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The answer to the question above is 240 watts of power. This assumes it is a 12V battery with 6 hours of sunlight available plus 20% extra watts to compensate for energy loss. It takes 5-6 hours to fully charge a 100ah battery depending on how depleted it is.
That scenario works if there are the aforementioned hours of sunlight, otherwise the numbers change. While you can get a 240 watt solar panel now and charge that battery, you may want to learn how to calculate solar panel size for any battery capacity. Here we also point out the best time to charge a 100ah battery and how long it will take under different conditions.
How to Calculate Solar Panel Size for Battery
Before we go over the formula, keep the following in mind:
Solar panel sizes are measured in watts
- W: watts
- kWh: kilowatt
- 1000 watts = 1 kilowatt
Battery capacity is measured in Ah or mAh:
- Ah: amp hour
- mAh: milliamp hour
- 1000 mAh = 1 Ah
The formula step by step is:
- Battery amp hour (Ah) x battery voltage = watts
- Watt hours / number of available sunlight hours = solar panel watts
- Solar panel watts + 20% extra panel power = solar panel watt size you require
Using the example above:
- 100 ah battery x 12V = 1200 watts
- 1200 watts / 6 hours of sunlight = 200W solar panel
- 200W + 20% extra watts = 240
With this calculation you can replace the battery capacity with any ah number. You can also adjust the number of full sunlight hours and how much reserve watt power you want, but the steps are still the same.
However there are several factors that affect the charging time of solar panels and batteries. These include sunlight availability, charge controller type and the solar panel efficiency rating.
Solar Panel Efficiency Rating
Even under ideal weather conditions, solar panels are not 100% efficient. There will be some energy lost during the accumulation phase. That is why you should get a larger solar panel than what your battery needs. So in this example you get a 240 or 250watt solar panel to charge a 100ah battery. Some would go further and buy a 300 watt solar panel. If you want to do this, we suggest the Renogy 300 watt solar panel kit as it also come with a 30A charge controller.
You can also try the following solar panel sizes and combos.
1 x 240 watt panel = 240 watts
2 x 120 watt panel = 240 watts
5 x 50 watt panel = 250 watts
3 x 100 watt panel = 300 watts
It does not matter which combination above you choose as long as it is 240 watts minimum. You can configure the panels in a series or parallel, just make sure they are close together and there are no obstructions. This is something to keep in mind if you are boondocking for instance.
The 5-6 hour charge time for a 100ah battery assumes there is nothing blocking the panel. The entire panel will be affected if even a single cell were to be blocked, so make sure there is nothing in their way. If something suddenly blocks the panel, the charge may slow down or stop completely.
Choose the Right Charge Controller Size
Your charge controller should be compatible with the amperage (A, amp) and watts going through the system. If you have a 20A 240 watt system, your charge controller must be able to match that. If you are running a 240 watt solar panel, you need at least a 300W capable charge controller.
There are two types of charge controllers, PWM and MPPT. PWM is the standard and works fine for most solar panels and batteries. MPPT charge controllers are more accurate and can handle greater power. But they are also more expensive. For charging a 100ah battery however, a 30A PWM controller the should be fine. Our choice is the EEEKit Solar Charge Controller as it does the job without any fuss.
How Long to Charge a 12V 100ah Battery?
- Total solar panel watts / battery voltage = amps
- Battery capacity / amps = time required to charge
So in our example:
- 240 watt solar panel / 12V battery = 20A (amps)
- 100 ah battery / 20A = 5 hours
Again, this assumes there is at least 5 hours of peak sunlight available. The calculation and charge time will be different if there are fewer hours of sunlight.
A 20 amp charge current average is required for a completely depleted 100ah battery The voltage panel itself should have a 14.5V capacity, even if the battery is 12V. Some voltage is inevitably lost during the transfer from the solar panel to the battery. This may increase the required panel voltage to 15.5V.
Keep in mind that very hot temperatures may lead to solar panel performance degradation. This results in a drop in voltage and a slower charge time. If you’re going to charge a 100Ah battery under hot conditions, you need a 360W solar panel with 20A, 18V power.
This can be avoided two ways: the first is not charge the panel under very hot temperatures. The second is to use a charge controller that optimizes energy transfer. MPPT charge controllers are designed for this, optimizing a solar panel’s power point to reduce watt requirement. Instead of a 360W solar panel can get away with a 300-320W solar panel.
There are a lot of myths about solar power, but its reliance on sunlight is 100% fact. In ideal weather conditions those numbers will hold. But if it is a cloudy day the charge time and solar energy production could slow by up to a third.
Even if it is a sunny day, a sudden passing cloud could slow the charge down. In the worst case scenario the panel stops charging and you have to recharge from 0%. Some solar panels fortunately, have a resume function so if a cloud passes by, the charge will stop then resume from where it left off.
When to Recharge a 100ah Battery?
The example scenario here assumes that the battery has been completely drained. In that case the 5-6 hour charge time is correct. But you should never let a battery drop to 0% because it isn’t good for the long term.
Lead acid batteries should be recharged when the power drops to 50%. The same with flooded batteries. With lithium ion you can let it drop to 35-40%, then recharge. Charging a battery from 0-100% repeatedly is going to weaken its internal circuits. So whenever possible, charge the battery before it falls below 50% for lead acid and before 35-40% for lithium ion.
Can a 200W Solar Panel Charge a 100ah Battery?
Recharging at the 50% level will prolong the life of your battery, and it shortens the charge time considerably. Instead of waiting 5 to 6 hours, it will only take 2 to 3 hours before the battery is fully charged. This does bring up the question: if you recharge the battery at 50%, do you still need 240W or 300W solar panels?
Technically, you can use a 200 watt solar panel to charge a 100ah battery if it is 50% full. But it will take about 5 hours or so. If the battery is at 0%, it will take all day. Even if it is at 50% level, you have to hope that the weather stays perfect for a fast charge.
As we have mentioned earlier, it is better to have a bigger solar panel than a small one. Even if the weather is perfect, there will be some energy loss due to poor wiring, dust on the solar panel, improper connection, or the panel orientation is not optimal.
Adding another 100 watt panel to your solar array is easy, and the cost has really dropped. The convenience you get is well worth the price. Just like the battery, you should never push your solar panels to their maximum capacity. Always leave some extra power as reserve.
Figuring out how many solar panels are needed to charge a 100ah battery is not difficult, as we have shown here. At first it seems like a lot of math is involved, but the calculation is really simple. Once you know the process you can replace the number with any battery size you want. Knowing how to do this eliminates guesswork.