What Size Solar Panel to Charge 12v Battery

Last Updated: April 25, 2022

Solar panels are perfect for you if you live in a van, cabin, or RV. Once you’ve shortlisted your kit, you need a battery to store the energy produced by it. How will you determine the charging need of the battery, and how will a solar panel be selected based on it? Let’s read ahead:

Battery type

These may be like car batteries, but they are actually different. They are designed for sustained energy provision over time. Deep cycle batteries can discharge up to 80% but discharging below 45% is not recommended. Going beyond this level regularly will shorten the battery’s lifespan.

Charging batteries with solar panels

Solar batteries easily store energy according to their capacity from the solar panels. An inverter, solar panels, and charge controller are required to use them with the solar installation. When solar batteries are used for home, you can store excess energy instead of sending it back to the grid.

First, you need to connect the solar panels to the charge controller to be able to monitor energy storage to prevent overcharging. If the batteries are depleted, the charge controller shuts down the system. Before turning the power on, the batteries must be connected to an inverter for DC to AC energy conversion.


Deep cycle batteries come with a specific amp hour rating. It is the range of current supplied by a battery over time. For example, a 200ah battery will provide 20 amps continuously for 10 hours.

Amperes in a 100-watt panel

The current can be calculated in amps by diving power (in watts) by voltage (in volts). A 12V solar panel is rated 100W, known as the instantaneous voltage rating. So, after meeting all test conditions, the output is measured, resulting in 18V. Now, watts are equal to volts times amps so the amperage will be 5.5 amps. Therefore, the panel is supposed to produce 5.5 amps/hour.

Charging time for the battery

The charging time will depend on the weather, type, and state of the battery. Once the battery is completely drained, the panel charges a battery in five to eight hours. Sun’s position in the sky will also affect the panel’s charging speed. Sunlight shining directly on the panel in mid-summer leads to a faster-charging rate. On cloudy days, the charging cycle is slower.


Charging a 12v battery requires the sizing of solar panels that depends on different factors. Battery capacity and discharge rate should be monitored to size accordingly. After defining these factors, the solar panel size will be resolute. The 12v battery capacity is listed on the specification sheet or printed on the unit’s exterior. The capacity is mentioned chiefly as amp-hours (Ah).

The discharge rate

Calculating the discharge rate is necessary if you want to use the batteries on charge. Some appliances run around the clock, including the air conditioner, refrigerator, lights, etc. It is better to determine their power draw to guarantee the power of solar panels to keep the operation of your appliances smooth and charge the battery banks.

The power draw is mentioned in watts, as the solar production capacity is. To understand the relationship of your battery capacity with the power draw, convert amp-hours into watt-hours.

If a 300Ah 12V battery is used, then after multiplying amp hours by voltage, the capacity can be determined in watt-hours – 3600 watt-hours (Wh). Such a battery bank can quickly run an appliance that demands 300 watts/12 hr.

The drawing of your appliances can also be determined. Some appliances indicate the expected consumption too. It can be present on the charging cord, the back of the appliances, or the charging block. It must show the wattage or the voltage and amp rating. If the latter is given, multiple volts and amps convert it into watts.

Once you get the power ratings of your appliances, the draw can be counted through these steps:

  • A refrigerator will take 150 watts per 4 hours, which makes 600 Wh
  • Around six LED lights consume 6 x watts per 6 hours, which produces 180 Wh
  • An air conditioner will take 1000 watts per 2 hours, which makes 2000 Wh
  • The total expected discharge daily would be 2780 Wh

The solar panel size can be determined after calculating the expected discharge, which is 2780 Wh.

Size of panels

As per the example stated above of a 300 Ah battery, the expected daily discharge is 2780 Wh; now, the size of the panel is easy to find out to charge battery banks and operate appliances. These appliances alone require 2780 Wh power.

In an ideal situation, the solar panels provide a system more than enough to meet your daily needs, so your batteries can charge maximally overnight to be used during the shade. The total battery capacity and the daily usage can be added up to give a figure of 6380 Wh, which will simultaneously allow charging of 12v batteries.

If you are working in an estimate of 6380 Wh, the power coming out of the panels can also be calculated. Solar panels are usually sold in the measure of watts. Although the calculation is straightforward, certain factors are to be remembered.

Solar panels don’t always work at their peak efficiency. Therefore a 70% production of power from the panels gives an accurate picture of their show during typical usage. To reach a maximum of 6380 Wh during 12 hours, we divide the desired energy production with the average sunlight hours daily to calculate the watts, which in this case are 531.67. Therefore, the solar panels must produce 532 watts of power in the safe zone to meet the goals and requirements.

Using the estimate of 70% from above, the percentage calculation brings us to the accurate sizing of the panel, which is 759.52 watts. This is the size of the solar panel required to power the 12v battery system for daily use.

Combination of Panels and battery systems

If there are no solar panels in the market that suit your need, multiple panels can be combined to reach the desired goal. A total of four 200-watt solar panels will be ideal to power the battery system we have under discussion. Similarly, two 400-watt solar panels will also be suitable.

The final output voltage is the critical focus factor when connecting the solar panels in series or parallel so that the solar charge controller can be selected accordingly. When these are connected in series, the total voltage is the sum of each panel voltage. However, the amperage is the same. On the other hand, total amperage is the sum of all panels and voltage stays the same in parallel.


It is a tiresome job to get at selecting solar panel and battery options. The deciphering and navigation of the amp hours and the voltages can be tricky. You will also get confused about the types of batteries that will suit you. Simple math can save you from a lot of hassle, and you can directly reach the best product waiting for you out there. We hope we helped take you to some point near your battery and solar journey. Happy shopping!

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