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- How to work out the size of solar panel needed
- How to estimate remaining battery capacity
- Convert depth of discharge in amp-hours to energy in watts
- How much power does a solar panel produce?
- What size solar panel do you need for your 12V battery?
- Do you need a charge controller?
- Related Questions:
Follow these steps to work out the best solar panel size for charging your 12V battery – this is what you need to know and do:
- What is the nominal terminal voltage, the capacity in amp-hours (Ah) and its State of Charge (SoC)?
- Convert the amp-hours discharged into energy measured in watt-hours (Wh)
- Calculate how much energy solar panels will generate in your location
- Note how fast you want the 12 volt battery to charge, and match the SoC to solar panel output.
In general, a solar panel with power rating in watts just above the battery’s amp-hour rating will charge a 25% discharged 12 volt lead-acid battery in 8 hours, It is recommended to use a solar charge controller with a solar panel rated more than 5 watts. Higher panel wattages may be used with a solar charge controller, in which case the charge time would be less.
Note: The following post assumes lead acid 12 volt battery at 25°C (77°F) and solar panel in direct sunlight.
Depth of discharge of a lead-acid battery can be assessed by measuring the terminal voltage with a multi-meter. This should be done:
- At a stable temperature preferable 25°C
- When the battery has been dormant for a few hours and settled, neither charging or discharging
Lead-acid battery depth of discharge chart
State Of Charge % (12 V Pb)
Battery terminal voltage
Convert depth of discharge in amp-hours to energy in watts
Why should we do this?
The battery capacity is measured in amp-hours while meaningful solar panel output energy is measured in watt-hours.
Battery amp-hour capacity doesn’t reflect the true power in the battery, because this is dependent on the battery voltage.
Converting to watt-hours is the only true way of comparing the energy of the battery compare to a solar panel output.
If you look at the specification sheet for a solar panel it will give you the maximum current it can generate.
However, this is in very special laboratory conditions. It’s quite rare to have sustained maximum power from a solar panel.
This is why instantaneous power at any time doesn’t really have any meaning. If a cloud passes, or the panel becomes shaded, then the current will drop.
By far the most sensible method of sizing solar panels to any application is to calculate how much energy it can generate over time in kWh, and then match this figure to the battery re-charge capacity needed.
Solar panel energy over time depends on irradiance, or peak-sun-hours, and it varies depending on your location.
The chart below shows average irradiance per day for various locations, but differences between summer and winter will also be significant.
Irradiance chart for 5 cities in USA and UK
San Fran., Ca
For example, Houston Texas has an irradiance of 5.79 peak sun hours in June and 2.58 in December, so you would need to take this into account with your calculations.
You can find out the peak-sun-hours for your location on this site. Simply enter your location and read of the direct sunlight reading as shown in the image below.
Irradiance is measure in kWh/m2/day or year, but the value is also known as peak-sun-hours and is used for sizing solar systems.
What size solar panel do you need for your 12V battery?
12 volt lead-acid batteries come in various capacities, from 5 amp-hours up to 200 amp-hours. You need to ask 4 questions before deciding what is the right size solar panel to charge your particular battery:
- What is your battery capacity in Ah?
- How much is it normally discharged?
- How fast do you need it charged?
- What is your location? (to find peak-sun-hours)
A 12 volt lead-acid battery has a fully charged capacity of 60Ah is discharged 25%. 8 hours charge time will be adequate. Location is Houston, Texas. Assume full sunshine.
Energy required = 60Ah x 25% x 12 volts = 180 watt-hours
Irradiance in Houston, Texas = 4.253 peak-sun-hours
A 80 watts solar panel will generate 80 x 4.253 peak-sun-hours = 196 watt-hours
So an 80 watt solar panel would be good enough to recharge this battery throughout the day, but it would be prudent to use a 100 watt or 120 watt rated panel.
Solar panel sizing chart for 12 volt battery charging
Use the chart to below to figure out the size of solar panel you would need for your battery:
12 Volt Lead-Acid Capacity in Amp-hours/Watt-hours
(assume 25% discharged) - 8 hours charge time - peak-sun-hours 4.0
Down - Solar Panel Size (watts)
X - adequate
X - preferred
Solar charge controllers are always recommended for battery charging applications unless the solar panel rating is so low that is cannot overcharge the battery.
Such devices are only suitable for trickle charging. A charge controller should be used in any case if using a solar panel over 10 watts.
Which is better PWM or MPPT?
There are two main types of solar charge controller – Pulse Width Modulation (PWM) and Maximum Power Point Tracking (MPPT).
The PWM is a dumb device and simply compares the voltage between the panel and the battery, and sets the output current by chopping up the output to achieve an average.
It’s quite inefficient, but adequate for small applications where efficiency is not the prime concern.
Maximum power can only be drawn from a solar panel when the load resistance matches the panel’s Characteristic Resistance. This is what the MPPT controller does.
An MPPT charge controller adjusts its own internal resistance until the panel’s voltage and current are at optimum levels. MPPT type charge controllers are up to 40% more efficient than PWM.
How long will a 100 watt solar panel take to charge a 12V battery?
A 100 watt panel can generate up to 400 watt-hours of energy per day with a maximum output current of 6 amp. A 60Ah battery with 25% discharge would be fully charged in 3 hours, assuming an MPPT solar charge controller was used.
How do you charge a 12 volt battery with a solar panel?
As a general rule, always use a solar charge controller when using a solar panel when charging batteries. In this way the battery will be protected from over-charging and a larger solar panel can be chosen to minimize charging time. A low-power solar trickle charger may not need a charge controller at all.
Can you charge a 12 volt battery with a 24 volt solar panel?
In general, always use an MPPT solar charge controller for battery charging applications. The input voltage is normally up to 60 volts and the voltage variations are irrelevant. MPPT controllers are very efficient, matching their internal resistance to the panel’s Characteristic Resistance and maximizing output watts.
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