What Solar Panel Size to Charge a 12V Battery?

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Solar panels and batteries work hand in hand. The solar panel converts the sun’s rays into electricity and the battery stores this electricity for use by appliances and gadgets. For this to work, the two components must be compatible. So it’s no surprise a common question is “what solar panel size to charge a 12V battery?”

A 300 watt solar panel can charge a 12V battery in 4 hours in favorable weather conditions. A smaller solar panel can be used with a 12V battery but it is going to require more sun hours to complete the charge.

What Solar Panel Size is Best for Charging 12V Battery?

There are many factors to consider when buying a solar panel, But for battery charging purposes it will depend on the battery size and how quickly you want it charged. it also depends on how many sunlight hours are available.

Suppose there are 6 hours of sunlight per day in your area. You have a 100ah 12V battery, which is equal to 1200 watts.

A 100 watt solar panel needs two days to charge a 12V 100ah battery. This is based on the assumption the panel produces 600 watts a day.

A 300 watt solar panel will charge the same 100ah battery in a single day. If the panel generates 250 watts an hour, it only needs 5 hours to recharge the battery. And if the module reaches peak output of 300 watts, the battery will be ready in four hours.

If you are looking for a 300 watt solar panel, we recommend the Rich Solar 200W Solar Panel Pack. it works great for various outdoor settings including RVs.

There are two kinds of solar panels, monocrystalline and polycrystalline. Monocrystalline solar panels are more energy efficient because they are made from a single source of silicon. Polycrystalline combine several silicon sources, which is inferior to silicon.

How big is the difference? It depends on the quality of the solar panel design and manufacturing process. In some cases monocrystalline panels are up to 15% more efficient, but in other cases it is negligible. It comes down to the manufacturing procedure.

How Long Does it Take to Charge a 12V Battery?

If you have a 12V 100Ah battery and a 300W solar panel, the charge time from 0% to 100% should be 5-6 hours, assuming there is 5-6 hours of available sunlight. it also helps if you have a fast charging battery like the Weize 12V 100 AGM so the process doesn’t take forever.

The calculation earlier gives you a charge time of 5 hours, but it could reach 6 depending on the sun. The charge time will also depend on the following factors:

  • Solar panel watts
  • Battery capacity
  • How much the battery is drained
  • Charge controller
  • Available hours of sunlight

We assume the panel generates 300 watts an hour. This is possible but if there are clouds or dirt on the solar panel, output will be lower. In some cases the solar panel will not reach 300 watts an hour, probably 250 watts or so.

If the output is les than 300 watts, battery charge time will take longer. By how much depends on when the skies clear up. keep in mind too that the sun’s position in the sky changes. As the afternoon draws to a close, the sun goes down the horizon and the solar panel receives less light. This is going to reduce power output and batteries will take longer to charge.

How Much Solar Power Do I Really Need to Charge Batteries?

The rule of thumb among solar power users is to add 10% to 20% more power than what your estimated needs are. First we will explain how to calculate solar power watts for 12V batteries.

Multiply the battery capacity (ah) x voltage.

The result is the watts. Divide the watts by the number of sunlight hours available.

With a 12V 100ah battery, we get the following: 100ah / 12V=1200 watts. 1200 watts / 6 hours of sunlight = 200 watts.

So now you have 200 watts. That should be enough to charge the battery, right? While 200W can work, you have to bet on ideal weather. Second, you’ll be pushing the solar panel to the limit. Not only the panel, but other components like the charge controller and battery. It is better to have a reserve or buffer present.

That’s why most solar power users add another 20% to the calculation. So if the calculation gives you 200W, add 20% (200 + 20%) to get 240 or better yet 300 watts. This will be more than enough to charge the battery even if the weather is less than ideal. if you have a 40W solar panel the charging will be longer.

Battery Capacity Explained

Solar battery capacity is measured in amp hours (ah). The battery stores the power generated by the solar panel and this current is converted by an inverter into usable AC electrical power for TVs, refrigerators, fans and other appliances.

1 ah is equal to 1000 mAh (milliamp hour). Power bank capacity for smartphones, mobile devices and portable solar generators are measured in mAh.

A lead acid battery at 0% takes 5 to 6 hours to charge based on the formula given above. If you recharge the battery at 50%, the charge time drops to 2.5 hours, assuming the weather is the same. If you do not let the battery level drop below 50%, you can charge it with a 200W solar panel.

But what if the battery is at 15% of its capacity? How do you calculate what solar panel watts are needed?

100 ah x 15% = 15 amps
15 amps x 12V = 180 watts
180W / .85 efficiency = 211 watts

You need a 211W solar panel to charge a lead acid battery that is at 15% of its capacity. You can replace the 15% with any number and adjust the efficiency accordingly. This is sufficient but as previously stated, having more watts never hurts.

If you have a 200ah battery, use the same calculation, just replace the 100ah with 200ah. Roughly you need around 400+W to fully charge a 200ah battery.

When to Recharge 12V Batteries

In the examples we have given, the charge time assumes the 12V battery is 100% drained. It can happen and it does, but experts strongly recommend you avoid this. Allowing a battery to completely drain shortens its lifespan and makes it less reliable.

We recommend recharging a lead acid battery when the capacity drops to 50%. For lithium ion, you can wait until it is around 35-40%. Yes you may have read lithium ion batteries can be used until it’s at 0%. But it is better to be safe and not let it drop that low.

Using only 50% of a battery’s capacity may seem like a waste. But the 50% level is a good trade-off between the price and lifespan. Utilizing only 50% prolongs the battery lifespan, so you save money by not having to buy a replacement sooner.

Putting the 50% charge rule into practice is easy. If you have a 100ah 12V battery it has a capacity of 1200W. Once you use up 600W, it is time to recharge. If you have a 200ah battery, recharge after using up 1200W and so on. By adhering to the 50% charge rule, you cut down the charge time and you get more out of the battery and the solar panel.

You should have 6 hours of sunlight available during the summer, enough to charge a 12V 100ah battery. Make sure your solar panel is properly oriented towards the sun for maximum effect. If the charge seems to take too long, check if there is anything blocking the panel, i.e., tree branches. For best results, set the solar panel in a clear area without any trees or buildings nearby.

The best solar panels won’t do much good if the cells are shaded. Always make sure the solar panel is in a clear spot so when you charge the battery, you get the best results.

Why You Need a Charge Controller

The charge controller does exactly what its name says, control the charge. When electricity flows from the solar panel into the battery, the charge controller makes sure there is no overcharging, overheating or overloading. Many charge controllers also have protection against short circuits and electrical discharge problems.

A 20A charge controller is enough for a 100Ah 12V battery. For a 200Ah battery, a 30A controller is better. When the controller runs, it charges the battery in 3 phases. Phase 1, where the controller puts in as much power as it can in the battery, followed by phase 2 which is absorption, a lengthy charge that lasts until the voltage is at maximum. Phase is the trickle charge and the battery is at full capacity.

Your charge controller should be compatible with your solar panel voltage. A typical 12V 100W solar panel comes with 30 or 32 cells generating 16 to 18V. The voltage goes down to about 15V during load, which is what a 12V battery needs. If you are charging a 100Ah battery you need two of these.


Learning how to use solar panels and batteries effectively is the key. As you can see from the examples here, it is not that difficult. Once you have an idea of how many solar panels are needed, you can charge any battery regardless of their capacity. Knowing how the math works makes it easy to optimize battery charging.