Characteristic Resistance.

The MPPT (Maximum Power Point Tracking) solar charge controller adjusts its own internal resistance to that of the panel. When this occurs, maximum current flows.

MPPT controllers can deliver up to 40% more charging current than the PWM type.

How do you connect two or more solar panels?

MPPT controllers can take higher voltage inputs than PWM – even low-power models allow up to 60V solar input.

The Voc (open circuit voltage) for a 36 cell 100 watt solar panel is about 21 volts . At a pinch, you could connect 3 together in series – see image below:

The voltage doubles when 2 solar panels are connected in series