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- What size solar panel do I need to charge a 100Ah battery?
- What’s the difference between a regular car battery and deep-cycle battery?
- What is Battery SOC and DOD, and why does it matter?
- How can you measure remaining battery capacity?
- Ah to Wh (amp-hours to watt-hours) – calculating battery capacity
- How to find daily solar irradiance data by location
- Can I connect a solar panel directly to a battery?
What size solar panel do I need to charge a 100Ah battery?
First, I would want to know – How much battery capacity has been used? If I know how much amp-hours have been taken out, or discharged, then I will know the amount of energy I have to put back in and size the solar panel accordingly.
I have never discharged a lead-acid battery 100% – that would be unheard of, unless it was unavoidable.
Some lithium-type batteries can be almost emptied, but lead-acid cells should only be discharged between 20% to 80%, depending if they are normal car-type batteries or deep-cycle design.
I’m going to talk about lead-acid batteries for the purposes of this post, as they are the most common type in use. In the lead-acid category there are two kinds of lead acid cell design.
In general, a 100Ah deep-cycle lead-acid battery would require 180 watts of solar panel to fully recharge from 50% Depth of Discharge (DOD) assuming 4.2 peak-sun-hours per day. It would take 8 hours to fully recharge with a clear sky.
What’s the difference between a regular car battery and deep-cycle battery?
Batteries known as ‘leisure’ or ‘marine’ batteries can be discharged to a much greater level than an ordinary car battery. That’s why they are much more expensive. They are known as ‘deep-cycle‘ batteries.
The two types of cell are designed to deliver power in very different ways. Deep-cycle batteries are made for applications that need medium level, steady currents for several hours at a time. Auto batteries need to deliver hundreds of amps fast for cranking heavy engines.
Their discharge capacities are different as well. You can discharge a deep-cycle battery down to 50% of it’s capacity and down to 80% if needed, without sustaining any damage.
That said, it’s best not to discharge a car battery more than 15%, or 20% at a maximum.
What is the difference between charging deep-cycle and auto batteries?
First, the 100Ah auto battery – let’s say it is regularly discharged down to 20%:
100Ah x 20% = 20Ah
A 10Ah deep-cycle leisure-type battery can be discharged regularly down to 50%:
100Ah x 50% = 50Ah
At 50% discharge, the deep-cycle battery will need more than twice as much solar power to fully recharge it.
I’ll assume the maximum discharge of 20% and 50% for each type in my calculations.
What is Battery SOC and DOD, and why does it matter?
SOC means State of Charge, while DOD are the initials representing Depth of Discharge. They are basically the same thing.
State of Charge is an indication of the amp-hours capacity left in the battery, and Depth of Discharge indicates how much power has been taken out, or discharged. DOD is more suitable for us, as this is the energy we need to replace with solar.
How can you measure remaining battery capacity?
A quick and dirty way to check the remaining amp-hour capacity of a lead-acid battery is by checking the terminal volts using a simple tester, such as a multi-meter.
The battery should be left in a dormant state for about 6 hours before taking the voltage reading, so there is no chemical reaction taking place.
Depth of discharge chart for a Lead-acid battery
Using the table below, match your terminal voltage to the percentage column to read off your battery’s remaining capacity. The chart is good for all types of lead-acid batteries.
State Of Charge % (12 V Pb)
Battery terminal voltage
Ah to Wh (amp-hours to watt-hours) – calculating battery capacity
Amp-hours (Ah) is a convenient way to measure battery capacity, but it can lead to an over-simplification of how batteries deliver current.
For example, the relationship between amps and time isn’t as straightforward as it seems. A 100Ah battery may deliver 10 amps for a while, but certainly not for 10 hours!
Even a deep-cycle model will only deliver 10 amps for 5 hours, and a car battery much less. Temperature also plays a part, as does the rate of current draw.
Energy in watt-hours (Wh) is a useful way of expressing battery capacity, because solar panel output over time is also in watt-hours – it makes complete sense to compare like with like.
Let’s look at the solar energy need to recharge each type of battery:
Car battery: (100Ah x 20%) x 12 volts = 20 x 12 = 240 watt-hours
Deep-cycle (leisure) battery: (100Ah x 50%) = 50 x 12 volts = 600 watts-hours
How much of the sun’s energy is available where you live?
