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How to choose a solar panel for a battery- Step by step guide

La energia solar es aprovechada utilizando paneles solares para transformar los rayos solares en electricidad.

Solar energy is harnessed by using solar panels to transform the sun’s rays into electricity.

Sometimes we don’t know how to charge a battery with solar panels, but here in this article I will explain how to choose a solar panel for a battery.

The correct selection of a solar panel is crucial for the operation of the entire photovoltaic system. It also affects the condition of the gel battery.

How to choose a solar panel for a battery

A panel that is too big or too small will either charge too quickly or not have the power to charge the battery, both of which have negative consequences for battery life. It is also important to choose a regulator. For a charge regulator that is too big, we will pay more and the excess energy will not be used, while a regulator that is too small will limit the flow of energy from the solar module or, in extreme cases, damage the regulator.

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How to choose a solar panel for a battery

Gel battery charging time.

The most optimal battery charging time is the so-called time of ten hours. This means that an exemplary battery with a capacity of 100 Ah must be charged with a current of 10 A for 10 hours. This is the most optimal battery charging time that ensures the longest battery life. Of course, in practice it may vary. PV systems are often selected to charge the system within 5-8 hours so that the batteries are fully recharged the next day after being fully discharged the next day. Of course, there is nothing wrong with it if we don’t shorten the loading time significantly.

The minimum charging time for  the gel battery  is 5 hours  . It should also be noted here that the nominal capacity of the battery decreases over time, which shortens the charging time. So how to choose the optimal solar panel for a gel battery. The easiest way is to reduce the battery capacity expressed in Ah (Ampere hour) to Wh (Watt hour), in this case we multiply the capacity of 100 Ah by 12 V (average) and obtain the battery capacity within the 1200Wh This means that our battery can work with a 100W charge for 12 hours and then it will discharge.

How to match the power of a solar panel with a battery

How to choose a solar panel for a battery? For our calculations we will use a 100W panel, this power is the nominal value and in real conditions, due to temperature and insolation, it can be even 10% lower, so it must be multiplied by 0.9. When calculating the charging time for a 100 Ah battery with a 100 W panel, use the formula. 1200Wh / (100 * 0.9) = 13.3 hours.

The calculation showed that with full sun exposure and a fully discharged battery, the charging time should be about 13.3 hours, which is an acceptable time. It should be remembered that the given formula is an estimate and not an accurate calculation because it does not take into account the variability of the sun, the resistance, the state of the battery, etc. It is also worth not taking the battery to maximum discharge. .

Using the above formula, we need to select the battery charging time to be no less than 5 hours and no more than 15 hours. However, if our calculations give us a time greater than 15 hours, it must be assumed that such a system should only work in a compensatory way. An example is a ship equipped with the aforementioned system, which waits in port all week and is used only on weekends. The standby time will be so long that the panel, even though it is too small, will eventually fully recharge the battery and keep it fully powered.

How to choose the type of charge regulator for the solar panel.

Depending on the solar panel we choose, we must choose the appropriate regulator for it. Working without a charge regulator will cause the battery to be charged with too high a voltage, which can lead to its destruction on the first day of use.

Depending on what interests us and for what purposes the system will be used, you should decide on one of the two types of regulator. The first, cheaper PWM is dedicated to small and undemanding photovoltaic systems with small panels, preferably up to 175W of power. These controllers charge the batteries in boost mode by shorting the panel to the battery. It is a good solution as long as we have full sun. However, if we want to work in the shade, we should choose the MPPT controller with power point tracking. Thanks to this system, we can gain up to 30% more power in the shade  or in unfavorable sunlight.

The power of the solar panels is selected according  to what the manufacturer declares  in the catalog file. This parameter is usually the power expressed in watts of a solar panel [Wp]. The manufacturer also declares the maximum operating voltage that must be respected. We pay less attention to the current because it is different on the solar panels and on the charge controller side.

Finally, it is worth adding that the panels connected in parallel (more with more and less with less) increase their current (A) but do not increase the voltage, and the panels connected in series (more less less) increase their voltage but do not increase the current. . Of course, in both cases, the power watts increase


The selection of the correct solar panel and charge regulator for the battery will not only guarantee the correct dynamics of the solar system, but will also increase its durability. When choosing a regulator, it is also worth considering the option of an LCD screen, which offers the possibility of monitoring the solar system. It should also be remembered that when choosing a voltage converter, it should also be selected appropriately to the size of the battery.

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