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In most solar power systems, the batteries run the appliances and the solar panels recharge the batteries. So you have to know how many solar panels are going to be needed for the charge, and how long it will take. This guide explains everything you need to know.
It is going to take 4 x 300W solar panels to charge four 100ah 12V batteries in 5 hours. The charge time is based on a 1200 to 1500W hourly output from the panels. Battery charging will take more time if the output is lower.
How to Calculate Solar Panel Battery Charge Time
Anyone who has dealt with solar power knows some math is involved. But the process is easy and you can apply these principles for any number of batteries. In this example we are using 12V 100ah batteries.
Solar panel watt output per hour x number of sun hours available = watt output per day
With this you can estimate how long your solar array will take to charge four batteries.
The number of solar panels needed depends on how quickly you need those batteries recharged. It also hinges on the battery depth discharge, whether it is fully discharged or at 50%.
Four batteries at 100ah and 12V is 4800 watts. A 300 watt solar array can produce 1500 watts a day with 5 sunlight hours available. You may try this with the Renogy Solar Panel Kit for example.
- You need 4 x 300W solar panels to recharge four batteries in 5 hours.
- If you only need those batteries every two days, you can recharge them over two days with 2 x 300W solar panels.
- If the batteries are only 50% discharged. the charge time is reduced to half.
Four 12V 100ah batteries at 50% DOD is 2400 watts. With 4 x 300 watt solar panels the charge time will be 2 to 3 hours. A single 300 watt solar panel can recharge four 100ah batteries at 50% DOD in 2 days with at least 5 sun hours availability.
Events That Affect Solar Battery Charging Time
The math is simple enough but there are many factors that determine how long charging will be. The most important are sunlight hours, battery depth discharge, battery usage and solar panel performance and efficiency.
Number of Sun Hours
Solar panels need sunlight to charge batteries, the more hours the better. Depending on where you are and the season, you might get 4 to 8 hours of sun per day.
You have four 200ah 12V batteries and they are completely discharged. You have five 250 watt solar panels. Let us compare the charge time with different sun hour availability.
4 x 200ah = 800ah
800ah = 9600W
Those 4 batteries need 9600 watts to recharge. If there are four sun hours available for your five 250 watt solar array, then:
250 x 5 = 1250
The panels can produce 1250 watts an hour.
1250 x 4 = 5000 watts
It would take two days for 5 x 250W solar panels to charge 4 x 200ah 12V batteries.
Now if you had 8 hours of sunlight:
1250 x 8 = 10000
The same solar array will do it one day. But that is assuming the solar panels produce maximum output. But even if it did not, charging time will still be faster.
Solar Panel Efficiency and Weather Variables
To perform these calculations we had to make some assumptions. The first is the number of sun hours and second, that the solar panels produce the maximum output. But that may not always be the case.
When we say that a 300 watt solar panel produces 1500 watts a day, it presumes there are 5 sun hours and the panel generates 300 watts every hour (300 x 5 = 1500).
But that output is only possible in ideal weather conditions. If clouds pass by and block the sun, the panel output will drop. If some foliage or debris lands on the panel, output will go down.
Solar panels can reach maximum output if they are properly oriented and the sun is at its highest point in the sky. But this isn’t constant so if you look at the solar panels, output changes throughout the day.
This is where solar panel efficiency matters. You will see solar panels advertised as 21%, 23% efficient and so on. Efficiency refers to the percentage of sun energy the panels convert into solar power.
The best solar panels have a 20% efficiency rating or higher. This is not the same as the output rating. That is, if a solar panel is 20% efficient, it does not mean it only produces 20% of its rated watt output. Rather it means 20% of the sun rays hitting the solar cells are turned into current.
What Solar Panel Size Do I Need to Charge Batteries?
The standard solar panel size today is 300 watts and for battery charging it works fine. You can use other solar panel sizes but 300Wis ideal for many reasons.
One, solar panels take up considerable space. Each one is 65 x 39 inches on average (5.4 x 3.2 ft.) and weighs 40 lbs.
If you have four batteries to charge, you would need four 300 watt solar panels. That is equal to 8 x 200W or 15 x 100W solar panels. If you want to use solar panels with different wattages, check out our guide here.
These three solar panel types do not differ much in physical size, so you should get the one that has the highest rated output. The same thing can be said for the weight as more panels mean more pressure on your rooftop.
Solar panels need space between each other, so the fewer modules the easier installation will be. Too many panels increases the chances of something going wrong with the wiring or connectors.
Fewer solar panels also makes cleaning and maintenance easier. In case there is a problem it is easier to troubleshoot four 300 watt panels than fifteen 100 watt panels.
How Long Will Batteries Last Per Charge?
The battery life per charge depends on its quality, usage, capacity and depth of discharge. All of these determine the charge frequency.
For example, your setup consists of four UPG 100ah batteries, 5 x 300W solar panels and an inverter. If you place a 4800 watt load on the inverter, the batteries will run out in an hour.
In fact the battery bank will probably stop running in less than that One, inverter inefficiency causes them to use more power than the load. Two, batteries lose discharge at a higher rate when more amps are used.
The fewer amps used per hour, the longer the batteries will last. The depth discharge is also a factor. Obviously a 50% DOD battery is going to charge faster than a fully discharged one. The disadvantage is you can only use half the capacity at a time.
Deep cycle lead acid batteries like gel and AGM have to be charged at 50%. Some lithium batteries have a 100% discharge rate, but most are at 80-85%. If you want full use of the batteries, buy lithium or purchase twice the capacity you need for lead acid batteries.
How to Make Solar Battery Charging Faster
- Position the solar panels to face true south. If you are in the southern hemisphere, the panels must be oriented true north.
- Make sure there is nothing to obstruct the panels’ path to the sun. No shading, no leaves, no debris etc.
- Connect the batteries and solar panels with the proper wiring. Short, thick cables are the best.
- Calculate ahead of time. Determine how many sun hours are available and when you need the batteries charged. This will tell you how many solar panels are going to be needed.
- For lead acid batteries, charge at 50% or higher. Do not fully charge it though, around 95% is enough.
- Perform proper battery maintenance. Lithium batteries do not need any, but lead acid batteries need water refilling every few weeks.
- Be mindful of the voltages. 24V batteries hold twice as many watts as 12V. If you have these, you can connect two 12V solar panels in a series so they become 24V and a match for the batteries.
There are a lot of factors that determine battery charge time, so you have to consider these ahead of time. The key is planning ahead and making sure you have some reserve power available.
I am an advocate of solar power. Through portablesolarexpert.com I want to share with all of you what I have learned and cotinue to learn about renewable energy.