How to find daily solar irradiance data by location
Although there are other things that affect solar panel efficiency, the biggest factor affecting solar panel power output is irradiance, which is the level of sun’s energy falling on the panels in your location.
Irradiance is measured in two ways:
- kilowatt-hours per square meter per day (kWh/m2/day)
- peak-sun-hours (basically the same units)
Irradiance according to your location and also with the seasons.
For instance, in Texas it’s a lot more than in London, UK – around 3 times more. This means that a solar panel with similar area will produce 3 times more power in Houston, Texas than London.
The easiest way to find the peak-sun-hours for your city is to visit the site Global Solar Atlas and use the historical irradiance for your location – the image below illustrates the process:
When you have the value of peak-sun-hours for your particular location, multiply by 100 and you’ll have the energy in watt-hours that a 100 watt solar panel will generate in your city.
A good average to use as a rule of thumb is that a 100 watt solar panel will generate about 400 watt-hours/day, which will vary according to where you are.
How to size solar panels power using peak-sun-hours
I’ll use 400Wh/day as an example, to show how it works. The calculation shows the number of 100 watt rated solar panels I would need to recharge each of the batteries:
Car battery: (100Ah x 20%) x 12 volts = 20 x 12 = 240 watt-hours.
One 100 watt solar panel could charge this battery in well under a day, about 5 hours.
Deep-cycle battery: (100Ah x 50%) = 50 x 12 volts = 600 watts-hours.
I would need 2 solar panels rated at 100 watts to recharge this battery in less than a day – 2 x 100 watts generates about 800 Wh/day.
Solar circuits always have losses, so I would add 30 to 50% to solar panel size you come up with. As well as the power lost due to inherent circuit losses, every passing cloud will reduce the energy produced.
This chart gives some idea of how the sun’s energy can vary between physical locations and how the number of peak-sun-hours affects solar panel sizing:
Solar panel rating in watts needed to fully charge each battery type in one day - full capacity 100Ah
Car Battery - 20Ah discharged (20%) - 240 Wh required
130 watts solar panel rating required
60 watts solar panel rating required
30 watts solar panel rating required
Deep-cycle - 50Ah discharged (50%) - 600Wh required
322 watts solar panel rating rerquired
149 watts solar panel rating required
75 watts solar panel rating required
Converting Ah to kWh is a good method in my opinion, as you’re comparing similar units of energy – the energy needed to recharge the battery expressed in kWh equals the kWh generated by the solar panels.
Instantaneous measures of current don’t relate to solar panel power output as it can change through the day according to the sun’s angle, and is also very much affected by any shading due to cloud cover.
Peak-sun-hours is a great average value of the energy put out by the sun through a day and is an accurate way to size solar panels for battery charging.
Can I connect a solar panel directly to a battery?
Never connect a solar panel directly to a battery, unless the panel power is so tiny that hardly any current is output. There are two choices for solar controllers and there basic operation is different.
MPPT vs PWM – which is better?
The Pulse Width Modulation (PWM) solar charger is the less efficient of the two models. It measures the battery volts and pulls the panel output voltage down to a value just above the battery volts.
The PWM principle of operation doesn’t take into account the Maximum Power Point of the solar panel. At the MPP of a solar panel, the panel voltage and the current are at maximum, and most charging power is generated.
The MPPT (Maximum Power Point Tracking) solar charge controller works in a different way.
Every circuit has its own internal resistance. In a solar panel it’s called the Characteristic Resistance.
An MPPT solar charger adjusts its own internal resistance so that it matches the the panel’s, so maximum power is generated.
MPPT solar charge controllers can provide up to 40% more power than PWM.
Can you connect two or more solar panels?
MPPT controllers can make use of higher voltage inputs than the PWM controllers. Even a 400 watt model can accommodate up to 60 volts. In fact, MPPT chargers are more efficient with higher solar input voltages.
Open circuit voltage for a 100 watt solar panel with 36 solar cells is about 21 volts. At a pinch, you could connect three together in a series configuration, as illustrated below:
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